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ContentsPage

90-2 Scope of the N.E.C. 4110-3(a)(5),(6), & (8) Requirements for Equipment Selection4110-3(b) Requirements for Proper Installation of Listed and Labeled Equipment4110-9 Requirements for Proper Interrupting Rating of Overcurrent Protective Devices 5110-10 Proper Protection of System Components from Short-Circuits 12110-22 Proper Marking and Identification of Disconnecting Means 15210-20(a) Ratings of Overcurrent Devices on Branch Circuits Serving Continuous and Non-Continuous Loads 15215-10 Requirements for Ground-Fault Protection of Equipment on Feeders 15230-82 Equipment Allowed to be Connected on the Line Side of the Service Disconnect 16230-95 Ground-Fault Protection for Services 16240-1 Scope of Article 240 on Overcurrent Protection 17

240-3 Protection of Conductors Other Than Flexible Cords and Fixture Wires 18240-4 Proper Protection of Fixture Wires and Flexible Cords 19240-6 Standard Ampere Ratings19240-8 &380-17 Protective Devices Used in Parallel19240-9 Thermal Devices19240-10 Requirements for Supplementary Overcurrent Protection 19240-11 Definition of Current-Limiting Overcurrent Protective Devices 20240-12 System Coordination or Selectivity 21240-13 Ground Fault Protection of Equipment on Buildings or Remote Structures 22240-21 Location Requirements for Overcurrent Devices and Tap Conductors 22240-40 Disconnecting Means for Fuses 25240-50 Plug Fuses, Fuseholders, and Adapters 25240-51 Edison-Base Fuses25240-53 Type S Fuses 25240-54 Type S Fuses, Adapters, and Fuseholders 26240-60 Cartridge Fuses and Fuseholders 26

240-61 Classification of Fuses and Fuseholders 26240-86 Series Ratings 27240-90 & 91 Supervised Industrial Installations 27240-92(b) Transformer Secondary Conductors of Separately Derived Systems 27240-92(b)(1) Short-Circuit and Ground-Fault Protection28240-92(b)(2) Overload Protection 28240-92(c) Outside Feeder Taps28240-100 Feeder and Branch Circuit Protection Over 600 Volts Nominal 28240-100(c) Conductor Protection28250-2(d) Performance of Fault Current Path 29250-90 Bonding Requirements and Short-Circuit Current Rating 29250-96(a) Bonding Other Enclosures and Short-Circuit Current Requirements 29250-122 Sizing of Equipment Grounding Conductors29310-10 Temperature Limitation of Conductors 30364-11 Protection at a Busway Reduction 31384-16 Panelboard Overcurrent Protection31430-1 Scope of Motor Article 31

430-6 Ampacity of Conductors for Motor Branch Circuits and Feeders 31430-8 Marking on Controllers 32430-32 Motor Overload Protection 32430-36 Fuses Used to Provide Overload and Single-Phasing Protection 32430-52 Sizing of Various Overcurrent Devices for Motor Branch Circuit Protection 33430-53 Connecting Several Motors or Loads on One Branch Circuit34430-71 Motor Control-Circuit Protection 34430-72(a) Motor Control-Circuit Overcurrent Protection 34430-72(b) Motor Control-Circuit Conductor Protection 34430-72(c) Motor Control-Circuit Transformer Protection 36430-94 Motor Control Center Protection 37430-109(a)(6)Manual Motor Controller as a Motor Disconnect 37440-5 Marking Requirements on HVAC Controllers37440-22 Application and Selection of the Branch Circuit Protection for HVAC Equipment 37450-3 Protection Requirements for Transformers37450-3(a) Protection Requirements for Transformers Over 600 Volts38450-3(b) Protection Requirements for Transformers 600 Volts or Less39

450-6(a)(3) Tie Circuit Protection 39455-7 Overcurrent Protection Requirements for Phase Converters 39460-8(b) Overcurrent Protection of Capacitors 39501-6(b) Fuses for Class I, Division 2 Locations 40517-17 Requirements for Ground Fault Protection and Coordination in Health Care Facilities 40520-53(f)(2) Protection of Portable Switchboards on Stage 40550-6(b) Overcurrent Protection Requirements for Mobile Homes and Parks41610-14(c) Conductor Sizes and Protection for Cranes and Hoists 41620-62 Selective Coordination of Overcurrent Protective Devices for Elevators 41670-3 Industrial Machinery 41700-5 Emergency Systems Their Capacity and Rating 42700-16 Emergency Illumination42700-25 Emergency System Overcurrent Protection Requirements (FPN) 42701-6 Legally Required Standby Systems Capacity and Rating 43702-5 Optional Standby Systems Capacity and Rating 43705-16 Interconnected Electrical Power Production Sources Interrupting and Short-Circuit Current Rating 43725-23 Overcurrent Protection for Class 1 Circuits 43

760-23 Requirements for Nonpower-Limited Fire Alarm Signaling Circuits 43

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Overcurrent Protection And The 1999 National Electrical Code

NE99Questions & Answers To Help You Comply

NE99

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2

A presentation format of questions and an-swers has been used in this bulletin to focus on

the factors which are pertinent to a basic

understanding and application of overcurrent

protective devices. Relevant sections of the

National Electrical Code are referenced and

analyzed in detail. Each section is translated

into simple, easily understood language,

complemented by one-line diagrams giving

sound, practical means of applying overcurrent

protection, as well as affording compliance with

the National Electrical Code. This Buss bulletin

is helpful to engineers, contractors, electricians,

plant maintenance personnel, and electrical

inspectors. It also should prove to be a valuable

training aid for formal and informal instruction.

National Electrical Code and N.E.C. are registered trademarks of the National Fire ProtectionAssociation (NFPA), Inc., Quincy, MA 02269. This bulletin does not reflect the official position ofthe NFPA.

Great care has been taken to assure the recommendations herein are in accordance with the N.E.C

and sound engineering principles. Bussmann cannot take responsibility for errors or omissions thatmay exist. The responsibility for compliance with the regulatory standards lies with the user.

Copyright March 2000 by Cooper Bussmann, Inc.

Printed U.S.A.

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90-2 Scope of the N.E.C.

What does this Section mean?90-2(b) covers installations that are not covered by requirementsof the N.E.C. However, the fine print note states that it is the intent

of this section that utility installed utilization equipment located onprivate property is subject to the National Electrical Code.

110-3(a)(5), (6) and (8) Requirements for Equipment Selection

What does 110-3(a)(5), (6) and (8) require?When equipment is selected its arc-flash protection capability andfinger-safe rating must be evaluated. When equipment isenergized, and the door is open the possibility exists that an

employee could accidentally create an arcing fault or come intocontact with a live part. Equipment must be evaluated for bothpossibilities, and be chosen for minimum employee exposure toeither danger.

110-3(b) Requirements for Proper Installation of Listed and Labeled Equipment

What is the importance of Section 110-3(b)?Equipment that is listed is subject to specific conditions ofinstallation or operation. The conditions must be followed for safeand proper operation.

What is the protection requirement of an air conditioner when its nameplate specifies Maximum Fuse Size Amperes?

Fuse protection in the branch circuit is mandatory to meet therequirements of the U.L. Listings and the National ElectricalCode.

Note that the U.L. Orange Book Electrical Appliance andUtilization Equipment Directory, April 1998, requires the followingfor central cooling, air conditioners: Such multimotor andcombination load equipment is to be connected only to a circuitprotected by fuses or a circuit breaker with a rating which does notexceed the value marked on the data plate. This marked protectivedevice rating is the maximum for which the equipment has beeninvestigated and found acceptable. Where the marking specifiedfuses, or HACR Type circuit breakers, the circuit is intended tobe protected only by the type of protective device specified. U.L.Standard 1995 also covers this subject.

What about a motor starter heater table (such as that shown below)which specifies Maximum Fuse?

Heater Full-Load Current Max.

Code of Motor (Amperes) Fuse

Marking (40C Ambient)XX03 .25- .27 1

XX04 .28- .31 3

XX05 .32- .34 3

XX06 .35- .38 3

XX14 .76- .83 6

XX15 .84- .91 6

XX16 .92-1.00 6

XX17 1.01-1.11 6

XX18 1.12-1.22 6

Above Heaters for use on Size 0

Like an air conditioner, use of fuse protection is mandatory. Also,the fuse must provide branch circuit protection and be no largerthan the specified size [430-53(c)]. The chart shown, for example,is typical for starter manufacturers and may be found on the insideof the door of the starter enclosure. (See starter manufacturer for

specific recommendations.)

8RY461M3-A

230

230

37

60

207

--

60

60

140

Typical Nameplate of a Central Air Conditioning Unit.

LISTED SECTION OF CENTRAL COOLING AIR CONDITIONER

ADME

812H

COMPRESSOR

FAN MOTOR

MINIMUM CIRCUIT AMPACITY

MAXIMUM FUSE SIZE AMPS

MINIMUM OPERATING V OLTAGE

FACTORY CHARGED WITH REFRIGERATORSEE CONTROL PANEL COVER FOR AOF SYSTEM REFRIGERANT

*COMPRESSOR RATED IN RLA

ELECTRICAL RATINGS

VAC PH CYC LRA

FOR OUTDOOR USE

UL TYPE NO.

CIRCUITBREAKERBRANCHCIRCUIT NON-FUSED

DISCONNECT

AIR CONDITIONERMARKED WITH"MAX" FUSE

Violates N.E.C. & Listing Requirements


BRANCHCIRCUITFUSEDDISCONNECT

FUSEDFEEDERCIRCUIT

Conforms to N.E.C. & Listing Requirements


NON-FUSEDDISCONNECT

FUSEDBRANCHCIRCUIT


BRANCHCIRCUITFUSEDDISCONNECT

CIRCUITBREAKER



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110-3(b) Requirements for Proper Installation of Listed and Labeled Equipment

What violation exists when a series-rated panelboard with a 42/10system rating has the potential to see a fault current less than 4 ft. fromthe loadside circuit breaker?

