AC feeder circuit breaker

[mlnk]

Main panel is 400 amps. There is a #4 cu feeder for one AC unit. The feeder goes to an EXO box, 60 amp rating with 45 amp fuses as required by manufacturer protecting #6 Cu wire to the AC unit . The full load amp rating is 41 amps. Can I install a 100 amp circuit breaker in the 400 amp box for the #4 wire feeder based on NEC 440-22, the 225% rule for AC? 41 x 225% = 92 amps, next size up is 100 amps. If not what circuit breaker should I use? Everything is 3 phase.

 

[augie47]

IMO, No !
For a couple of reasons, the 225% figure, which I assume you are obtaining from 440.22, is only allowed if the 175% number is not adequate. Secondly that section applies only to the individual motor compressor and I assume you unit incorporates a condensor fan and/or blower, and most importantly 440.22(C) will not allow you to exceed the manufacturers values.

 

[iwire]

Why would you want to use a breaker so much larger than the fuse size?

I would likely use a 60 or 50 amp breaker.

 

[mlnk]

The manufacturer of these new AC units wants a 45 amp fuse and I am providing that. The #4 feeder with the 100 Zinsco breaker is upstream from the EXO and was installed about 20 years ago. I am wondering if it was to Code at that time.