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© CCG Facilities Integration Incorporated
March 2016
UNDERGROUND DUCTBANK HEATING CONSIDERATIONS
A Practical Approach to Determining UG Electrical Ductbank Ampacity
Mike Mosman, PECCG Facilities Integration Incorporated
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Topics
� PHYSICS of HEAT in DUCTBANKS
� NEC and DUCTBANK AMPACITY
� TIPS for ACCURATE AMPACITY CALCULATIONS
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Ductbanks Have Issues3
� What’s wrong with this picture?� Maybe lots of things, maybe nothing.
� One needs to know purpose and usage of ductbank before design is deemed suitable.
The Physics of Underground Ductbank Heating and Ampacity Calculations
PART ONE4
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Basic Thermodynamics� Heat moves from hot
things to cold things.
� Heat flows through liquids, solids and gasses by various mechanisms.
� Heat transfer rate depends on temperature difference and thermal resistance.
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Heat Generation6
i = amps
Vin Vout
� Watts (W) = i2 x R = i x (Vin – Vout)� 1 Watt-second (W-s) = 1 Joule (J)� 1055.06 J = 1 BTU (British Thermal Unit)� Q = Heat, measured in Joules or BTU’s (and
sometimes calories)
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Heat Flow7
� Q = Heat flow rate = ∂Q/∂t in J/s or BTU/h� Q = k x A x ∆T /L, with units in Watts (J/s) when:
� k = Thermal conductivity in W/ºC-cm� ∆T = Thot – Tcold in ºC� Dimensions are in centimeters (cm)
� 1/k = Thermal resistance, Rho (ºC-cm/W)
A = Area (cm2)L = Length (cm)
Q
kThot Tcold
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Special Considerations8
Wat
ts
Amps
Rel
ativ
e Im
peda
nce
Temperature (degrees C)
SilverCopper
Aluminum
x 2x
y
4y
� Conductor heat dissipation is not linear to the load. It varies with the square of the current.
� Conductor impedance increases with operating temperature. (Be aware of temperature correction factors.)
� These facts can have significant implications in ductbank designs.
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Heat Transfer Mechanisms9
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Representative Ductbank10
AMBIENT
SLABB
NATIVE SOIL
BACKFILLB
ENCASEMENTB
DUCTB
INSULATION
CONDUCTOR
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Typical Ductbank Heat Flow
� Encasement conducts heat from source (wires in duct).� Ultimately, almost all heat from encasement flows to surface.� Most heat flows path of least thermal resistance, which makes
backfill very important.
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FINISHED SURFACE
NATIVE SOIL
ENCASEMENT
BACKFILL
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Types of Heat Flow in Ductbanks12
� Radiation and conduction from surface to ambient environment. (Lower ambient produces greater heat flow.)
� Conduction through slab or paving, if present. (Often ignored.)
� Conduction through backfill and native soil. (Thermal resistivity, rho, of soil and backfill often considered equivalent. Lower rho produces greater heat flow.)
� Conduction through encasement. (Thermal resistivity, rho, of concrete often set at 55. However, hardness and water content affect rho values.)
� Convection, radiation and conduction pass heat from wire to duct. (Duct temperature assumed to be that of cable surface.)
� Conduction through wire insulation. (Includes shields and outer coverings. Codes differ for LV and MV cable types.)
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Equivalent Circuit13
VsVeVdVi
Insulation
Current SourceAmps = Heat Generated
Duct Encasement SlabBackfill/Native Soil
Ground = Ambient Temperature
WireSurface
Temperature
DuctSurface
Temperature
EncasementSurface
Temperature
SlabUnderside
Temperature
� Volts ≈ Temperature Above Ambient (∆T)� Impedance ≈ Thermal Resistivity (rho)� Amps ≈ Heat Flow (Q)
Zi Zd Ze Zs
Zb
Zn
Vc
Conductor Temperature
Typical Equivalent Impedance
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Thermal Models14
VO
LTS
TIME
SW ON
GROUND POTENTIAL (AMBIENT)
VOLTSAMP
SOURCE ZSW V
TEM
PE
RAT
UR
E
TIME
COFFE TEMP
AIR TEMP (AMBIENT)
SW OFF
A STEAMIN’ CUP’A JOE LEFT ON THE TABLE.
SIMPLIFIED EQUIVALENT CIRCUIT.
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Temperature vs Time Domain
� This is a typical conductor temperature vs. time curve when a load is turned on and off, and is constant while on.