U.L. 489 Series Rating tests allow a maximum of 4 ft. of rated wireto be connected to the branch circuit breaker. Whenever thepotential for a fault exists closer than 4 ft. from the circuit breaker,i.e., where the #12 wire leaves the enclosure, or a maintenanceman is working on the equipment hot, a violation of 110-3bexists, as does a potentially hazardous condition. In this situation,the interrupting capacity of the circuit breaker may not equal itsmarked interrupting rating!

110-9 Requirements for Proper Interrupting Rating of Overcurrent Protective Devices

What is the importance of Section 110-9?

Equipment designed to break fault or operating currents musthave a rating sufficient to withstand such currents. This articleemphasizes the difference between clearing fault level currentsand clearing operating currents. Protective devices such as fusesand circuit breakers are designed to clear fault currents and,therefore, must have short-circuit interrupting ratings sufficient forfault levels. Equipment such as contactors and switches haveinterrupting ratings for currents at other than fault levels. Thus, theinterrupting rating of electrical equipment is divided into two parts.

Most people are familiar with the normal current carrying ampere ratingof a fuse or circuit breaker; however, what is a short-circuit interruptingrating?It is the maximum short-circuit current that an overcurrentprotective device can safely interrupt under specified testconditions.

What is a devices interrupting capacity?The following definition of Interrupting Capacity is from theIEEE Standard Dictionary of Electrical and Electronic Terms:

Interrupting Capacity: The highest current at rated voltage thatthe device can interrupt.

Because of the way fuses are short-circuit tested (withoutadditional cable impedance), their interrupting capacity is greaterthan or equal to their interrupting rating. Because of the way circuitbreakers are short circuit tested (with additional cableimpedance), their interrupting capacity can be less than, equal to,or greater than their interrupting rating.

How does Section 110-9 pertain to services?Service equipment must be able to withstand available short-circuit currents. More specifically, the service switchboard,panelboard, etc., and the protective devices which theyincorporate must have a short-circuit rating equal to or greater

than the short-circuit current available at the line side of theequipment.

In this circuit, what must be the short-circuit rating of the switchboard?

At least 100,000 amperes.

What must be the interrupting rating of the fuses?

100,000 amperes or greater. (Most current-limiting fuses have aninterrupting rating of 200,000 or 300,000 amperes.)

In this circuit, what must be the interrupting capacity of the main circuitbreaker, and the short-circuit rating of the switchboard?

At least 100,000 amperes.

As shown in the circuit, can fuses be used to protect circuit breakers witha low interrupting rating.

Yes. Properly selected fuses can protect circuit breakers as wellas branch circuit conductors by limiting short-circuit currents to alow level even though available short-circuit current is as high as100,000 amperes. (Buss LOW-PEAK YELLOW or T-TRON

fuses give optimum protection.)100,000Aavailablefault current

Fuses must have100,000 amperesinterrupting ratingor greater

100,000Aavailablefault current MAIN

BREAKER

SWITCHBOARD

FEEDER CIRCUIT BREAKERS

100,000Aavailablefault current

200 ampere service entrance panelmust have a short circuit ratingequal to or greater than 100,000amperes

10,000A.I.C.breakers

#12 CuWIRE

10KA.I.R.20A CB's

200A Panelboard

Branch Circuit

Fault

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Can cable limiters protect service entrance equipment from short-circuitcurrents?

Current-limiting cable limiters not only can be used to isolate a

faulted service cable, but also can help to protect utility meterswith low withstand ratings against high short-circuit currents. (SeeSection 230-82).

Application Note:Residential100 ampere and 200 ampere fused main-branchcircuit breaker panels are commercially available. These loadcenters incorporate the small-sized T-TRON JJN fuses whichmake it possible to obtain a 100,000 amperes short-circuit currentrating. Mobile home meter pedestals are also availableincorporating the T-TRON JJN fuses in a Fuse Pullout Unit.

Apartment ComplexesHave high densities of current and,therefore, high short-circuit currents for the typical meters.

Grouped meter stacks are commercially available using the T-TRON JJN fuses (up to 1200 amperes) to give the proper short-circuit protection. Meter stacks are also available with Class T fuse

pullouts on the load side of each meter.

What happens if a fault current exceeds the interrupting rating of a fuseor the interrupting capacity of a circuit breaker?It can be damaged or destroyed. Severe equipment damage andpersonnel injury can result.

In this circuit, what interrupting rating must the fuse have?

At least 50,000 amperes. (Class R, J, T, L and CC fuses have anInterrupting Rating of at least 200,000 amperes. The interruptingrating of a fuse and switch combination may also be 200,000amperes. . .well above the available short-circuit current of 50,000amperes. The interrupting rating of Class G fuses is 100,000amperes; K1 and K5 fuses can be 50,000, 100,000, or 200,000amperes.)

In this circuit, what interrupting rating must the circuit breaker have?

Some value greater than or equal to 50,000 amperes. Seediscussion on circuit breaker interrupting rating in Section 110-10for a further evaluation. (Faults within four feet of the breaker couldcause complete destruction of the breaker if it is applied where theavailable fault current approaches the tested interrupting capacityof the breaker.)

Section 110-9 also requires the overcurrent device to have asufficient interrupting rating for both phase voltage and phase-to-ground voltage.

What is the significance of this requirement?Certain molded case circuit breakers have lower single-poleinterrupting ratings than their multi-pole interrupting rating.

What are the single-pole interrupting ratings for overcurrent devices?

What are the single-pole interrupting ratings for overcurrentdevices? Modern current limiting fuses such as Class RK1, J andL have single-pole interrupting ratings of at least 200,000 amperesRMS symmetrical. For example, per UL/CSA 248-8, a 600 voltClass J fuse is tested at a minimum of 200,000 amperes at 600volts across one pole. Bussmann has recently introduced theabove fuse types with 300,000 ampere single-pole interruptingratings. Per ANSI C37.13 and C37.16, an airframe/power circuitbreaker has a single-pole rating of 87% of its three-pole rating.Listed three- pole molded case circuit breakers have minimumsingle-pole interrupting ratings according to Table 7.1.7.2 of U.L.489. The following table indicates the single-pole ratings of variousthree-pole molded-case circuit breakers taken from Table 7.1.7.2of U.L. 489. A similar table is shown on page 54 of the IEEE BlueBook (Std 1015-1997). Molded-case circuit breakers may or maynot be able to safely interrupt single-pole faults above thesevalues since they are typically not tested beyond these values.

If the ratings shown in this table are too low for the application,the actual single-pole rating for the breaker must be ascertained toinsure proper application. Or, modern current limiting fuses orairframe/power circuit breakers can be utilized.

CABLE LIMITER

UNDERGROUND CABLE

(Residential and lightcommercial buildings)

METER

METERS

METERS

JJN FUSE(up to 1200A)

JJN FUSE(up to 1200A)

CLASS T FUSES

Available fault current50,000 amperes

Available fault current50,000 amperes

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As an example of single-pole interrupting ratings in a typicalinstallation, consider a common three-pole, 20 amp, 480 voltcircuit breaker with a three-pole interrupting rating of 65,000amperes. Referring to the table, this breaker has an 8,660 amperesingle-pole interrupting rating for faults across one pole. If theavailable line-to-ground fault current exceeds 8660 amps, theMCCB may be misapplied. In this case, the breaker manufacturer

must be consulted to verify interrupting ratings and properapplication.

Single-Pole Interrupting Ratings for Three Pole Molded Case CircuitBreakers (ANY I.R.)

FRAME RATING 240V 480/277V 480V 600/347V 600V

100A Maximum 4,330

250V Max

100A Maximum 10,000 8,660 10,000 8,660

251-600V

101 - 800 8,660 10,000 8,660 10,000 8,660

801 - 1200 12,120 14,000 12,120 14,000 12,120

1201 - 2000 14,000 14,000 14,000 14,000 14,000

2001 - 2500 20,000 20,000 20,000 20,000 20,000

2501 - 3000 25,000 25,000 25,000 25,000 25,000

3001 - 4000 30,000 30,000 30,000 30,000 30,000

4001 - 5000 40,000 40,000 40,000 40,000 40,0005001 - 6000 50,000 50,000 50,000 50,000 50,000

How much short-circuit current will flow in a ground fault condition?The answer is dependent upon the location of the fault withrespect to the transformer secondary. Referring to Figures 3 and4, the ground fault current flows through one coil of the wyetransformer secondary and through the phase conductor to thepoint of the fault. The return path is through the enclosure andconduit to the bonding jumper and back to the secondary throughthe grounded neutral. Unlike three-phase faults, the impedance ofthe return path must be used in determining the magnitude ofground fault current. This ground return impedance is usuallydifficult to calculate. If the ground return path is relatively short (i.e.close to the center tap of the transformer), the ground fault currentwill approach the three-phase short-circuit current.

Theoretically, a bolted line-to-ground fault may be higher than

a three-phase bolted fault since the zero-sequence impedancecan be less than the positive sequence impedance. The groundfault location will determine the level of short-circuit currentavailable. However, to insure a safe system, the prudent designengineer should assume that the ground fault current equals atleast the three-phase current and should assure that theovercurrent devices are rated accordingly.

How does a solidly grounded wye system affect the requirements forsingle-pole interrupting ratings?The Solidly Grounded Wye system shown in Figures 1 and 2 is byfar the most common type of electrical system. This system istypically delta connected on the primary and has an intentionalsolid connection between the ground and the center of the wyeconnected secondary (neutral). The grounded neutral conductorcarries single-phase or unbalanced three-phase current. Thissystem lends itself well to industrial applications where 480V(L-L-

L) three-phase motor loads and 277V(L-N) lighting is required.

Figure 1 - Solidly Grounded WYE System - Circuit Breakers

Figure 2 - Solidly Grounded WYE System - Fuses

If a fault occurs between any phase conductor and ground(Figures 3 and 4), the available short-circuit current is limited onlyby the combined impedance of the transformer winding, the phaseconductor and the equipment ground path from the point of thefault back to the source. [Some current (typically 5%) will flow inthe parallel earth ground path. Since the earth impedance istypically much greater than the equipment ground path, current

flow through earth ground is generally negligible.]

