Motor Feeder Circuit Breaker

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dnyman

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The NEC says to calculate the Motor Feeder current by 125% the FLA of the largest Motor, plus 100% FLA of the remaining motors, plus other loads.
To calculate the feeder circuit breaker, its the largest branch breaker, plus 100% FLA of the remaining motors. There is no mention of "other loads". I'm looking at the other loads being PLC, control transformers, dc power supply, and convenience receptacle. I'm thinking that by using the NEC tables for the motor FLA, this has enough current over the actual FLA of the motor to account for this additional load. Question is - Does the NEC intend that the "additional loads " are not used to calculate the Feeder circuit breaker size? Anyone have any thoughts? I would appreciate the feedback.
Thanks.
 
dnyman said:
The NEC says to calculate the Motor Feeder current by 125% the FLA of the largest Motor, plus 100% FLA of the remaining motors, plus other loads.
To calculate the feeder circuit breaker, its the largest branch breaker, plus 100% FLA of the remaining motors. There is no mention of "other loads". I'm looking at the other loads being PLC, control transformers, dc power supply, and convenience receptacle. I'm thinking that by using the NEC tables for the motor FLA, this has enough current over the actual FLA of the motor to account for this additional load. Question is - Does the NEC intend that the "additional loads " are not used to calculate the Feeder circuit breaker size? Anyone have any thoughts? I would appreciate the feedback.
Thanks.
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Depends on how the motor starts, and what the OCP device is. CBs can be sized up to 250% of motor FLA. I use fuses sized at perhaps 125 %. But the amount of power used by a control transformer is minimal for my motors which are typically 50 hp or 65 amps, an amp at most for the ancilliary loads is not a factor.
 
Thanks for the reply

Thanks for the reply

Thanks for the reply.

I think the 250% is for the branch circuit over current protection. The motors I use are fractional to 5 HP, so the control circuit current is a higher percent.
 
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