OT: Multiple motor FLA computation

[theripley]

Basically, you want to consider how the motor loads start up. Any motors that ALWAYS start at the same time should be lumped together. Having a PLC-controlled start-up timer that controls how the motors start can reduce the total breaker size and the maximum start-up current.

Thanks a lot for the reply. Thank you PLCS.

 

[T Gibbs]

NEC says multiply largest motor current by 125% then add the sum of all other loads to get WIRE size. Then multiply the largest motor by table 430.52 (inverse time circuit breaker is 250%), then add the sum of all other loads for BREAKER size.

 

[Lancie1]

You are talking about the motor short-circuit and ground fault protection (which varies from 150% up to 1100%). For motor overload protection, Article 430.32 shows generally 125% times the nameplate Full Load Amps.

 

[T Gibbs]

OVERLOAD protection is part of the starter. The breaker, which is what the OP is asking about, is the short circuit and ground fault protection.

 

[Lancie1]

OVERLOAD protection is part of the starter.

Maybe so, but it seems funny tht the NEC used a chapter to discuss overload protection for motors. Read it yourself.

 

[T Gibbs]

Yes, but that chapter is not relevant to the OP's question. 430.62A covers the breaker size for motor feeders.

 

[JRB]

Motor overload size selection is determined from motor nameplate current, however, motor conductor
ampacity and fuse or CB size is determined by motor currents listed in NEC tables 430-(147-150), not
motor nameplate FLA.

 

[T Gibbs]

Motor overload size selection is determined from motor nameplate current, however, motor conductor
ampacity and fuse or CB size is determined by motor currents listed in NEC tables 430-(147-150), not
motor nameplate FLA.

exactly