©ted “smitty” smith 2008. this speed drill is designed to help you practice calculations. the...

©Ted “Smitty” Smith 2008

This speed drill is designed to help you practice calculations. The slides will transition in three and one half minutes instead of two minutes.

Attempt to answer the questions and then when the slide show completes, Check your answer. When you reach the end of the drill all of the slides will be repeated without the timer so you will have time to check and verify your answers at that time.

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What is the minimum sized THHN wire that can be used to feed the following motor:

Three Phase, squirrel cage, 30 HP, 480 Volts, 33.6 amperes FLC, 1.15 marked service factor and Design K.

6 AWG THHN Based on the 60 degree column of

310.16 based on 110.14(C)

A 1/0 AWG THHN conductor installed in an ambient temperature of 103 degrees F. will have an ampacity of _________.

Copyright 2005 Ted "Smitty" Smith

• 154.70 Amperes

What is the minimum sized THHN copper conductor that can be used to feed six arc welders with the following ratings:

1. 50 A at 30 Duty Cycle 2. 60 A at 40 duty cycle 3. 100 at 10 duty cycle 4. 100 A at 10 duty cycle 5. 75 A at 50 duty cycle 6. 44.6 A at 60 duty cycle

250 KCMIL Determined by 6301.11 (A )and (B) Table 310.16 Column 60 degrees C

based on 110.14(C)

What is the ampacity of a 250 kCMIL aluminum THW conductor installed in an ambient temperature of 44 degrees Celsius?


168.100 amperes

• What is the volume allowance required for the following junction box which has a total of nine 12 AWG THHN conductors entering the junction box, one of which is an equipment grounding conductor. The box will contain one duplex receptacle, has no internal clamps, and no luminairie studs or hickeys.

• What is the demand on the main service entrance of a range that has a nameplate rating of 240V and 9.5 kW?

8000 watts

A load of 23 amperes is being fed by a circuit consisting of 10 AWG THHN copper conductors. The conductors are installed in a conduit that is installed on a rooftop in the direct sunlight. The conduit is installed 12” above the roof from the bottom of the conduit. The average ambient temperature for the area in which the conduit is installed is 87 degrees F. The conduit contains a total of three current carrying conductors. Is this an acceptable installation?


Yes, it is rated to carry 32.8 amperes.

Three 10 AWG THHN, four 6 AWG THW and five 8 AWG TW conductors are installed in a one and one half inch electrical metallic tubing conduit that is 280 feet in length. Is this an acceptable installation?


Yes, the wires make up a volume of .5722 and at 40% fill an EMT 1 1/2” conduit is .814.


What is the ampacity of 2 AWG TW aluminum conductor installed in a conduit that is installed in a boiler room with an ambient temperature of 103 degrees F and the conductor is installed in a conduit with a total of 17 current carrying conductors?

30.75 Amperes

What is the demand on the main service entrance of a dwelling unit for a range that has a nameplate rating of 14.5 kW at 240 volts?

9200 watts

A 4 x 1 ½ square metal box is installed with two duplex receptacles. The box has a total of three 12 AWG MC cables entering the box, each MC cable is a 12/2 with ground. The box contains no internal cable clamps, and no luminairie studs or hickeys. Is this an acceptable installation?

No, the conductors and devices require a volume allowance of 24.75 and Table 314.16(A) lists a 4 x 1 ½” metal box as having a volume of 21.0

Is it allowable to add three 10 AWG THHN conductors to an existing conduit that is 150 feet in length and currently has eight 6 AWG THWN conductors and one 12 AWG THHN conductor installed? The conduit is a 1 ¼” inch electrical metallic tubing.

No. The total area used by the conductors once the three 10 AWG conductors are added will be .4822 inches and a 1 ¼” EMT at 31% fill has an allowance of only .464 inches.

What is the demand on the main service entrance of a single family dwelling for a range that has a rating of 14.2 kW at 240 volts?

8800 VA

What is the demand on the main service entrance of a multifamily dwelling for the following ranges:

9.6 kW at 240 V 12.9 kW at 240 V 16.8 kW at 240 V 21.2 kW at 240 V 11.5 kW at 240 V

How many 15 ampere lighting circuits will be required for a single family dwelling that is 2650 square feet to meet the minimum requirements of the National Electric Code?

5

2650 x 3= 7950/120=66.25 amperes/ 15= 4.41

Total of 5 circuits

What is the demand on the main service entrance of a multifamily dwelling for 15 electric clothes dryers that are rated at 4.4 kW at 240 volts?

32250 VA

5000 x 15= 75000 75000 x .43= 32250 VA

What is the demand on the main service entrance of a multifamily dwelling for the lighting and receptacle loads for 20 apartments if each apartment is 1300 square feet?

55950 VA

3 x 1300 x 20= 78000 4500 x 20= 90000 78000 + 90000= 168000- 3000=

165000 165000- 117000= 48000 117000

x .35= 40950 48000 x .25= 12000 12000 + 40950 + 3000= 55950 VA

A 39.6 ampere load is fed with a 6 AWG THW conductor that is installed in a conduit with a total of 7 current carrying conductors. The conduit is installed in a room that has an ambient temperature 38 degrees Celsius. Is this an acceptable installation?

Yes it is acceptable. The conductor is rated at 40.04 amperes.

What is the maximum sized instantaneous trip breaker that can be used to protect the following motor for short circuit protection:

50 HP, 208V, three phase, squirrel cage motor, 116.8 amperes FLC, marked service factor of 1.15?

1200 Amperes Based on T430.52 and 240.6 for

standard sized fuses and circuit breakers.

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