Figure 3 - Single-Pole Fault to Ground - Circuit Breakers

Figure 4 - Single-Pole Fault to Ground - Fuses


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In solidly grounded wye systems, the first low impedance fault toground is generally sufficient to open the overcurrent device on thefaulted leg. In Figures 3 and 4, this fault current causes the branchcircuit overcurrent device to clear the 277 volt fault. This systemrequires compliance with single-pole interrupting ratings for 277volt faults on one pole. If the overcurrent devices have a single-

pole interrupting rating adequate for the available short-circuitcurrent, then the system meets Section 110-9 of the NationalElectrical Code .

Although not as common as the solidly grounded wyeconnection, the following systems are typically found in industrialinstallations where continuous operation is essential. Wheneverthese systems are encountered, it is absolutely essential that thesingle-pole ratings of overcurrent devices be investigated. This isdue to the fact that full phase-to-phase voltage can appear acrossjust one pole. Phase-to-phase voltage across one pole is muchmore difficult for an overcurrent device to clear than the line-to-neutral voltage associated with the solidly grounded wye systems.

How does a corner-grounded-delta system affect the requirements forsingle-pole interrupting ratings?The systems of Figures 5 and 6 have a delta-connected secondaryand are solidly grounded on the B-phase. If the B-phase should

short to ground, no fault current will flow because it is alreadysolidly grounded.

Figure 5 - B-Phase Grounded (Solidly) System - Circuit Breakers

Figure 6 - B-Phase Grounded (Solidly) System - Fuses

If either Phase A or C is shorted to ground, only one pole of theovercurrent device will see the 480V fault as shown in Figures 7and 8. This system requires compliance with single-poleinterrupting ratings for 480 volt faults on one pole.

Figure 7 - B-Phase Solidly Grounded System - Circuit Breakers

Figure 8 - B-Phase Solidly Grounded System - Fuses

A disadvantage of B-phase solidly grounded systems is the

inability to readily supply voltage levels for fluorescent or HIDlighting (277V). Installations with this system require a 480-120Vtransformer to supply 120V lighting. Another disadvantage, asgiven on page 33 of IEEE Std 142-1991, Section 1.5.1(4) (GreenBook) is the possibility of exceeding interrupting capabilitiesof marginally applied circuit breakers, because for a groundfault, the interrupting duty on the affected circuit breaker poleexceeds the three-phase fault duty.

How does a resistance-grounded system affect the requirements forsingle-pole interrupting ratings?Low or High resistance grounding schemes are found primarily inindustrial installations. These systems are used to limit, to varyingdegrees, the amount of current that will flow in a phase to groundfault.Low resistance grounding is used to limit ground fault current tovalues acceptable for relaying schemes. This type of grounding is

used mainly in medium voltage systems and is not widely installedin low voltage applications (600V or below).The High Resistance Grounded System offers the advantage thatthe first fault to ground will not draw enough current to cause theovercurrent device to open. This system will reduce the stresses,voltage dips, heating effects, etc. normally associated with highshort-circuit current. Referring to Figures 9 and 10, HighResistance Grounded Systems have a resistor between the centertap of the wye transformer and ground.

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With high resistance grounded systems, line-to-neutral loadsare not permitted per the (1999) National Electrical Code, Section250-36(4)

Figure 9 - Resistance Grounded System - Circuit Breakers

Figure 10 - Resistance Grounded System - Fuses

When the first fault occurs from phase to ground as shown inFigures 11 and 12, the current path is through the grounding

resistor. Because of this inserted resistance, the fault current is nothigh enough to open protective devices. This allows the plant tocontinue on line. NEC&RM 250-36(3) requires ground detectorsto be installed on these systems, so that the first fault can be foundand fixed before a second fault occurs on another phase.

Figure 11 - First Fault in Resistance Grounded System - Circuit Breakers

Figure 12 - First Fault in Resistance Grounded System - Fuses

Even though the system is equipped with a ground alarm, theexact location of the ground fault may be difficult to determine. Thefirst fault to ground MUST be removed before a second phasegoes to ground, creating a 480 volt fault across only one pole ofthe affected branch circuit device. Figures 13 and 14 show how the480 volt fault can occur across the branch circuit device.

Figure 13 - Second fault in Resistance Grounded System - Circuit Breakers

Figure 14 - Second fault in Resistance Grounded System - Fuses

The magnitude of this fault current can approach 87% of the L-L-Lshort-circuit current. Because of the possibility that a second faultwill occur, single-pole ratings must be investigated. The IEEE RedBook, Std 141-1993, page 367, supports this requirement, Onefinal consideration for resistance-grounded systems is thenecessity to apply overcurrent devices based upon their single-pole short-circuit interrupting rating, which can be equal to or insome cases less than their normal rating.

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How does an ungrounded system affect the requirements for single-poleinterrupting ratings?The Ungrounded Systems of Figures 15 and 16 offer the sameadvantage for continuity of service that are characteristic of highresistance grounded systems.

Figure 15 - Ungrounded System Circuit Breakers

Figure 16 - Ungrounded System - Fuses

Although not physically connected, the phase conductors arecapacitively coupled to ground. The first fault to ground is limitedby the large impedance through which the current has to flow(Figures 17 and 18). Since the fault current is reduced to such alow level, the overcurrent devices do not open and the plantcontinues to run.

Figure 17 - First Fault to Conduit in Ungrounded System Circuit Breakers

Figure 18 - First Fault to Conduit in Ungrounded System - Fuses

As with High Resistance Grounded Systems, ground detectorsshould warn the maintenance crew to find and fix the fault before asecond fault from another phase also goes to ground (Figures 19and 20).

Figure 19 - Second Fault to Conduit in Ungrounded System - Circuit Breakers

Figure 20 - Second Fault to Conduit in Ungrounded System - Fuses

The second fault from Phase B to ground (in Figures 19 and 20) willcreate a 480 volt fault across only one pole at the branch circuitovercurrent device. Again, the values from Table 1 must be usedfor molded case circuit breaker systems as the tradeoff for theincreased continuity of service. Or, properly rated current limitingfuses and air frame/power circuit breakers can be utilized to meetthe interrupting rating requirements. The IEEE Red Book, Std141-1993, page 366, supports this requirement, One finalconsideration for ungrounded systems is the necessity to applyovercurrent devices based upon their single-pole short-circuitinterrupting rating, which can be equal to or in some cases lessthan their normal rating.

A simple solution exists to insure adequate interrupting ratingsboth in present installations and in future upgrades. Moderncurrent-limiting fuses are available that have tested single-poleinterrupting ratings of 300,000 amps. Air frame/power circuitbreakers are also available that have tested single-pole interruptingratings that are 87% of the published three-pole rating.

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Does an overcurrent protective device with a high interrupting ratingassure circuit component protection?No. Choosing overcurrent protective devices strictly on the basisof voltage, current, and interrupting rating alone will not assurecomponent protection from short-circuit currents. High interruptingcapacity electro-mechanical overcurrent protective devices,(circuit breakers) especially those that are not current-limiting, may

not be capable of protecting wire, cable, starters, or othercomponents within the higher short-circuit ranges. See discussionof Sections 110-10 and 240-1 for the requirements that overcurrentprotective devices must meet to protect components such asmotor starters, contactors, relays, switches, conductors, and busstructures.

Note: Breaking current at other than fault levels.

The rating of contactors, motor starters, switches, circuit breakersand other devices for closing in and/or disconnecting loads atoperating current levels must be sufficient for the current to beinterrupted, including inrush currents of transformers, tungstenlamps, capacitors, etc. In addition to handling the full-load currentof a motor, a switch and motor starter must also be capable ofhandling its locked rotor current. If the switch or motor starter hasa horsepower rating at least as great as that of the motor, they will

adequately disconnect even the locked rotor current of the motor.

It is necessary to calculate available short-circuit currents at variouspoints in a system to determine whether the equipment meets therequirements of Sections 110-9 and 110-10. How does one calculate thevalues of short-circuit currents at various points throughout a distributionsystem?There are a number of methods. Some give approximate values;some require extensive computations and are quite exacting. Asimple, usually adequate method is the Buss Point-To-Pointprocedure presented in Buss bulletin SPD, Selecting ProtectiveDevices. The point-to-point method is based on computation of thetwo main circuit impedance parameters: those of transformers andcables. Of these two components, the transformer is generally themajor short-circuit current factor for faults near the serviceentrance. The percent impedance of the transformer can varyconsiderably. Thus, the transformer specification should always be

checked. As shown in the illustration of a typical transformernameplate, % impedance is specifically designated.

Given the f ull-load transformer secondary amperage and percentimpedance of a transformer, how can you compute the level of short-circuit amperes that can be delivered at the secondary terminals(Assuming an infinite, unlimited, short-circuit current at the primary)?

ISCA = (F.L.A.) x 100

%Z x .9

Given: 1.3% impedance from nameplate of 500 KVA transformerwith a 480V secondary601 Full-Load Amperes (from Table below)

ISCA =601 x 100

= 51,368 Amperes1.3 x .9

What are typical values of transformer short-circuit currents?

