Motor Protected Circuit Breaker

[Dennis Alwon]

I got this PM from a new member named mdb443.

I'd like your opinion. Would it be code compliant to use a MPCB (motor protector circuit breaker) to protect a motor, but also to tap off load side of MPCB to feed a .5kva control transformer BCP (branch circuit protector)?

The MPCB is cULus Listed as a Circuit Breakers and the BCP is UL489 listed. The tap to primary of control transformer is less than 3ft and does not exit the enclosure, but control transformer secondary could control motor contactor coil in the enclosure and a couple solenoid valves outside the enclosure. Is this code compliant?

Instead should I use a MCCB as feeder to motor contactor (with OLR on load side) and control transformer BCP?

 

[augie47]

IMO, since the MPCB is limited for use in a listed combination motor controller any alteration would be a violation as it would be altering a listed assembly.

 

[templdl]

MCPs are magnetic only devices of course with no thermal protection. They realy don't "protect" motors at all. If one does trip it is as a result of a failing motor. They do two things, as a motor fails it prevents fires and limits damage to the motor.
It also protects the upstream distribution system from being affected by the failed motor.
It is the job of the properly sized motor starter over load relay to protect the motor from being overloaded.

 

[petersonra]

Would it be code compliant to use a MPCB (motor protector circuit breaker) to protect a motor, but also to tap off load side of MPCB to feed a .5kva control transformer BCP (branch circuit protector)?

I do not believe there is a problem tapping off the load side of a MPCB to a branch CB that feeds the control xfmr. He can certainly tap off the line side of it just to be sure.

I don't know quite what the OP means by a BCP though.

 

[petersonra]

IMO, since the MPCB is limited for use in a listed combination motor controller any alteration would be a violation as it would be altering a listed assembly.

By your description I think what you are calling a MPCB is actually a MCP.

A motor circuit protector (MCP) is a thermal magnetic CB with no thermal element installed. They can only be used as part of a listed assembly.

My understanding is that MPCB (motor protection CB) are actually UL listed as branch circuit breakers.