Service feeder sizing

[Shockedby277v]

208/120v 3 phase system. 400 amp service 500 Cu. Where does it say its ok to run a 500 Cu rated at 380 amps for a 400 amp service?? Thanks

 

[charlie b]

It doesn't. What it does say is that if the load is under 380 amps, you can use 500 MCM copper conductors, and protect them with a 400 amp breaker. This does not appear to fit your description, and wouldn't, unless you have a calculation that concludes that the load really is 380 or lower. You have to go two places to get this information: 240.4, and 240.4(B).

That said, if you are talking about a dwelling unit for which the conductors in question supply the entire load, then table 310.15(B)(6) will let you use 400 MCM conductors.

 

[Smart $]

... You have to go two places to get this information: 240.4, and 240.4(B).

...

Also 230.90 Exception No. 2

 

[augie47]

It doesn't. What it does say is that if the load is under 380 amps, you can use 500 MCM copper conductors, and protect them with a 400 amp breaker. This does not appear to fit your description, and wouldn't, unless you have a calculation that concludes that the load really is 380 or lower. You have to go two places to get this information: 240.4, and 240.4(B).

That said, if you are talking about a dwelling unit for which the conductors in question supply the entire load, then table 310.15(B)(6) will let you use 400 MCM conductors.

Fine point note: True, generally speaking, but not with the OP's 208/120 3 phase service, residential or not.
(I know your post didn't infer that, but I added the note to prevent misunderstanding)