� T0 is ambient (or starting) temperature. T2 is maximum conductor temp when thermal equilibrium is reached at t2.
� Load is turned off at point of thermal equilibrium and cools to ambient at t3. (t3 - t2 = t2 - t0)
� t1 is the “time constant” of this curve type. (T1 = 63% of T2)
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TIME
TEM
PE
RAT
UR
E
t1 t2
T1
T2
Load OffLoad On
T0 t3t0
The National Electrical Code and Underground Ductbank Calculations
PART TWO16
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NEC Article 31017
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NEC 310.15(A)(1)� Note – This is for low
voltage wires only.� Two methods of
calculating wire ampacities is allowed:� Tables in 310.15(B) which
are familiar to every engineer, or
� Under engineering supervision per 310.15(C) which is basically the Neher-McGrath formulas.
� Note the reference to Annex B.
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NEC 310.15(A)(2)
� There is an important Exception in 310.15(A)(2). It will come in handy in all sorts of situations.
� Note the reference to termination limitation. 90�C wire ampacity cannot be used with 75�C rated terminations.
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NEC 310.15(A)(3)
� This paragraph in the code states that the manner of use of a conductor has a bearing on the selection of its maximum allowable ampacity.
� It is incumbent on the Engineer to determine the purpose of the conductors in UG ductbanks, and perform appropriate ampacity calculations that find the most economical design that results in safe operation.
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NEC 310.15(C)
� This is the basis for use of the Neher-McGrath.� All is fairly simple except for determining Rca.� Thus the popularity of ampacity software.
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NEC 310.60
� This part of the code is for medium voltage cables.� It also allows “engineering supervision,” i.e. Neher-McGrath
calculations.
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NEC Annex B
� When you don’t have the software and want to do quick calculations on simple ductbanks, Annex B is a good tool.
� Annex B is information and not part of the required code.� It applies to low voltage wiring (up to 2000 volts) and is not
used for MV ductbanks.
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Table B.310.15(B)(2)(7)� This table is used
more than all others together.
� It’s limited to just three ductbank configurations.
� It uses “standard” ductbank cross-sections shown in Figure B.310.15.
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Figure B.310.15(B)(2)(2)
� But what if your ductbank doesn’t look like these?
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B.310.15(B)(5)
� What about a 5-way ductbank? Interpolation between the 4-way (calculated) and the 6-way In chart is fairly accurate.
� What about larger ductbanks?
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AMPACITY = 2 x .88 AMAPACITY OF 1 DUCT
AMPACITY = 4 x .94 AMPACITY OF 1 DUCT IN
3-WAY DUCTBANK
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Figure B.310.15(B)(2)(3) INFO� This figure give us a 9-way ductbank. Again, interpolation for
7-way and 8-way ductbanks is fairly accurate.
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� Computers required beyond this.
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Figure B.310.15(B)(2)(1)� If you know how to use this
chart, you’re already expert.� The bottom half allows you to
select a different Rho value and load factor than those given in the Tables.
� The upper half is derived from the amperages given in the Tables, I1 being the larger amperage and I2 being the smaller amperage of the three columns of amperages.
� Note the dotted line is I1, the larger amperage.
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B.310.15 (B)(3)(a)
� Does this mean if the ductbank was 400’ long a deeper part could be 100’? No. It’s purpose is to avoid obstructions, not to avoid ampacity adjustments.� This applies to MV ductbanks as well.
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25’100’
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B.310.15.(B)(3)(b)30
-30”
BAD GOOD BETTER, BUT THE NEC DOESN’T CARE.
� This applies to MV ductbanks as well.
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Table B.310.15(B)(2)(11)� Account for the neutral wire if it’s current carrying.� Conductor count and ambient temp corrections must both be
applied. Why?
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� Current carrying neutral adds to Q, the heat being generated.
� Higher ambient temp lowers ∆T which reduces Q, the heat flow.
� More heat + less heat flow = higher conductor temps.
Tips for Making Accurate and Economical Underground Ductbank Ampacity Calculations
PART THREE32
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Typical Service Entrance?33
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Complex Ductbank Problem34
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Is the Code Conservative?� Read this excerpt from the NEC Handbook – I’ll wait.
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Rho Values36
� The following (from Annex B) is a “suggestion” by the NEC, but is it appropriate?
� It’s better to verify actual conditions to be found on site. Ask for official reports.
