Sizing of Feeder OCD for multiple branch circuits with motor loads

[easy]

Could use some help with this one. My electrical engineer is telling me that we need to reduce the size of our main breaker. He is reviewing our electrical design that has a 250 A breaker. without getting into too much detail, here is our installation:
the 600V main breaker feeds a distribution panel with 5 breakers. 4 of these are for heating loads and 1 is for a Control panel that feeds multiple motors. Each of the motors is individually protected by a combination motor starter, overload, overcurrent device. he is sizing the main breaker off of the sizing rules for motor overcurrent devices telling me it needs to be reduced to 200 A.

this makes no logical sense to me. it would be like telling me we have to resize our main breaker in the summer when we are not running the heating loads. or telling a homeowner to reduce his main breaker because he is not running his hot tub anymore.

Can someone please give me some backup to ask the right questions here. in my mind the main breaker is simply protecting the bus bars in the 600V distribution panel which are rated for more than 250A.

thanks

E

 

[petersonra]

I think you are Ok but given the sparse details I reserve judgement. a sketch would help as i am not entirely clear on just what you have. where is the main breaker (250A) located that you are mentioning?

you have not mentioned any wire sizes or loads either which might make a difference.

 

[easy]

I think you are Ok but given the sparse details I reserve judgement. a sketch would help as i am not entirely clear on just what you have. where is the main breaker (250A) located that you are mentioning?

you have not mentioned any wire sizes or loads either which might make a difference.

thanks for the reply.

i'm trying to avoid wire size details and load details here, its just more of a general question as to whether or not the main breaker can be oversized. my engineer is claiming this sizing is due to the motor loads, he isn't looking at conductor sizes at all. that is my fundamental disagreement with him here. i think this breakers only purpose is protection of the CDP and it can be as large as I like as long as the CDP is rated for that size breaker.
the main breaker is in the CDP feeding the 5 other 600V breakers.

 

[Smart $]

thanks for the reply.

i'm trying to avoid wire size details and load details here, its just more of a general question as to whether or not the main breaker can be oversized. my engineer is claiming this sizing is due to the motor loads, he isn't looking at conductor sizes at all. that is my fundamental disagreement with him here. i think this breakers only purpose is protection of the CDP and it can be as large as I like as long as the CDP is rated for that size breaker.
the main breaker is in the CDP feeding the 5 other 600V breakers.

It depends on wire size. Where motors are involved, breakers can and often are rated greater than the ampacity of conductors. However, in no case is a feeder SC/GFP/OCPD oversized if rating is not greater than conductor ampacity. Now when you get to individual motor protection, it can be oversized.

 

[easy]

makes sense, now we are getting somewhere. so if the individual motors have their own overcurrent and overload protection is it possible to have a main breaker feeding these motors that is oversized? or can it be as large as i like given the feeder conductors are sized appropriately?

 

[Smart $]

makes sense, now we are getting somewhere. so if the individual motors have their own overcurrent and overload protection is it possible to have a main breaker feeding these motors that is oversized? or can it be as large as i like given the feeder conductors are sized appropriately?

Yes and yes. The feeder breaker cannot exceed the maximum permitted unless the feeder conductors are sized such that the breaker protects them. Otherwise, it is the motor overload protection and other load OCPD's that protect the feeder conductor. But as I said earlier, if the breaker rating does not exceed the conductor ampacity [aside from the next size up provision of 240.4(B)], it can be as large as you want.