calculating instantaeous trip breakers and inverse time breakers

[2571]

Good morning,

I'm calculating the size of an Instantaneous or a inverse time breaker for a 200 hp motor, according to NEC 430.52 I'm to multiple my FLA by 1100% for instantaneous and 250% for inverse time. PLease correct me if I'm wrong.

221(fla) X 11.00= 2431 instaneous trip breaker
221(fla) X 2.5= 552.6 inverse time breaker

I'm assuming these has standard size as fuses do.

thank you for your help
2571

 

[wbd]

I would say that you would need a 250A bkr thermal mag with an instantaneous range that has a range that includes something close to 2400 A.

 

[GGOSS]

Hi 2751,

Subject to cable sizing of-course I would normally use a 400A thermal magnetic breaker for a 150kW / 200hp motor.

Some breakers provide adjustments for both the themal and magnetic/instantaneous trips and this can be very usefull. Others may provide adjustments for thermal trips only, and generally on these I have found that the magnetic/instantaneous trip occurs somewhere in the range of 11 to 13 x rated current.

Specs will vary between manufacturers. I would therefore suggest your supplier of circuit breakers is in the best possible position to advise you further.

Regards,
GGOSS