MCP must be as part of a combo motor starter?

[Isaiah]

My understanding is if a MCP is used in a MCC it must be part of a combination starter; and can’t be used as a ‘stand alone’ breaker to a load, even if the load has overload protection


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[don_resqcapt19]

Correct, but not aware of any MCPs that are not magnetic only.

 

[Isaiah]

Correct, but not aware of any MCPs that are not magnetic only.

I always thought MCPs lacked thermal protection and were magnetic only?


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[Isaiah]

Correct, but not aware of any MCPs that are not magnetic only.

Don if the RVSS is installed externally to the MCC does the thermal magnetic CB need to be sized per 430.52 at 250% of FLC instead of 125%?


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[don_resqcapt19]

I would treat this like a VFD. The input conducts sized not less the the rated input current of the RVSS, and the branch circuit short circuit and ground fault protection sized not more than 250% of the motor current rating as found in the code tables.
There is no requirement that you size it at 250%, just can't be greater than that. I see no issue with 125% for a soft start.

 

[Isaiah]

I would treat this like a VFD. The input conducts sized not less the the rated input current of the RVSS, and the branch circuit short circuit and ground fault protection sized not more than 250% of the motor current rating as found in the code tables.
There is no requirement that you size it at 250%, just can't be greater than that. I see no issue with 125% for a soft start.

Thanks Don
- there is an exception in 430.52 that allows you to go to 400%


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[don_resqcapt19]

Thanks Don
- there is an exception in 430.52 that allows you to go to 400%


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You would never use that for a VFD or a soft start. That is for high efficiency motors started across the line where the inrush current exceeds the instantaneous trip point of a breaker sized at 250%. You can only use that if the 250% sized breaker will not permit the motor to start. In my opinion, you must try the 250% rated breaker and show it will not permit the motor to start before going up to a 400% rated breaker.

 

[Isaiah]

You would never use that for a VFD or a soft start. That is for high efficiency motors started across the line where the inrush current exceeds the instantaneous trip point of a breaker sized at 250%. You can only use that if the 250% sized breaker will not permit the motor to start. In my opinion, you must try the 250% rated breaker and show it will not permit the motor to start before going up to a 400% rated breaker.

Thanks again Don!


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