NEC Cable Ampacity Calculation
NEC Cable Ampacity Calculation
(OP)
a) Required Cable Ampacity = FLC X 1.25
(OR)
b) Required Cable Ampacity = FLC / DF (Ambient temp) x DF (Cables in raceway)
(OR)
C) Required Cable Ampacity = FLC x 1.25 / DF (Ambient temp) x DF (Cables in raceway)
(OR)
d) Required Cable Ampacity = Largest of a), b) & C).
(OR)
e) Required Cable Ampacity = Largest of a) & b)
Which one of the above is the correct method to calculate the required ampacity?
(OR)
b) Required Cable Ampacity = FLC / DF (Ambient temp) x DF (Cables in raceway)
(OR)
C) Required Cable Ampacity = FLC x 1.25 / DF (Ambient temp) x DF (Cables in raceway)
(OR)
d) Required Cable Ampacity = Largest of a), b) & C).
(OR)
e) Required Cable Ampacity = Largest of a) & b)
Which one of the above is the correct method to calculate the required ampacity?






RE: NEC Cable Ampacity Calculation
More than 3 wires in a raceway or cable the ampacity is derated.
So; derated ampacity times 1.25.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: NEC Cable Ampacity Calculation
xnuke
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RE: NEC Cable Ampacity Calculation
For 215.2(A)(1)(a):
Load = 100% * noncontinuous + 125% * continuous
Conductor ampacity (at any rated temperature) must be >= load
For 215.2(A)(1)(b)
Load = 100% * (continuous + noncontinuous)
Cable_rating_new = Cable_rating_old (at any rated temperature) * DF(amb temp) * DF (cables in raceway)
Cable_rating_new must be >= load
For 110.14(C)
The cable ampacity (without application of derating factors) at the termination's temperature rating (usually 60C or 75C but not always) must be >= the equipment termination's rating. For a OCPD on a feeder, see 215.3 for calculating the rating: it is 100% * noncontinous + 125% * continuous.
So I would say e) is the closest.
RE: NEC Cable Ampacity Calculation
Under the codes that i am subject to:
Cable ampacity = listed ampacity x derating for more than four cables in a raceway or cable and any further derating for temperature.
This is the new ampacity and this is the ampacity that that factor of 1.25 will be applied to.
Possibly I did not explain myself well.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: NEC Cable Ampacity Calculation
In 2014, the NEC clarified that it wasn't 1.25 AND application of derating factors, but rather 1.25 OR derating factors for continuous loads, like wroggent and I stated. A lot of people were using the first method for sizing. I don't know if the CEC followed suit with that clarification.
We also both mentioned that per the NEC, terminal temperature ratings must also be accounted for when sizing (which was added to the CEC in 2012, I believe).
xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: NEC Cable Ampacity Calculation