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calculating instantaeous trip breakers and inverse time breakers

calculating instantaeous trip breakers and inverse time breakers

calculating instantaeous trip breakers and inverse time breakers

(OP)
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

RE: calculating instantaeous trip breakers and inverse time breakers

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.

RE: calculating instantaeous trip breakers and inverse time breakers

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

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