breaker too small?

[curlybillynilly]

we have older allen bradley mcc with updated mechanical i.e. fans, mills, conveyer belts etc.
some years back the motors have been changed (hp increase) example: 100 hp motor for a coal mill with fla of 124 and breaker size of 150. can not increase breaker frame size to 250 without serious modification to starter cabinet. table 430.52 is max and excpeption in 430.52(C)1. what is permissable min and what is standard for high torque equipment?

 

[bob]

Re: breaker too small?

The code does not specify a minimum. AS you stated the MAx OC device is listed in 430.

 

[templdl]

Re: breaker too small?

The only other thing to be concerned about is nuisance tripping which you would consider to occur when the motor is energized. I doubt if the breaker would trip while starting due to long acceleration times. LRA one would expect to be somewhere around 7x the FLA which would trip the breaker on thermal if it were to be extended for a long enough period of time. However, breakers if sized too small are often prone to trip instantaneously as a result of the magnetizing inrush current which often reaches 13x the FLA for the first 1/2 cycle for energy efficient motors.
124x13=1612a
Knowing that the magnetic calibration of a common 150a industrial breaker is 10x the breaker rating +-20% (1200-1800a)the application may be marginal. Since breaker tend to lean toward the high side of the calibration and that you may not have a high efficiency motor that approaches the 13x it is of my opinion that the odds are in your favor that you will not have a nuisance tripping problem.