When Calculating Feeders and Subfeeder conductors, do I apply a 80% continous duty deration factor into my calculation? Or can I just size my conductors to the entire ampacity? What I am getting at is this: I have a 400amp transfer switch, with feeders (500/3 XHHW-2 TC), but if I go to T.310.16, the permitted ampacity is 380, without any deration included. If I apply an 80% Continous Load, (210.19(a), 215.2(a)(1), and 230.42) Ampacity correction factor, that Ampere rating then drops to 304 amps. Which I can only terminate on the Next size higher Breaker at 350 amps, ( 240.4(b)), therefore I am not supposed to terminate 500kcmil xhhw-2 on a 400 amp Breaker, am I correct here? Or is it Legal ?
Derating Feeders and Subfeeders?
- Thread starter sirjaxx
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Re: Derating Feeders and Subfeeders?
The answer is yes, and no.
The total feeder load will consist of BOTH types, continuous and non-continuous.
Lights on a commercial location are continuous because they will run more than 3 hours at a time. Many recepticals are not, like one with a sewing machine plugged in.
Do the derating or demand calc's individually then add the totals to get feeder total.
The answer is yes, and no.
The total feeder load will consist of BOTH types, continuous and non-continuous.
Lights on a commercial location are continuous because they will run more than 3 hours at a time. Many recepticals are not, like one with a sewing machine plugged in.
Do the derating or demand calc's individually then add the totals to get feeder total.
- Location
- Lockport, IL
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- Semi-Retired Electrical Engineer
Re: Derating Feeders and Subfeeders?
I agree with what has been said, but let me say it another way. You can put every bit of 380 amps through the 500 kcmil. You don?t de-rate from Table 310.16 UNLESS (1) The ambient temperature is above 30C, or (2) You have more than 3 current-carrying conductors in the same raceway. So do what bob and wolfman56 have said: calculate the load, accounting for continuous and non-continuous loads. Then look at the final answer. If is it at or below 380, then you can use the 500 kcmil cable.
I agree with what has been said, but let me say it another way. You can put every bit of 380 amps through the 500 kcmil. You don?t de-rate from Table 310.16 UNLESS (1) The ambient temperature is above 30C, or (2) You have more than 3 current-carrying conductors in the same raceway. So do what bob and wolfman56 have said: calculate the load, accounting for continuous and non-continuous loads. Then look at the final answer. If is it at or below 380, then you can use the 500 kcmil cable.
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