Short-Circuit Currents Available from Various Size TransformersVoltage+ KVA Full- % Short-

and Load Impedance CircuitPhase Amperes (Name plate) Amperes

25 104 1.58 11,574

371/2 156 1.56 17,351

120/240 50 209 1.54 23,122

1 ph.* 75 313 1.6 32,637

100 417 1.6 42,478

167 695 1.8 60,255

150 416 1.07 43,198

225 625 1.12 62,004

300 833 1.11 83,383

500 1388 1.24 124,373

120/208 750 2082 3.5 66,095

3 ph. 1000 2776 3.5 88,127

1500 4164 3.5 132,190

2000 5552 5.0 123,377

2500 6950 5.0 154,444

1121/2 135 1.0 15,000150 181 1.2 16,759

225 271 1.2 25,082

300 361 1.2 33,426

277/480 500 601 1.3 51,368

3 ph. 750 902 3.5 28,410

1000 1203 3.5 38,180

1500 1804 3.5 57,261

2000 2406 5.0 53,461

2500 3007 5.0 66,822

Three-phase short-circuit currents based on "infinite" primary.* Single-phase values are L-N values at transformer terminals. These figures are based

on change in turns ratio between primary and secondary, 100,000 KVA primary, zerofeet from terminals of transformer, 1.2 (%X) and 1.5 (%R) multipliers for L-N vs. L-Lreactance and resistance values, and transformer X/R ratio = 3.

U.L. listed transformers 25KVA or greater have a 10% impedance tolerance. Short-Circuit Amperes reflect a worst case scenario.

+ Fluctuations in system voltage will affect the available short-circuit current. Forexample, a 10% increase in system voltage will result in a 10% increase in theavailable short-circuit currents shown in the table.

H2

H1 H3X1 X3

X2H0X0

0 ANGULAR DISP.

X3X2X1H0X0H1H2H3

%Zor

PercentageImpedance

COOPERPower Systems Division

THREE PHASE

VOLTAGE

RATING

CATNO%IMP

TRANSFORMER 60 HERTZ65CRISE

BIL-KVFULL-WAVE

WDG.MAT LV

HV GALOILCLASS

OA

LV ENCLOSURE LBS.

KVA 500

12470GRD. Y/7200480Y/277

PCWN 416124-500-L1

LBS. TOTAL

LBS. OIL

LBS.

LBS.

TANK & FITTINGS

UNTANKING

1.3HV

SER.

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110-10 Proper Protection of System Components from Short-Circuits

What is the importance of Section 110-10?The design of a system must be such that short-circuit currentscannot exceed the short-circuit current ratings of the componentsselected as part of the system. Given specific system componentsand level of available short-circuit currents which could occur,overcurrent protective devices (mainly fuses and/or circuitbreakers) must be used which will limit the energy let-through of

fault currents to levels within the short-circuit current ratings of thesystem components. (Current- limitation is treated under 240-11 ofthis bulletin). The last sentence of Section 110-10 emphasizes therequirement to thoroughly review the product standards and toapply components within the short-circuit current ratings in thosestandards.

What is component short-circuit current rating?It is a current rating given to conductors, switches, circuit breakersand other electrical components, which, if exceeded by faultcurrents, will result in extensive damage to the component. Therating is expressed in terms of time intervals and/or current values.Short-circuit damage can be heat generated or the the result ofelectro-mechanical force of high-intensity, magnetic fields.

Conductor Protection

How is the component withstand rating of conductors expressed?As shown in the table below, component withstand of conductorsis expressed in terms of maximum short-circuit current vs. cycles(or time).

TableCopper, 75 Thermoplastic Insulated Cable Damage Table*(Based on 60 HZ).Copper Maximum Short-Circuit Withstand Current

Wire Size in Amperes

75 For For For For

Thermoplastic 1/2 Cycle** 1 Cycle 2 Cycles 3 Cycles**

#14 2,400 1,700** 1,200** 1,000

#12 3,800 2,700** 1,900** 1,550

#10 6,020 4,300 3,000 2,450

#8 9,600 6,800 4,800 3,900

#6 15,200 10,800 7,600 6,200

#4 24,200 17,100 12,100 9,900

Footnotes*Reprinted from ICEA. **From ICEA formula

In this circuit, what is the maximum permissible available short-circuitcurrent?

2700 amperes. Since the protective device is not current-limiting,the short-circuit current must not exceed the one cycle withstandof the #12 conductor, or 2700 amperes.

In this 20 ampere circuit with a non-current-limiting protective device,what would be the smallest size conductor that would have to be used?

No. 4 wire. Since the protective device is not current-limiting, thewire selected must withstand 12,000 amperes for one cycle.

In this circuit, what type of protective device must be used?

It must be current-limiting. When the available short-circuit currentexceeds the short-circuit current rating of the wire, a protectivedevice such as a current-limiting fuse, properly selected, will limit

fault current to a level lower than the wire short-circuit currentrating (3,800 amperes for 1/2 cycle). (See Section 240-1 FPN.) Forinstance, a LOW-PEAK YELLOW LPN-RK20SP fuse will limit the12,000 amperes available short-circuit to less than 1000 amperesand clear in less than 1/2 cycle.

Protection of Motor Controllers, Contacts and Relays

In this circuit, what kind of fuse must be used to provide adequateprotection of the starter?

A current-limiting fuse, such as the Buss LOW-PEAK YELLOWor FUSETRON dual-element fuse. Such a fuse must limit faultcurrents to a value below the withstand rating of the starter andclear the fault in less than 1/2 cycle.

PROTECTIVE DEVICE(1 cycle opening time;not current-limiting)

2' #12 Cu(75C thermoplasticinsulated Cu)Available

Short-CircuitCurrent

Short-Circuit

PROTECTIVE

DEVICE(20A, 1 cycle opening time;not current limiting)

?12,000Aavailablefault current

Short-Circuit

PROTECTIVEDEVICE

2' #12 Cu12,000Aavailablefault current

Short-Circuit


Size 1 Starter(Tested by UL with 5000A available)

Short-Circuit

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What is Type 2, motor starter protection?UL 508E and IEC 947-4-1 have test procedures designed to verifythat motor controllers will not be a safety hazard and will not causea fire.

These standards offer guidance in evaluating the level ofdamage likely to occur during a short-circuit with various branch

circuit protective devices. They address the coordination betweenthe branch circuit protective device and the motor starter. Theyalso provide a method to measure the performance of thesedevices should a short-circuit occur. They define two levels ofprotection (coordination) for the motor starter:

Type 1. Considerable damage to the contactor and overloadrelay is acceptable. Replacement of components or a completelynew starter may be needed. There must be no discharge of partsbeyond the enclosure.

Type 2. No damage is allowed to either the contactor oroverload relay. Light contact welding is allowed, but must beeasily separable.

Where Type 2 protection is desired, the controller manufacturermust verify that Type 2 protection can be achieved by using aspecified protective device. Many U.S. manufacturers have boththeir NEMA and IEC motor controllers verified to meet the Type 2requirements. Only current-limiting devices have been able to

provide the current-limitation necessary to provide verified Type 2protection. In many cases, Class J, Class RK1, or Class CC fusesare required, because most Class RK5 fuses and circuit breakersaren't fast enough under short-circuit conditions to provide Type 2protection.

Type 2 protection is defined and suggested in the notes toTable 1 of NFPA 79 (Industrial Machinery).

Protection of Circuit Breakers

There are several key concepts about the protection of circuitbreakers that need to be understood.

1. The user should be aware of the potential problemsassociated with series-rated circuit breakers. The engineercan not always "engineer" the installation as beforebecause,

2. A molded case circuit breaker's interrupting capacity may

be substantially less than its interrupting rating, and3. Some molded case circuit breakers exhibit "dynamic"

operation that begins in less than 1/2 cycle. This makesthem more difficult to protect than other static electricalcircuit components.

The most practical and reliable solution is to specify a fully-rated fusible system.

Molded Case Circuit BreakersU.L. 489 and CSA5 Test ProceduresU.L. 489 requires a unique test set-up for testing circuit breakerinterrupting ratings. Figure F illustrates a typical calibrated testcircuit waveform for a 20 ampere, 240 volt, 2-pole molded casecircuit breaker, with a marked interrupting rating of 22,000amperes, RMS symmetrical.

Figure F

Figure G illustrates the test circuit as allowed by U.L. 489.

Figure G

Standard interrupting rating tests will allow for a maximum 4 ft.

rated wire on the line side, and 10 in. rated wire on the load side ofthe circuit breaker. Performing a short-circuit analysis of this testcircuit results in the following short-circuit parameters, as seen bythe circuit breaker.

Actual short-circuit RMS current = 9900 amperesRMS symmetrical

Actual short-circuit power factor = 88% Actual short-circuit peak current = 14,001 amperes

P.F. = 20%IRMS = 22,000 Amps

IRMS = 22,000A

Ip = 48,026A

Time

Amps

S.C. P.F. = 20%S.C. Avail. = 22,000A

RLINE XLINERCB XCB

20A

RLOAD XLOAD

RS

XS

SOURCE: 4' Rated Wire (#12 Cu)

Note: For calculations, RCB and XCB are assumed negligible.

10" Rated Wire (#12 Cu)

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14


Following is an example of a partial table showing the actual IPand IRMS values to which the circuit breaker is tested.

240V2-Pole MCCB INTERRUPTING CAPACITIES (KA)

CB 10KA 14KA 18KA 22KA

RATING Ip Irms Ip Irms Ip Irms Ip Irms

15A 7.2 5.1 8.7 6.1 9.3 6.6 9.9 7.020A 8.9 6.3 11.4 8.1 12.6 8.9 14.0 9.9

25A 10.7 7.5 14.2 10.1 16.5 11.7 19.9 13.5

30A 10.7 7.5 14.2 10.1 16.5 11.7 19.9 13.5

40A 11.7 8.3 16.0 11.3 19.2 13.6 22.7 16.1

50A 11.7 8.3 16.0 11.3 19.2 13.6 22.7 16.1

60A 12.5 8.8 17.3 12.2 21.3 15.1 25.6 18.1

70A 13.0 9.2 18.1 12.8 22.6 16.0 27.4 19.4

80A 13.0 9.2 18.1 12.8 22.6 16.0 27.4 19.4

90A 13.2 9.3 18.3 12.9 23.0 16.3 27.9 19.7

100A 13.2 9.3 18.3 12.9 23.0 16.3 27.9 19.7

These values are known as the circuit breakers interruptingcapacities.

What about the bus shot tests? Wont those prove that circuit breakerscan safely and properly interrupt their marked interrupting rating?

