Circuit Breaker Question 1

[acfm]

I believe this was inapropriately posted in Electric Motors and Motors Control Engineering so I am reposting it here.

FORGIVE ME FOR DOUBLE POSITNG!

I have a customer asking for assistance with selecting the correct main circuit breaker for a 10HP 460V triplex vacuum system. Depending on the demand, only one unit could be in operation or all three could be operating at the same time. Motor FLA is 14 each (w/0 1.15 SF).

Cutler Hammer Motor Circuit Selector says the following:

1. Instantaneous trip circuit breaker GMPC/HMCP: 30 AMPS and
2. Circuit breaker trip rating: 35 AMPS

A.) Can someone provide a briefly explain the difference between #'s 1 and 2? and
B.) Should the breaker be sized based on all three units in operation (10HP x 3 = 30HP)?

Just trying to help a guy out!

Thanks!

 

[waross]

Is it possible for two or three motors to start simultaneously?
yours

 

[dpc]

If you are wondering about using an HMCP as a main breaker to a 480V panel - you can't do that. The HMCP is magnetic-only breaker - no thermal element. It can only be used as part of a combination starter for a motor.

For a main breaker, you will want a standard thermal-magnetic breaker.

 

[acfm]

Waross,

Thanks for responding.

The possibility that all three units could start at once does exist.

 

[alehman]

The HMCP is what's called a motor circuit protector. As dpc mentioned, it is basically a molded case circuit breaker with only a magnetic trip element. it is intended for short circuit protection only and can only be used in direct combination with appropriately listed motor starters (which provide overload protection). Each motor should have it's own protection consisting of a starter for overload and a fuse, circuit breaker or MCP for short circuit protection.

All three motors can be supplied from a common feeder. The circuit size for three motors would be 14 * 1.25 + 14 + 14 = 45.5 amps. The NEC allows the overcurrent protection for the feeder to be sized at a maximum of the sum of (the permissable overload protection for the largest motor) + (sum of the full load currents for the other motors). Got all that? It's somewhat complicated and should be evaluated by a licensed engineer.

 

[Senselessticker]

Remember that the purpose of a circuit breaker is to protect the conductor servicing the load (not necessarily the load). Make sure that the feeder is sized properly before selecting the breaker size.