Motor OCPD

[cppoly]

The OCPD for multiple motors is the next computed DOWN for the OCPD of the largest motor + the sum of the remaining motor FLCs. Does this apply only to motors on the same pull box / junction box / motor control center OR to motors on multiple random circuit breakers on a 42-pole panelboard as well?

For instance, lets say 3 small 1 HP motors are connected on circuits 3, 19, and 41 on a 150 A main circuit breaker 42-panelboard. Each of the individual branch circuit circuit breakers are sized at 250% motor FLC. But the main 150 A circuit breaker is going to be a lot larger than the allowed "next size down" rule.

Is the main circuit breaker excluded from this next size down rule? Is there a place in the code that calls this out?

 

[david luchini]

You are referring to the Motor Feeder Short-Circuit and Ground-Fault Protection from Part V of Art. 430.

This protection is specific to the Feeder Conductors, not the motors. The first part of 430.62(A) says "A feeder supplying a specific fixed motor load and consisting of conductor sizes based on 430.24...."

Your example of the Main C/B in the panelboard is not supplying specific fixed motor loads, nor is the panelboard busbars sized based on 430.24, so 430.62(A) does not apply.

Also, look at 430.62(B), where the rating of the OCDP shall be permitted to be based on the ampacity of the feeder where the feeder has an ampacity greater than required by 430.24.

 

[cppoly]

Ok that makes sense and that's what I've seen in Mike Holt's illustrative examples. I didn't see any illustrations for a general panelboard, but that clears it up. Thanks.