next size up rule for service entrance conductors

[mshields]

240.4 allow one to use the "next size up" rule for feeders up to and including 800A. 240.21(C) says that one MAY NOT apply this rule on transformer secondaries.

What does the code say about service entrance conductors. i.e. I'm specifying an 800A service (happens to be 208Y/120V if that matters). So in this case, I'm on the secondary side of utility owned pad mounted transformer which will be whatever size they deem appropriate. May I apply the 240.4, next size up rule and therefore use 2 sets of 500kcmil with a nominal rating of 760A with an 800A service disconnect at the building OR must I use 2 sets of 600kcmil such that the ampacity in keeping with 240.21(C) of my feeder matches (in this case exceeds) the 800A service disconnect?

Thanks,

Mike

 

[augie47]

Provided your conductors are large enough to carry the calculated load, you may use 240.4 from a utility transformer.

 

[mshields]

thanks

thanks

I actually just found that listed as one of the exceptions to 230.90

 

[infinity]

Provided your conductors are large enough to carry the calculated load, you may use 240.4 from a utility transformer.

I agree we see many 400 amp services fed with 500 kcmil THHN.

 

[steve66]

Of course, there is always the "local amendment" caveat. There happens to be one in my area - here service conductor ampacity must match the service overcurrent device.

So for an 800 amp service, I must spec. 2 sets of 600KCM cu.

 

[jumper]

Of course, there is always the "local amendment" caveat. There happens to be one in my area - here service conductor ampacity must match the service overcurrent device.

Does that apply to residential services also? Just curious.

 

[steve66]

Does that apply to residential services also? Just curious.

Apparently so. Here is the text of their amendment, and like most local amendments, it really seems kind of vague.

A. All newly installed electrical services shall be in conformance with the 2011 edition of the National Electrical Code and with the following:
......
(5) Sizing of service entrance conductors shall be one of the following: (a) to main breaker with no reduction of ampere rating; (b) to approved calculated load where multiple disconnects are used, with no reduction of ampere rating; (c) to rated buss of equipment when no calculated load is provided with no reduction of ampere rating.