According to 310.15(B)(6), I can feed a 120/240 single phase service with 400 MCM copper. Am I correct that for a 4 wire, 3 phase, 120/240 service that I have to derated per 310.15(B)(2)(a)? If so, I would need 500 MCM, correct? Thanks, Merlin
310.15(B)(6) is for single phase dwelling units service and feeders. Since that you are bringing in a 3 phase service, you can not derate. If you were to then run single phase feeders to like apartment units, those could be derated. Otherwise no derating is allowed.
I stand corrected, yes as far as your original yes. The service wires by code are only required to be big enough to the load caulated in Article 220. However if you are building something off of an engineer's drawing, you need to use was was spec'ed of do a RFI with the designer of record. There may be also local ordinaces. I typically size my feeders based on my main breaker size. This way if the loads increase on the future, the customer will not have to pay twice.
Am I allowed to size according to breaker size at 80%? Therefore allowing me to use 350 MCM. Or would I have to use 100% rating? Thats how I read the code one minute, but the next I'm second guessing myself!