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400 amp 3 phase 120/240 question. #191899 01/17/10 08:10 PM
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Merlin Offline OP
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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

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Re: 400 amp 3 phase 120/240 question. [Re: Merlin] #191900 01/17/10 08:26 PM
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sparkyinak Offline
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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.


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Re: 400 amp 3 phase 120/240 question. [Re: sparkyinak] #191901 01/17/10 08:33 PM
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Merlin Offline OP
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Thanks Sparky. So if that is the case, then I have to run 500 MCM conductors then. Correct?

Re: 400 amp 3 phase 120/240 question. [Re: Merlin] #191902 01/17/10 08:51 PM
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sparkyinak Offline
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yep


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Re: 400 amp 3 phase 120/240 question. [Re: sparkyinak] #191903 01/17/10 09:03 PM
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sparkyinak Offline
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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.


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Re: 400 amp 3 phase 120/240 question. [Re: sparkyinak] #191904 01/17/10 09:12 PM
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Merlin Offline OP
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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!

Re: 400 amp 3 phase 120/240 question. [Re: Merlin] #191906 01/17/10 09:49 PM
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frenchelectrican Offline
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you can run at 100% however if you going to make single phase feeder network you can not reduce the neutal conductor at all espcally feeded from 3 supply.

Merci,Marc


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