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Concrete Rho Values (typical rpt.)
� Note variation of rho values with concrete encasement hardness and water content.
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(Courtesy Near-Mcgrath.com)
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Soil Rho Values (typical report)
� Soil rho values are rarely consistent and depend heavily on water content.
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Project
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A Word About Water Content� True: It never rains under a building.� False: That means it’s dry under there.� Q: Why do they put a vapor barrier under the most concrete
slabs on grade?� A: To keep the moisture from coming through the slab from
below.
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Parking lot @ 150º
Air-conditioned building @ 70º
Slab
Water evaporates under hot paving
...and condenses under cool slab.
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“Dry” vs “Wet” Soil� Conduction of heat in soil occurs
at contact points between soil particles.
� Water in soil aids in conduction of heat.
� Saturated soils have all air gaps filled with water.
� As soil dries some water remains. Due mainly to capillary action the remaining water collects around particle contact points.
� Even a small amount of residual water aids conduction at particle contact points.
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SATURATED SOIL
“DRY” SOIL
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Appropriate Ambient Temperature
� Heat generally flows toward surface.� Ductbanks under large buildings generally remain a stable
20ºC or close to the building’s interior temperature.� Ductbanks outdoors under heat-gathering surfaces will have
higher ambient temperatures.
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Parking lot @ 150º
Air-conditioned building @ 70º
Ambient 35ºC (or higher?)
Ambient normal 20ºC
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Temp Adjustments in Portions� Wire ampacity at termination must be based on 75ºC wire, not
90ºC, due to termination.� In portion of duct near heat source temperature deration must
be applied, but it may be applied to 90ºC wire rating instead of 75ºC wire rating.
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75ºC TERMINATION
90ºC WIRE
HEAT SOURCE
10’
PORTION 1 PORTION 2
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Load Definition43
NEC 100
NEC 220.60
NEC 210.19
� These definitions imply a Load Factor effect on ampacity, but Load Factor is not defined anywhere in the NEC.
� Load Factor impact on conductor ampacity depends on what time duration is used to define it. 3 hours? 24 hours? A week? A year?
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Beware the PVC Duct Limitation44
� Most PVC conduits are UL listed for 90ºC max wires.� 105ºC MV cables may be loaded only to 90ºC when in PVC.
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Good Ductbank Design Practices45
� Stack ducts 2-high maximum. All ducts get proximity to the encasement surface to facilitate heat flow.
� Uneven number of ducts? Leave the blank at the bottom middle. That’s the hottest position.
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Good Ductbank Design Practices46
� The code allows (in Annex B) that mutual heating of ductbanks is negligible if edges of encasements are 4’ apart or nearest conduits are 5’ apart.� Only for ductbanks up to 2000 volts.� This may conflict with many computerized programs.
� This implies that feeders 5’ or more apart in a wide ductbank will not mutually heat each other.
4’
5’OR
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Non-concurrent Redundancy47
� When non-concurrent loads appear in same ductbank, the code allows the calculation to be made with only the greater load.� Normal and emergency feeders to ATS’s.� UPS input and bypass feeders.� “A” and “B” circuits to double-corded computer equipment.� Utility and backup EG feeders.
� Interleave “A” and “B” circuits for cooler ductbanks.
A BBA
AB B A
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Short Duration Loads
� Short duration or time-limited loads may benefit from a temperature vs. time (time-domain) ampacity calculation.� Backup generator feeders, maintenance bypass feeders, etc.
� T0 is ambient (or starting) temperature. T2 is maximum conductor temp when thermal equilibrium is reached at t2.
� If load is turned off before thermal equilibrium at t2, the maximum conductor temperature T1 will be lower than T2.
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TIME
TEM
PE
RAT
UR
E
t1 t2
T1
T2Time-domain curve
T0 t0
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Ampacity Software� When the design is complex, computerized ampacity
programs are a must. Common software programs are:� ETAP (etap.com)
� Requires add-on package for underground cable thermal calculations.� Add-on includes time-domain (transient) calculations.
� AmpCalc (calcware.com)� Single-purpose software.� Inexpensive and easy to use.� Does not perform time-domain calculations.
� CymCap (cyme.com)� Single-purpose software.� Performs time-domain calculations.
� All above programs based on Neher-McGrath equations.
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Michael Mosman, PEVP, CTOCCG Facilities Integration IncorporatedBaltimore, MD(410)[email protected]
QUESTIONS & COMMENTS50