Motor branch-circuit short-circuit and ground fault protection

[Erik Jurgens]

There is a difference between the Ugly's calculation for breaker size and my calculation when dealing with a three phase motor. I referenced table 40.250 to determine the FLA of a 7.5 HP motor, 460 volts (11 amps). I then used table 430.52 and the Inverse time circuit breaker column for a polyphase motor (250 % of FLA) to come up with 27.5 amps. Therefor, it would appear to me that the appropriate size CB would be a three phase 30A. In the Ugly's book, on pg. 40, it shows a 7.5 HP motor, same FLA, but with a 20A circuit breaker. Why is this different? Thank you!

 

[Jraef]

The rule on the 250% is MAXIMUM. You can use anything SMALLER than that, but do so at your own risk of nuisance tripping.

 

[Erik Jurgens]

That makes sense. So, Ugly's is looking at the 27.5 amp calculation I came up with and taking it down to the next available size, but that would really be a 25 amp not a 20, and I brought it up to 30 amp. It seems that either way is still compliant with the code due to 430.52 (c)(1) exception No.1.

 

[Jraef]

Correct. You are good to go.