Feeder Calculation

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Colorado
I am working on a clean up project where the origional E.C. installed 1,520amps worth of wire capacity from a 2000amp MCB to a 2000amp MLO motor distribution switch board. This is in direct violation of NEC 240.4(C). If at all possible the owner wants to reuse the existing feeder. The solution took me to NEC 240.4(G)which in turn led to 430.62 and then inturn to 430.24. If I size the feeder per 430.24 I have a calculted ampacity of 1,400 amps thus the 1520 amp wire is adaquate. If I size the feeder with 430.24 then I have to size the MOCP per 430.62 which calculates out to be 1850amps. Thus I need to change the MCB from a 2000amp unit to a 1600amp unit. In doing this I still am in violation of 240.4(C), however comply with 240.4(G). Does this make sense to anyone? In this instance I beleive it is ok to have 1520amps worth of wire on a 1600amp overcurrent device, does anyone see an issue with this solution?
 

bob

Senior Member
Location
Alabama
Re: Feeder Calculation

240.4C requires that the conductor ampacity be equal to or greater the overcurrent device. There are no exceptions. You may be able to get a 2000 amp breaker and use a 1500 amp plug. In sizing the conductor I think 430.24 and 430.90 should apply.
430.62 says that the protective device must have a
rating NOT GREATER THAN the largest branch ckt device plus the sum of the other motors. You don't have to increase the 1500 amp breaker if it will allow all the motors to start.
Regardless you seem to be stuck with a conductor capacity of 1500 amps unless you add another circuit, which might be the best thing to do.

[ September 04, 2003, 08:26 PM: Message edited by: bob ]
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: Feeder Calculation

Bob,
If this is a motor feeder, the rule in 240.4(C) does not apply. 240.3 and Table 240.3 say that the overcurrent protection for motor circuits is per Article 430 and not 240.
Don
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: Feeder Calculation

Chuck,
As long as the loads are motors, and your calculations are correct, I see no problems.
Don
 

bob

Senior Member
Location
Alabama
Re: Feeder Calculation

Don
Thanks for your attention to the problem. Your
response is always helpful.
Bob
 
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