Nec 230.42 & 240.4(c)

[steve66]

Steve you need to clear this up. 500 kcm terminating in a 400 amp breaker is done all the time. Of course the load could not exceed 380 amps.

Sorry for the confusion. Now that we know it's a POCO transformer, and the customers wire, this doesn't apply. But, if these "service conductors" were actually feeders, then the tap rules would govern the wire between the transformer and the overcurrent protection.

I was refering to 240.21(B):

Feeder Taps....The provisions of 240.4(B) shall not be permitted for tap conductors."

But, you guys know that - I just wasn't making myself clear.

I think Ron's reference to 230.90 is exactly what you are looking for:

230.90(A) Such protection shall be provided by an overcurrent device...that has a rating not higher than the allowable ampacity of the conductor

And we know none of the exceptions apply to this case. (Although it is kind of interesting to note that although you can't use a 1600A breaker on (4) sets of 500, you can use (2) 800 amp breakers in parallel.)

Steve