This doesnt seem right.
I think that makes good sense given the cost of wire these days. Just so you will know the big box will carry NM-B #6 awg copper cable, the NEC requires you to limit the ampacity to 55 amps but you are allowed to breaker it at 60 amps. It's a special requirement for nm-b cable that you must use the ampacity in the 60C column of table 310.16 (see NEC art. 334.80). If you can get an SER cable in copper #6 you can use 65 amps and protect it with a 70 amp breaker. Otherwise a #4 SER aluminum cable will give 65 amps and you can put it on a 70 amp breaker. The reason for this is an ocpd rule we call the "next size-up rule". There are no 55,65,75,85,95,105,115... circuit breakers (see NEC art. 240.6)(A) so when your ampacity falls in between for example 50 and 60 amps you are allowed to go to the next size up which would be a 60 amp breaker. Most cables (other than nm-b or uf-b) are allowed to use the 75C column of 310.16 for ampacities. Basically if the wire has insulation matching the types in the 75C column or the 90C column it can use the 75C ampacities.... other than the exceptions noted. Do not use the 90C column ampacities.
Last edited by Stubbie; 01-25-2007 at 11:47 AM.