Residential neutral size

[cornsyrup]

I'm working on a bid for a 13 meter service for an apartment building. I figured load calcs. and determined 400A for the mains. It's a 3 wire 120/240V
service.
Using table 310.15 (b) (6) it's 400 kcm for the ungrounded conductors; any suggestions for the neutral and the ground would be appreciated.

 

[Ben Franklin]

310.15(B)(6) can only be used for each individual dwelling unit and not the whole service of the multi-family building.

In any case, 220.61 and 250.24(C)(1) are used to size the grounded service conductor (neutral) and 250.66 is used to determine the size of the grounding electrode conductor(s).

 

[physis]

It's 250.66 for the GEC. I don't like using little neutrals so I'll have to look for that one.

 

[Dennis Alwon]

I would normally just downsize one size for the neutral but you can go as small as a #2 copper if your calculated load is very low on the neutral. I have never seen it done but it is legal.

 

[augie47]

I would normally just downsize one size for the neutral but you can go as small as a #2 copper if your calculated load is very low on the neutral. I have never seen it done but it is legal.

Dennis, rare, but I'd have to disagree. For this 400 amp service he has to use 310.16. The neutral will have be be no smaller than allowed by 250.66, probably a 1/0 Cu or 2/0 Al.(Ref 250.24(c){1} )