Motor Question

[Jimmy7]

My drawings have the following information, and I'm trying to figure out how the engineer came up with this breaker size.

Exhaust Fan - 40 HP, 52 FLA, 480 V, 3p - 125A Breaker

I get, 40 HP , In 430.250 - 52 Amps, 52 x 250% (430.52) = 130 Amps, In 240.6 (A) - Breaker size 150 Amps

Am I correct?

 

[Jraef]

150A would be the MAXIMUM allowable size. 125A is fine, so long as it holds during startup. Since the EE called for 125, I would put in 125 and if it nuisance trips, you get paid to replace it.

That said, 52A FLA, starting current (assuming Across-the-Line) can be 600% (306A) and magnetic inrush can be as high as 1020A. Mag trip settings (if adjustable) on a 125A breaker would be up to 1250A, it should be fine. If the 125A breaker does not have adjustable mag trips, it LIKELY is fixed at 10x the rating, but you should check the tech data on whatever breaker you use.

 

[petersonra]

It will probably work OK. If it does not, you get paid to replace it.