I have an 800 A continuous load. My circuit breaker is an 800AF/800AT 100% rated circuit breaker. According to the exception to 215.2 (A)1, my conductors only have to be rated for 800A not 1.25*800, therefore (2)600kcmil/phase will do. Is this correct? Note my feeder terminates at an MCC rated for 800A continuous.
100% rated CB and feeder sizing
- Thread starter hopkins
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- Massachusetts
Re: 100% rated CB and feeder sizing
I think you are right, you might even be able to use 2 sets of 500s by using 240.4(B) Next higher device allowed up to 800 amps.
I think you are right, you might even be able to use 2 sets of 500s by using 240.4(B) Next higher device allowed up to 800 amps.
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- Illinois
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- retired electrician
Re: 100% rated CB and feeder sizing
Bob,
While you can often round-up to the next standard size of OCPD, you can't do it in this application. 215.2(A)(1) requires that the conductors have an ampcaity of not less than the load to be served. Parallel 500s would only have an ampacity of 760 amps and the load is 800. Parallel 600s have an ampacity of 840 amps. This assumes that the conductors do not require derating.
Don
Bob,
While you can often round-up to the next standard size of OCPD, you can't do it in this application. 215.2(A)(1) requires that the conductors have an ampcaity of not less than the load to be served. Parallel 500s would only have an ampacity of 760 amps and the load is 800. Parallel 600s have an ampacity of 840 amps. This assumes that the conductors do not require derating.
Don
- Location
- Massachusetts
Re: 100% rated CB and feeder sizing
Thanks Don I was not sure on that.
I am wondering what sort of load is exactly 800 amps continuous, seems odd to come out to a standard OCPD, perhaps a test question?
Thanks Don I was not sure on that.
I am wondering what sort of load is exactly 800 amps continuous, seems odd to come out to a standard OCPD, perhaps a test question?
Re: 100% rated CB and feeder sizing
In using the 100% of the continuous load + 100% of the noncontinuous load in applying the 100% rated breaker it may make it unnecessary to move up to the next frame size when campared to sizing a breaker with 125%+100% with standard breakers. This also applies with cable sizing. Without the need for that extra 25% it is also may be possible to downsize the cable. However the cable must be 90degC rated cable applied at 75degC.
In using the 100% of the continuous load + 100% of the noncontinuous load in applying the 100% rated breaker it may make it unnecessary to move up to the next frame size when campared to sizing a breaker with 125%+100% with standard breakers. This also applies with cable sizing. Without the need for that extra 25% it is also may be possible to downsize the cable. However the cable must be 90degC rated cable applied at 75degC.
Re: 100% rated CB and feeder sizing
Also, 100% rated breakers cannot be magically substituted for standard breakers are installed anywhere you want. The enclosure must be listed for that application. It had been a favorite of mine in the past to be required to supply a 100% rated breaker in an MCC as a customer spec. required only to be applied a 80% because the MCC wasn't listed for a 100% breaker application. It has been a common misconception that 100% rated breakers a better than standard breakers so customers specifies them. 100% rated breakers are no better than standard breakers other than the additional testing that is required for that application to allow them to be listed as such and how they are applied.
Also, 100% rated breakers cannot be magically substituted for standard breakers are installed anywhere you want. The enclosure must be listed for that application. It had been a favorite of mine in the past to be required to supply a 100% rated breaker in an MCC as a customer spec. required only to be applied a 80% because the MCC wasn't listed for a 100% breaker application. It has been a common misconception that 100% rated breakers a better than standard breakers so customers specifies them. 100% rated breakers are no better than standard breakers other than the additional testing that is required for that application to allow them to be listed as such and how they are applied.
Re: 100% rated CB and feeder sizing
Bob,
Its not a "test question". I am feeding a new mcc for a new production line from a substation breaker. The total design load is ~750A which prevents me from using an 80% rated 800A breaker. I just want to make sure that 600kcmil is the correct wire size.
Bob,
Its not a "test question". I am feeding a new mcc for a new production line from a substation breaker. The total design load is ~750A which prevents me from using an 80% rated 800A breaker. I just want to make sure that 600kcmil is the correct wire size.
- Location
- Massachusetts
Re: 100% rated CB and feeder sizing
The only reason I thought it might be a test question was because of the fact 800 amp is the point where 240.4(B) changes.
With a true load of 750 Amps you could use 2 sets of 500s, as Don pointed out that would give you a capacity of 760 Amps, in separate raceways.
I would stay with the 600s, easy for me to say, I am not paying the bill.
Good luck and I hope you did not take offense to the test remark, a lot of test questions end up here.
Bob
The only reason I thought it might be a test question was because of the fact 800 amp is the point where 240.4(B) changes.
With a true load of 750 Amps you could use 2 sets of 500s, as Don pointed out that would give you a capacity of 760 Amps, in separate raceways.
I would stay with the 600s, easy for me to say, I am not paying the bill.
Good luck and I hope you did not take offense to the test remark, a lot of test questions end up here.
Bob
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