How should I size my feeder breaker? I have a 50hp motor. Using table 430-150 the amps are 65A. The wire size is #3AWG. Using NEC 430-52 the MCP should be sized no greater than the value calculated using table 430-152. This value is 65A X 2.5 = 162.5. Likewise, the using 430-62 the same applies. This rating can not exceed the settings for the branch circuit, short circuit and ground fault protection device which was calculated using table 430-152. If this is correct, can my MCP and Feeber Breaker sized the same, 150AMPS? What is industry standards? Am I on the right track?

What else is on this feeder?

As far as the branch circuit is concerned, you can round up to 175 amps by applying the exception to 430.52(C)(1), it is possible to go larger if the motor will not start without tripping this breaker, see exception #2.

I've led a sheltered life, can you tell me what an MCP is?

MCP = Motor circuit protector. I'm an amature still, but here goes. The MCP based on how I've seen it used, provides instantaneous trip and overcurrent protection for your motor. It's jsut a breaker that has thermal overloads for overcurrent protection and I think the instantaneous trip.

As far as I know MCPs are always fully magnetic without any thermal element(they do have an adjustment on the instantaneous trip unit that permits motor starting currents to be tolerated).
They are usually found in Motor Control Centers to provide Short-circuit & Ground-fault protection for the motor circuit.
The motor controller's overload relay (with properly sized heaters) will provide overload protection, therefore no thermal component is needed in the MCP.

[This message has been edited by Redsy (edited 12-20-2002).]

If this is a true instantaneous trip breaker in a listed motor control center or listed starter, then this breaker can be set at 800%.

If you are feeding an individual motor starter (with or without an MCP), the breaker you are using will be thermal/magnetic & should be sized according to the calculations you've already done.