Sizing Circuit Breaker in Different motor starter

[Markdegracia0370]

Hi everyone,Is the Sizing of CB NEC standard 250% of FLC of motor will also be applied in different motor control?We have 250HP motor, 480V that has FLC of 414A.If NEC Standard be applied the CB will be 800A, but we've installed it with soft starter control and we only put 400A CB and worked just fine.Is this 250% NEC standard doesn't applied to Soft starter, VFD or wye delta?Your response is very much appreciated.

 

[Jraef]

Read it carefully. The NEC is giving you the MAXIMUM size of the breaker. If it is at or under the maximum and works, it works.

 

[kwired]

What Jraef said plus - if you have a VFD or soft starter - you won't have as high of a current surge at starting which is the main reason we are allowed to size overcurrent devices like we do on across the line started motors.

To be completely code compliant a VFD supply needs to be in accordance with the drive instructions as far as conductor ampacity and overcurrent protection goes. Reality is they almost never draw more then FLC during starting or at least not significantly more and if motor is never loaded to it's full load rating you probably can go with breaker less then the drive rating and never trip it during normal operations.

Soft starter likely has higher starting surge then a VFD, but is still significantly less then across the line starting surge would be.