No! Beginning 10/31/2000, UL 489 will require circuit breakersrated 100A and less to additionally be tested under bus barconditions. In this test, line and load terminals will be connectedto 10" of rated conductor. For single pole circuit breakers, these10" leads are then connected to 4' of #1 for connection to the teststation. For multipole circuit breakers, the 10" line side leads areconnected to the test station through 4' of #1. The load side isshorted by 10' leads of rated conductor per pole. These busshots still do not fully address the situation where a fault canoccur less than 4'10" from the circuit breaker. For example,7.1.11.6.3.1 of UL 489 states The inability to relatch, reclose, orotherwise reestablish continuity shall be considered acceptablefor circuit breakers which are tested under bus bar conditions.This says the circuit breaker doesnt have to work after a close-infault occurs, and is in violation of the 1999 NEC requirement for acircuit breaker which is found in the definition. The NEC defines acircuit breaker as A device designed to open and close a circuit

by nonautomatic means and to open the circuit automatically on apredetermined overcurrent without damage to itself when properlyapplied within its rating.

Protection of Bus Structures

In the circuit below, what must be the busway short-circuit bracing?

100,000 amperes, because the overcurrent device is not current-limiting.

In this circuit, what would the busway short-circuit bracing have to be?

36,000 amperes (as shown in the Minimum Bracing Table). Withan available short-circuit current of 100,000 amperes, the LOW-PEAK YELLOW KRP-C1600SP fuse will only let-through anequivalent of 36,000 amperes, RMS symmetrical.

Minimum Bracing Required for Bus Structures at 480V.(Amperes RMS Symmetrical)Rating*

Busway Fuse Available Short-Circuit Amperes RMS Sym.

25,000 50,000 75,000 100,000 200,000

100 100 3,400 4,200 4,800 5,200 6,500

225 225 6,000 7,000 8,000 9,000 12,000

400 400 9,200 11,00 13,000 14,000 17,000

600 600 12,000 15,000 17,000 19,000 24,000

601 601 11,000 14,500 17,000 18,000 24,000

800 800 14,200 17,500 20,000 23,000 29,000

1200 1200 16,000 22,500 26,000 28,000 39,000

1600 1600 22,500 28,500 33,000 36,000 46,000

2000 2000 25,000 32,000 37,000 40,000 52,000

3000 3000 25,000 43,000 50,000 58,000 73,000

4000 4000 25,000 48,000 58,000 68,000 94,000

*Fuses are: 100-600 AmpereLOW-PEAK YELLOW Dual-Element FusesLPS-RK_SP (Class RK1) or LPJ_SP (Class J); 800-4000 AmpereLOW-PEAK

YELLOW Time-Delay FusesKRP-C_SP (Class L). (LOW-PEAK

YELLOW fuses are current-limiting fuses.)

NON-CURRENT-LIMITING DEVICE

1600A BUSWAY100,000Aavailablefault current

KRP-C1600SP FUSE(Current-limiting)

1600A BUSWAY100,000Aavailablefault current

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110-22 Proper Marking and Identification of Disconnecting Means

What does this new Section require?

Labeling ConsiderationsN.E.C. Sections 110-22 and 240-83 require special marking for atesting agency listed series-rated systems.

On listed series-rated systems, the downstream equipment willbe marked by the manufacturer per applicable standards [240-86(a)]. The N.E.C. requires that the main or upstream protectivedevice be marked with a field installed label per N.E.C. Section110-22. This is the responsibility of the electrical contractor.

Short-circuit calculations must be performed at panel

locations where series-rated systems are specified.

215-10 Requirements for Ground-Fault Protection of Equipment on Feeders

What is the importance of this Section?

Equipment classified as a feeder disconnect, as shown in theseexamples, must have ground fault protection as specified inSection 230-95.

G.F.P. is not required on feeder equipment when it is provided onthe supply side of the feeder (except for certain Health CareFacilities requirements, Article 517).

Additionally, the requirements of this section do not apply to firepumps or to a continuous industrial process where a nonorderlyshutdown will introduce additional or increased hazards.

See Section 230-95 for an in-depth discussion of Ground FaultProtection.

Ground fault protection without current-limitation may not protect systemcomponents. See Section 110-10.

What is the importance of this Section?The overcurrent protective device provided for branch circuits,must not be less than the total non-continuous load, plus 125% ofthe continuous load (defined as a load that continues for 3 hoursor more).

Rating not less than = [(10A) x 1.0] + [(8A) x 1.25]= 20A

EXAMPLE

The branch circuit rating shall not be less than 20 amperes.

210-20(a) Ratings of Overcurrent Devices on Branch Circuits Serving Continuousand Non-Continuous Loads

20A Rating

Non-Continuous10A

Continuous Load8A

VIOLATION

High VoltageService 4160V

480Y/277V

Feeder W/OG.F.P.

1000Aor Greater

COMPLIANCE

High VoltageService 4160V

480Y/277V

FeederProvidedw/G.F.P.

1000Aor Greater

COMPLIANCE Feeder of any ratingno G.F.P. Required

(Except Per Article 517)

480Y/277V

G.F.P.

1000AorGreater

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230-82 Equipment Allowed to be Connected on the Line Side of the

Service Disconnect

230-95 Ground Fault Protection for Services

What are the advantages of using cable limiters on the supply side of theservice disconnect.Typical cable installations are shown in the illustration below. Thebenefits of cable limiters are several:1. The isolation of a faulted cable permits the convenient

scheduling of repair service.2. Continuity of service is sustained even though one of morecables are faulted.3. The possibility of severe equipment damage or burn down as aresult of a fault is greatly reduced. (Typically, without cable limitersthe circuit from the transformer to the service equipment isafforded little or no protection.).4. Their current-limiting feature can be used to provide protectionagainst high short-circuit currents for utility meters and providecompliance with Section 110-10.

COMMERCIAL/INDUSTRIAL SERVICE ENTRANCE

(Multiple cables per phase)

RESIDENTIAL SERVICE ENTRANCE

(Single cable per phase)

What do (7) and (8) mean?The control circuit for power operable service disconnectingmeans and ground fault protection must have a means fordisconnection and adequate overcurrent protectioninterruptingrating and component protection.

ServiceDisconnect

(Open) (Open)

Faulted cable isolated; only the cablelimiters in faulted cable open; othersremain in operation

OpenFaulted cable isolated; the otherservices continue in operationwithout being disturbed

RESIDENCES

#4

#3

#2

#1

What is the importance of this section?This section means that 480Y/277 volt, solidly grounded wyeonly connected service disconnects, 1000 amperes and larger,

must have ground fault protection in addition to conventional over-current protection. Ground fault protection, however, is notrequired on a fire pump or a service disconnect for a continuousprocess where its opening will increase hazards. All deltaconnected services are not required to have ground faultprotection. The maximum setting for the ground fault relay (orsensor) must be set to pick up ground-faults which are 1200amperes or more and actuate the main switch or circuit breaker todisconnect all phase conductors. A ground fault relay with adeliberate time delay characteristic of up to 3000 amperes for 1second can be used. (The use of such a relay greatly enhancessystem coordination and minimizes power outages).

Under short-circuit conditions, unlike current-limiting fuses,ground fault protection in itself will not limit the line-to-ground orphase-to-phase short-circuit current. When mechanical protectivedevices such as conventional circuit breakers are used withG.F.P., all of the available short-circuit current will flow to the point

of fault limited only by circuit impedance. Therefore, it isrecommended that current-limiting overcurrent protective devicesbe used in conjunction with G.F.P. relays.

In this circuit, what protection does the fuse provide in addition to thatprovided by the ground fault equipment?

Current limitation under short-circuit conditions and high-levelground-faults.

In this circuit, is protection provided against high magnitude ground-faults as well as low level faults?

No, it is not. There is no current-limitation.

Is G.F.P. required on all services?No. The following do not require G.F.P.:1. Continuous industrial process where non-orderly shutdown

would increase hazard.2. All services where disconnect is less than 1000 amperes.3. All 120/208 volts, 3, 4W (wye) services.4. All single-phase services including 120/240 volt, 1, 3W.5. High or medium voltage services. (See N.E.C. Sections 240-13and 215-10 for equipment and feeder requirements.)6. All services on delta systems (grounded or ungrounded) suchas: 240 volt, 3, 3W Delta, 480 volt, 3, 3W Delta, or 240 volt, 3,4W Delta with midpoint tap.7. Service with 6 disconnects or less (Section 230-71) where eachdisconnect is less than 1000 amperes. A 4000 ampere servicecould be split into five 800 ampere switches.8. Resistance or impedance grounded systems.

1000 ampereswitch & fuseor larger

480Y/2773, 4WService

Ground faultprotectionrequired

SWBD

1000 amperecircuit breakeror larger

480Y/2773, 4WService

Ground faultprotectionrequired

SWBD

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230-95 Ground Fault Protection for Services

240-1 Scope of Article 240 on Overcurrent Protection

What are some of the problems associated with G.F.P.?Incorrect settings, false tripping and, eventually, disconnection.(The knocking-out of the total building service or large feeders asa result of minor faults or nuisance tripping cannot be tolerated inmany facilities). Unnecessary plant down time is often morecritical, or even more dangerous, than a minor ground fault.

Note: G.F.P. without current limitation may not protect systemcomponents. See Section 110-10 and 250-2(d).

How can ground faults be minimized?1. To prevent blackouts, make sure that all overcurrent protectivedevices throughout the overall system are selectively coordinated.When maximum continuity of electrical service is necessary,ground fault protective equipment should be incorporated infeeders and branch circuits. [Per Section 230-95 (FPN No. 2).]2. Insulating bus structures can greatly minimize the possibility offaults. The hazard of personnel exposure to energized electricalequipment is also reduced with insulated bus structures.3. Specify switchboards and other equipment with adequateclearance between phase conductors and ground. Ground faultsare rare on 120/208 volt systems because equipment manufactur-ers provide ample spacing for this voltage. Insist on greaterspacing for 277/480 volt equipment and the likelihood of ground

faults will be greatly reduced.4. Avoid unusually large services; split the service wheneverpossible.5. Adequately bond all metallic parts of the system to enhanceground fault current flow. Then, if a ground fault does occur, it ismore likely to be sensed by fuses or circuit breakers.

