Breaker size

[surineman]

Everyone,

I have an existing MCC Panel.
One of the branches is 400A breaker feeding another MCC in other building Which its main incoming breaker is 500A.
Does anybody can see a reason behind that?
what If the total current of second MCC is more than 400?

Regards,

 

[mpparent]

The 400A breaker in the first MCC you mentioned will trip.

Mike

 

[MrBananas]

I would guess that for some reason whoever designed the system wanted any problem to trip the breaker in the main MCC.

Typically the circuit breakers on the destination side of branches are rated less than the source so that the breaker closest to the problem blows first.

 

[dpc]

There could be a lot of reasons, including using on-hand breakers instead of buying new ones. If the 400 A breaker is adequate to carry the load, then I don't see any concern.

 

[RonShap]

If it is a manual transision, then maybe it is planned maintenance and load shed of some load at the 2nd MCC

 

[dpc]

Or the feeder is only good for 400A and the inspector made them put in a smaller breaker

 

[wschel]

Chances are the 500A CB is designed to serve only as a local disconnect switch at the MCC. The 400A CB serves as the overcurrent protection for the 500 kCM feeder. The idea being that the 400A CB would trip before the 500A CB. Of course for most faults, both CBs will trip.
I have seen older facilities where the 500A CB is a molded case switch so that only one CB has to be reset. These usually have to be replaced because of the low withstand ratings of molded case switches.