To respond properly to a line-to-ground type fault, what should be thesetting of a ground fault relay located on the main disconnect?The setting should allow the feeder circuit (or preferably thebranch) overcurrent protective devices to function withoutdisturbing the G.F.P. relay.

How is a G.F.P. setting determined?By making a coordination study. Such a study requires the plottingof the time-current curves of the protective devices.

A simple solution to the problem of coordinating ground fault

relays with overcurrent protective devices is shown in the systemrepresented in the graph at right. The G.F.P. relay coordinates withthe feeder fuses KTS-R 250. The G.F.P. relay with a degree ofinverse time characteristics provides coordination with feeder

fuses in order to avoid outages. (Section 230-95 permits aninverse time-delay relay with a delay of up to 1 second at 3000amperes.)

Conventional mechanical tripping overcurrent protectivedevices often do not permit a selectively coordinated system* andBLACKOUTS can occur. For ground faults (and short-circuitcurrent as well) of current magnitude above the instantaneous trip

setting on the main circuit breakers overcurrent element, the mainwill nuisance trip (open) causing a blackout even though the faultis on a feeder or branch circuit. Appropriate selection of current-limiting fuses with proper G.F.P. settings can provide the highestdegree of coordination and prevent blackouts.

A system wherein only the protective device nearest the fault operates andnone of the other protective devices in the system are disturbed.

6

4

3

2

1.8

.6

.4

.3

.2

.1

.08

.06

.04

.03

.02

.01

.05

.5

5

100

200

300

400

500

600

800

1,

000

2,

000

3,

000

4,

000

5,

000

6,

000

8,

000

10,

000

20,

000

30,

000

10

8

20

30

405060

80100

200

300

KRP-C160

KTS-R

KTS-R125

KTS-R250

GFPsetat

1200 AMPSPICK UP &0.5 SEC.

KRP-C1600SP

KTS-R125

CURRENT IN AMPERES

TIMEINSECONDS

*

What is the importance of this Section?The basic purpose of overcurrent protection is to open a circuitbefore conductors or conductor insulation are damaged when anovercurrent condition exists. An overcurrent condition can be theresult of an overload or a short-circuit. It must be removed before

the damage point of conductor insulation is reached. Conductorinsulation damage points can be established from availableengineering information, i.e., Publication P-32-382, Short-CircuitCharacteristics of Cable, ICEA, (Insulated Cable EngineersAssociation, Inc.), IEEE Color Books, Canadian Electrical Code,and IEC Wiring Regulations.

When selecting an overcurrent protective device to protect a conductor,is it adequate to simply match the ampere rating of the device to theampacity of the conductor?No. Although conductors do have maximum allowable ampacityratings, they also have maximum allowable short-circuit currentwithstand rating. Damage ranging from slight degradation ofinsulation to violent vaporization of the conductor metal can resultif the short-circuit withstand is exceeded. (See Section 110-10.)

Why, in the circuit below, is the #10 wire protected even though theavailable short-circuit current exceeds the wire withstand? The #10conductor can withstand 4300 amperes for one cycle and 6020 amperesfor one-half cycle.**

**FootnoteFrom ICEA tables and formula.

Under short-circuits, the LOW-PEAK YELLOW Dual-Elementfuse (30 ampere) is fast acting. It will clear and limit (cut off) short-circuit current before it can build up to a level higher than the wirewithstand. The opening time of the fuse is less than one-half cycle(less than 0.008 seconds). In this particular example, theprospective current let-thru by the fuse is less than 1850 amperes.Thus, opening time and current let-through of the fuse is far lower

30ALow-Peak YellowClass RK1 Dual-ElementFuse

#10 THW COPPER WIRE40,000Aavailable

Short-Circuit

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240-1 Scope of Article 240 on Overcurrent Protection

240-3 Protection of Conductors Other Than Flexible Cords and Fixture Wires

than the wire withstand. (Conductor protection is not a problemwhen the conductor is protected by current-limiting fuses whichhave an ampere rating that is the same as the conductor. In thecase of short-circuit protection only, fuses can often be sizedmany times higher than the wire current rating, depending uponthe current-limiting characteristics of the fuse.)

Does the circuit below represent a misapplication? (#10 THW insulatedcopper wire can withstand 4300 amperes for one cycle and 6020amperes for one-half cycle).

Yes. The 40,000 ampere short-circuit current far exceeds the with-stand of the #10 THW wire. Note the table and chart which follow.

What can be done to correct the above misapplication?

There are two possible solutions:1. Use a larger size conductor (i.e., 1/0), one with a withstandgreater than the short-circuit for one cycle (see chart below).2. Use an overcurrent protective device which is current-limitingsuch as that shown in the previous question.

The following table is based on Insulated Cable EngineersAssociation, Inc. (ICEA) insulated cable damage charts inPublication 32-382. This table assumes that the conductor ispreloaded to its ampacity before a short-circuit is incurred. Theformula that was used to develop the ICEA Damage Charts isgiven following the table. This formula can be used to extrapolatewithstand data for wire sizes or time durations not furnished in theICEA Publication 32-382 charts. A sample chart is shown at right.

The mechanical overcurrent protective device opening timeand any impedance (choking) effect should be known along withthe available short-circuit current and cable withstand data to

determine the proper conductor that must be used.

Insulated Cable Damage Table (60Hz)Wire Size Maximum Short-Circuit Withstand Current Amperes)

(THW Cu) at Various Withstand Times

1 Cycle 1/2 Cycle 1/4 Cycle 1/8 Cycle

#14 1,700* 2,400* 3,400* 4,800*

#12 2,700* 3,800* 5,400* 7,600*

#10 4,300 6,020* 8,500* 12,000*

#8 6,800 9,600* 13,500* 19,200*

#6 10,800 15,200* 21,500* 30,400*

#4 17,100 24,200* 34,200* 48,400*

See Insulated Cable Engineers Association, Inc., Short-Circuit Characteristics ofCable, Pub. P-32-382, and circuit breaker manufactures published opening times forvarious types of circuit breakers.

CopperThermoplastic AluminumThermoplasticConductor Insulation Conductor Insulation

Where:I = Short-Circuit CurrentAmperesA = Conductor AreaCircular Milst = Time of Short-CircuitSecondsT1 = Maximum Operating Temperature75CT2 = Maximum Short-Circuit Temperature150C

Note: ICEA (Insulated Cable Engineers Association) is the most widely acceptedauthority on conductor short-circuit withstand ratings.

Conductor mustbe protected forits entire length

30A MECHANICAL OVERCURRENTPROTECTIVE DEVICE(Clearing time 1 cycle;not current-limiting)

40,000Aavailable

Short-Circuit

#10 COPPER WIRE(THW insulation)

I 2t = 0.0297 log

T2 + 234 A T1 + 234

I 2t = 0.0125 log

T2 + 228 A T1 + 228

1CYC

LE-0.0167

SE

COND

40,000 Amps - 1 Cycle

4,300Amps - 1Cycle

Conductor-CopperInsulation-ThermoplasticCurves Based on Formula

I

A

2

t = .0297 logT2 + 234

T1 + 234

IAt

T1

T2

= Short-Circuit Current - Amperes= Conductor Area - Circular Mils= Time of Short-Circuit - Seconds= Maximum Operating Temperature -

75C= Maximum Short-Circuit Temperature -

150C

Where

SHORTCIRCU

ITCURRENTTHOUSANDSOFAMPERES

100

80

60

50

40

30

20

10

8

6

5

4

3

2

1.8

.6

.5

.4

.3

.2

.110 8 6 4 2 1

1/0

2/0

3/0

4/0

AWG

250MCM

500

1000

CONDUCTOR SIZE

2CYC

LE-0.0333

SEC

OND

4CYC

LE-0.0667

SEC

OND

8CYC

LE-0.1333

SEC

OND

16CYC

LE-0.2667

SEC

OND

30CYC

LE-0.50

00SEC

OND

60CYC

LE-1.0000

SEC

OND

100CYC

LE-1.6667

SEC

OND

What is the meaning of 240-3(b) and 240-3(c)?Where the ampacity of a conductor does not correspond with astandard rating (240-6) of a fuse, the next standard rating may beused as long as the fuse is not above 800 amps and theconductors are not part of a multi-outlet branch circuit supplyingreceptacles for cord and plug-connected portable loads.

What does 240-3(f) mean?Conductors fed from single-phase, 2-wire secondary transformersand three phase, delta-delta connected transformers with three-wire (single-voltage) secondaries can be considered protected by

the primary side fuses if the transformer is properly protected inaccordance with Section 450-3. The primary fuse must be lessthan or equal to the secondary conductor ampacity times thesecondary-to-primary transformer voltage ratio.

What is the definition of a tap conductor?A tap conductor is defined in 240-3(e) as a conductor, other thana service conductor that has overcurrent protection ahead of itspoint of supply, that exceeds the value permitted for similarconductors that are protected as described elsewhere in thissection.

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240-4 Proper Protection of Fixture

Wires and Flexible Cords

240-6 Standard Ampere Ratings

What is the importance of this section?Flexible cords and extension cords shall have overcurrent protec-tion rated at their ampacities. Supplementary fuse protection is anacceptable method of protection. For #18 fixture wire 50 feet orover, a 6 ampere fuse would provide necessary protection, and for

#16 100 feet or over, an 8 ampere fuse would provide thenecessary protection. #18 extension cords must be protected by a7 ampere fuse.

Also, Section 760-23, covering special non-power-limited firealarm circuits, requires 7 ampere protection for #18 conductorsand 10 ampere protection for #16 conductors.

240-6 Standard Ampere Ratings

What is the importance of this section?In addition to the standard ratings of fuses and circuit breakers,this section states that the rating of an adjustable trip circuitbreaker is considered to be the highest possible setting. Thisbecomes important when protecting conductors or motor circuits.For example, if a copper 75C conductor is required to carry 200amperes continuously, a 250 kcmil cable might be chosen. If acircuit breaker were chosen to protect this cable with an external

adjustable trip from 225 through 400 amperes, the rating of thebreaker would be 400 amperes, and 500 kcmil cable wouldtherefore be required, increasing costs significantly. However, ifthis adjusting means is restricted, such as behind a boltedequipment door, behind locked doors accessible only by aqualified person, or if a removable and sealable cover is over theadjusting means, then the rating can be considered to be equal to

the adjusted setting.

Note: Standard ampere ratings for fuses and inverse time circuitbreakers are 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100,110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600,700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000 and6000 amperes. In addition, standard fuse ratings are 1, 3, 6, 10and 601.

240-8 & 380-17 Protective Devices

Used in Parallel

What do these sections mean?

There are cases in which an original equipment manufacturer, forvarious reasons, must parallel fuses and receive an appropriateequipment listing. For example, this would be the case of somesolid-state power conversion equipment. However, for thestandard safety switch, conventional branch circuit applications,switch-boards, and panelboards, the use of parallel fuses is notallowed.

240-9 Thermal Devices

What does this section mean?Thermal overload devices generally can neither withstand openinga circuit under short-circuit conditions nor even carry short-circuitcurrents of higher magnitudes. When using thermal overloadprotective devices, the use of a current-limiting fuse will not onlyprovide short circuit protection for the circuit but for the thermaloverload device as well.

240-10 Requirements for

Supplementary Overcurrent Protection

What is the importance of this section?Supplementary fuses, often used to provide protection for lightingfixtures, cannot be used where branch circuit protection isrequired.

What are the advantages of supplementary protection?The use of supplementary protection for many types ofappliances, fixtures, cords, decorator lighting (Christmas treelights. . .)*, etc., is often well advised. There are several

advantages:1. Provides superior protection of the individual equipment bypermitting close fuse sizing.2. With an occurrence of an overcurrent, the equipment protectedby the supplementary protected device is isolated; the branchcircuit overcurrent device is not disturbed. For instance, the in-l ine-fuse and holder combination, such as the Type HLRfuseholder with Type GLR or GMF fuses, protects and isolatesfluorescent lighting fixtures in the event of an overcurrent.3. It is easier to locate equipment in which a malfunction hasoccurred. Also, direct access to the fuse of the equipment ispossible.

FootnoteSupplementary protection for series connected decorator lighting sets andparallel sets (Christmas tree string lights) was required in 1982. Manufacturers haveimplemented this requirement.

Violation(EXTENSION CORD)

Receptacle

20ABranch Circuits

#18 Extension Cord

Receptacle

20ABranch Circuits

Compliance(EXTENSION CORD)

#18 Extension Cord7 AmpFuse

#16 Fixture Wire100 ft. or over

Violation(FIXTURE WIRE)

20A FuseTo load

BRANCHCIRCUIT


Compliance(FIXTURE WIRE)

8A FuseTo load

BRANCHCIRCUIT


Violation(FIXTURE WIRE)

20A Fuse

To load

BRANCHCIRCUIT


Compliance(FIXTURE WIRE)

6A Fuse

To load

BRANCHCIRCUIT

*

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240-11 Definition of Current-Limiting Overcurrent Protective Devices

What is the importance of this Section?

ACTION OF NON-CURRENT-LIMITING CIRCUIT BREAKER

ACTION OF CURRENT-LIMITING FUSE.

Simply stated, a current-limiting protective device is one whichcuts off a fault current in less than one-half cycle. It thus preventsshort-circuit currents from building up to their full available values.

The greatest damage done to components by a fault currentoccurs in the first half-cycle (or more precisely, the first majorloop of the sinewave). Heating of components to very hightemperatures can cause deterioration of insulation, or evenexplosion. Tremendous magnetic forces between conductors cancrack insulators and loosen or rupture bracing structures.

The levels of both thermal energy and magnetic forces areproportionate to the square of current. Thermal energy isproportionate to the square of RMS current; maximum magneticfields to the square of peak current. If a fault current is 100 timeshigher than normal current, its increased heating effects equals(100)2 or 10,000 times higher than that of the normal current. Thus,to prevent circuit component damage, the use of current-limitingprotective devices is extremely important, particularly sincepresent-day distribution systems are capable of delivering highlevel fault currents.

Footnote: The more technical definition of a current-limiting protective device is expressedby 240-11.

To further appreciate current-limitation, assume for example,that the available prospective short-circuit current in a circuit is50,000 amperes. If a 200 ampere LOW-PEAK YELLOW fuse isused to protect the circuit, the current let-through by the fuse willbe only 6500 amperes instead of 50,000 amperes. Peak currentwill be only 15,000 amperes instead of a possible 115,000

amperes. Thus, in this particular example, currents are limited toonly 13% of the available short-circuit values.

As is true of fuse application in general, the application ofcurrent- l imiting fuses in respect to current- l imitation andcomponent protection (110-10) is quite simple. Graphs or tablessuch as the one shown below permit easy determination of thelet-thru currents that a fuse will pass for various levels ofprospective short-circuit currents. For example, the table belowshows that the 200 ampere LOW-PEAK YELLOW fuse will let-through 6500 amperes when prospective short-circuit current is50,000 amperes.

For the above circuit, the Size 1 Starter has a short-circuitwithstand rating of 5000 amperes.* The question is, with the25,000 ampere available short-circuit current, will a LOW-PEAK

YELLOW fuse provide adequate protection of the starter? Byreferring to the table below, it can easily be seen that for aprospective short-circuit current of 25,000 amperes, fuses withratings of 100 amperes or less will limit fault currents to below the5000 ampere withstand of the starter and, thus, provide adequateprotection.

Current-Limiting Effects of RK1 LOW-PEAK YELLOW Fuses.Prospective Let-Through Current (Apparent RMS Symmetrical)

Short-Circuit LPS-RK_SP (600V) Fuse Ratings

Current 30A 60A 100A 200A 400A 600A

5,000 980 1,600 2,100 3,200 5,000 5,000

10,000 1,200 2,000 2,550 4,000 6,750 9,150

15,000 1,400 2,300 2,900 4,800 7,850 10,20020,000 1,500 2,500 3,150 5,200 8,250 11,300

25,000 1,600 2,650 3,400 5,450 9,150 12,200

30,000 1,650 2,850 3,550 5,650 9,550 12,800

35,000 1,750 2,950 3,750 5,850 10,000 13,500

40,000 1,850 3,100 3,900 6,100 10,450 13,900

50,000 1,950 3,300 4,150 6,500 11,300 15,000

60,000 2,050 3,500 4,350 6,950 11,950 16,100

80,000 2,250 3,850 4,800 7,850 13,000 17,400

100,000 2,450 4,050 5,200 8,250 13,900 18,700

150,000 2,750 4,800 6,100 9,550 15,900 21,300

200,000 3,000 5,200 6,500 10,000 17,400 23,500

RMS Symmetrical Amperes

*Footnote: See discussion on Section 110-10 in this bulletin.

The reader should note that much of the current-limitation

claimed by small ampere circuits breakers is actually the result ofthe significant impedance added to the circuit breaker test circuitafter the circuit has been calibrated. Refer to the circuit breakerprotection portion of Section 110-10 for further information oncircuit breaker test circuits.

Circuit breaker tripsand opens short-circuitin about 11/2 cyclesInitiation of

short-circuit current

Normalload current

Areas within waveformloops represent destructiveenergy impressed uponcircuit components

Fuse opens and clearsshort-circuit in lessthan 1/2 cycle


Size 1 Starter(Tested with 5000A available)

Short-Circuit

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240-12 System Coordination or Selectivity

What is the importance of this section?Whenever a partial or total building blackout could causehazard(s) to personnel or equipment, the fuses and/or circuitbreakers must be coordinated in the short-circuit range. It isacceptable for a monitoring system to be used to indicate anoverload condition, if the overcurrent protective devices cannot be

coordinated in the overload region. However, in the vast majorityof cases, both circuit breakers and fuses will be able to becoordinated in the overload range, so the monitoring systems willseldom be required. Typical installations where selectivecoordination would be required include hospitals, industrial plants,office buildings, schools, government buildings, mil itaryinstallations, high-rise buildings, or any installation wherecontinuity of service is essential.*

*Footnote: See also Section 4-5.1 of NFPA 110 (Emergency and Standby PowerSystems) and Sections 3-3.2.1.2(4) & 3-4.1.1.1 of NFPA 99 (Health Care Facilities) for

additional information on selective coordination.

VIOLATION

Fault exceeding the instantaneous trip setting of all 3 circuit breakers in series willopen all 3. This will blackout the entire system.

COMPLIANCE

Fault opens the nearest upstream fuse, localizing the fault to the equipmentaffected. Service to the rest of the system remains energized.

If the ampere rating of a feeder overcurrent device is larger than therating of the branch circuit device, are the two selectively coordinated?No. A difference in rating does not in itself assure coordination. Forexample, a feeder circuit breaker may have a rating of 400amperes and the branch breaker 90 amperes. Under overloadconditions in the branch circuit, the 90 ampere breaker will openbefore, and without, the 400 ampere breaker opening. However,under short-circuit conditions, not only will the 90 ampere deviceopen, the 400 ampere may also open. In order to determinewhether the two devices will coordinate, it is necessary to plot theirtime-current curves as shown. For a short-circuit of 4000 amperes:

1. The 90 ampere breaker will unlatch (Point A) and free thebreaker mechanism to start the actual opening process.2. The 400 ampere breaker will unlatch (Point B) and it, too, wouldbegin the opening process. Once a breaker unlatches, it will open.The process at the unlatching point is irreversible.3. At Point C, the contacts of the 90 ampere breaker finally open

and interrupt the fault current.4. At Point D, the contacts of the 400 ampere breaker open. . .theentire feeder is blacked out!

Example of Non-Selective System.

Now, lets take the case of fuse coordination. When selectivecoordination of current-limiting fuses is desired, the SelectivityRatio Guide (next page) provides the sizing information necessary.In other words, it is not necessary to draw and compare curves.

Current- l imiting fuses can be selectively coordinated bymaintaining at least a minimum ampere rating ratio between themain fuse and feeder fuses and between the feeder fuse andbranch circuit fuses.

These ratios are based on the fact that the smaller downstreamfuses will clear the overcurrent before the larger upstream fusesmelt. An example of ratios of fuse ampere ratings which provideselective coordination is shown in the one-line circuit diagram.

1000AI.T.=10x

225AI.T.=8x

Opens

Opens

20AI.T.=8x

Opens

22,000 AmpShort-Circuit

22,000 AmpShort-Circuit

Opens20A

225ANotOpen

1000ANotOpen

19-B

.001

.002

.003

.004

.006

.008

.08

.01

.02

.03

.04

.06

.1

.2

.3

.4

.6

.81

2

34

6

810

20

3040

80

60

100

200

300400

600800

1,000

100

200

300

600

400

800

1,

000

2,

000

3,

000

4,

000

6,

000

8,

000

10,

000

20,

000

30,

000

30,

000

TIMEINSECONDS

CURRENT IN AMPERES

POINT D

POINT C

POINT B

POINT A

400 AMP

Circuit BreakerI.T. = 5X

90 AMPCircuitBreaker

90A

Short-Circuit

400A

2:1 (or more)

LPS-RK400SPLPS-RK90SP

Short-Circuit

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240-12 System Coordination or Selectivity

240-13 Ground Fault Protection of Equipment on Buildings or Remote Structures

240-21 Location Requirements for Overcurrent Devices and Tap Conductors

*Selectivity Ratio Guide (Line-Side to Load-Side) for Blackout Prevention.

Circuit Load-Side Fuse

Current Rating 601-6000A 601-4000A 0-600A 601-6000A 0-600A 0-1200A 0-600A 0-60A

Type Time- Time- Dual-Element Fast-Acting Fast-Acting Time-Delay Delay Time-Delay Delay

Trade Name & LOW-PEAK LIMITRON LOW-PEAK FUSETRON LIMITRON LIMITRON T-TRON LIMITRON SCYELLOW YELLOW

Class (L) (L) (RK1) (J)** (RK5) (L) (RK1) (T) (J) (G)

Buss KRP-CSP KLU LPN-RKSP LPJSP FRN-R KTU KTN-R JJN JKS SCSymbol LPS-RKSP FRS-R KTS-R JJS

601 to Time- LOW-PEAK KRP-CSP 2:1 2.5:1 2:1 2:1 4:1 2:1 2:1 2:1 2:1 N/A6000A Delay YELLOW (L)

601 to Time- LIMITRON KLU 2:1 2:1 2:1 2:1 4:1 2:1 2:1 2:1 2:1 N/A4000A Delay (L)

LOW-PEAK LPN-RKSP 2:1 2:1 8:1 3:1 3:1 3:1 4:1YELLOW LPS-RKSP

0 Dual (RK1)to Ele- (J) LPJSP** 2:1 2:1 8:1 3:1 3:1 3:1 4:1

600A ment FUSETRON FRN-R 1.5:1 1.5:1 2:1 1.5:1 1.5:1 1.5:1 1.5:1(RK5) FRS-R

601 to LIMITRON KTU 2:1 2.5:1 2:1 2:1 6:1 2:1 2:1 2:1 2:1 N/A6000A (L)

0 to Fast- LIMITRON KTN-R 3:1 3:1 8:1 3:1 3:1 3:1 4:1

600A Acting (RK1) KTS-R

0 to T-TRON JJN 3:1 3:1 8:1 3:1 3:1 3:1 4:1

1200A (T) JJS

0 to LIMITRON JKS 2 :1 2:1 8:1 3:1 3:1 3:1 4:1600A (J)

0 to Time- SC SC 3:1 3:1 4:1 2:1 2:1 2:1 2:160A Delay (G)

Note: At some values of fault current, specified ratios may be lowered to permit closer fuse sizing. Plot fuse curves or consult with Cooper Bussmann.

General Notes: Ratios given in this table apply only to Buss

fuses. When fuses are within the same case size, consult Cooper Bussmann.

Consult Cooper Bussmann for latest LPJSP ratios.

.

Line-S

ideFuse

*

**

What does this section require?Equipment ground fault protection of the type required in Section230-95 is required for each disconnect rated 1000 amperes ormore in 480Y/277V systems that will serve as a main disconnectfor a separate building or structure. Refer to Sections 215-10 and230-95.

Note: G.F.P. that is not current-limiting may not protect systemcomponents. See Section 110-10 and 250-1 (FPN).

High VoltageService

Building A

800A480Y/277V

Building B

1000A or Greater480Y/277V

G.F.P. NotRequired

G.F.P. NotRequired

G.F.P.Required

What are the requirements of 240-21(b)(1)?The basic content of this section remains unchanged. However, ithas been rewritten to improve readability and comprehension.Typically, fuses must be installed at point where the conductorreceives its supply, i.e., at the beginning or line side of a branchcircuit or feeder. There are installations where this basic rule maynot have to be followed.

Fuses are not required at the conductor supply if a feeder tapconductor is not over ten feet long; is enclosed in raceway; doesnot extend beyond the switchboard, panelboard or control devicewhich it supplies; and has an ampacity not less than thecombined computed loads supplied, and not less than the ratingof the device supplied, unless the tap conductors are terminatedin a fuse not exceeding the tap conductor's ampacity [240-21(b)].For field installed taps, where the tap conductors leave theenclosure, the ampacity of the tap conductor must be at least 10%of the overcurrent device rating. See the following example.

M M M M M M

ComplianceViolation240V, 3600A

EquipmentRoomWireway

1HP

5HP

7.5HP

15HP

20HP

25HP

10'TAP

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240-21 Location Requirements for Overcurrent Devices and Tap Conductors

In the previous diagram, the feeder overcurrent devices aresized per the N.E.C for the load served.

All taps to the motors are 10 foot taps.Three of the motors are smaller motors:

one motor is a 1 HP motor,one motor is a 5 HP motor,and one motor is a 71/2 HP motor.

The 1, 5 and 71/2 HP motors will require a minimum of #14, #12and #10 75C conductors, respectively. For field wiring, these 10foot taps are not permitted since the line side overcurrent device is600 amperes. Section 240-21(b)(1)(d) requires that the maximumovercurrent protection for field installations shall not exceed1000%, or 10 times the ampacity of the tap conductor, forexample:

#14 conductor, 20 amperes ampacity, maximum line sideovercurrent protection is 200 amperes.#12 conductor, 25 amperes ampacity, maximum line sideovercurrent protection is 250 amperes.#10 conductor, 35 amperes ampacity, maximum line sideovercurrent protection is 350 amperes.To tap the above conductors to a 600 amperes feeder

overcurrent device would be a violation of Section 240-21(b)(1)(d)of the Code.

The solution is to feed the smaller motors from a branch circuit

panel or from a smaller feeder where the feeder overcurrentprotection does not exceed the 10 times rating of the tapconductors ampacity.

The smallest of the three larger motors is a 15 HP motor whichrequires a branch circuit conductor with a minimum ampacity of52.5 amperes and which could be tapped to the 600 amperefeeder since a No. 6 75 conductor has an ampacity of 65amperes and 10 x 65 = 650. In other words the No. 6 75conductors could be tapped to an overcurrent device as high as650 amperes.

Motor tap conductors that have a 60 ampere ampacity orgreater could be tapped to a 600 ampere feeder overcurrentprotective device.

What are the requirements of 240-21(b)(2)?

Fuses are not required at the conductor supply if a feeder tapconductor is not over 25 feet long, is suitably protected fromphysical damage; has an ampacity not less than 1/3 that of thefeeder conductors or fuses from which the tap conductors receivetheir supply; and terminate in a single set of fuses sized not morethan the tap conductor ampacity. See "Note".

What are the requirements of 240-21(b)(3)?Fuses are not required at the conductor supply where theconductor feeds a transformer and the primary plus secondary isnot over 25 ft. long and where all of the following conditions aremet. (Any portion of the primary conductor that is protected at itsampacity is not included in the 25 feet)(1) The primary conductor ampacity must be at least 1/3 of therating of the fuse protecting the feeder

(2) The secondary conductor ampacity when multiplied by thesecondary to primary voltage ratio must be at least 1/3 of therating of the fuse protecting the feeder(3) The primary and secondary conductors must be protectedfrom physical damage(4) The secondary conductors terminate in a single set of fusesthat will limit the load to the ampacity of the secondary conductors.

What are the requirements of 240-21(b)(4)?Fuses are not required at the conductor supply if a feeder tap isnot over 25 feet long horizontally and not over 100 feet long, totallength, in high bay manufacturing buildings where only qualifiedpersons will service such a system. Also, the ampacity of the tapconductors is not less than 1/3 of the fuse rating from which theyare supplied, the size of the tap conductors must be at least No. 6AWG copper or No. 4 AWG aluminum. They may not penetratewalls, floors, or ceilings, may not be spliced, and the taps are

made no less than 30 feet from the floor.

What are the requirements of 240-21(b)(5)?Fuses are not required at the supply for an outside tap of unlimitedlength where all of the following are met:(1) The conductors are outdoors except at the point of termination(2) The conductors are protected from physical damage(3) The conductors terminate in a single set of fuses that limit theload to the ampacity of the conductors.(4) The fuses are a part of or immediately adjacent to thedisconnecting means.(5) The disconnecting means is readily accessible and is installedoutside or inside nearest the point of entrance.

Note: Smaller conductors tapped to larger conductors can be aserious hazard. If not adequately protected against short-circuitconditions (as re

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