Sizing a circuit breaker for a motor starter

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ktm400sx

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I have a 100hp pump panel fed by #2 thhn wire. The contacter has heaters and there is a 300 amp mag break circuit breaker in the pump panel - so far, so good. (Motor is protected)

The circuit breaker that I would like to use to feed this pump panel and protect the #2 wire is a GE TED136150 (150 Amp thermal magnetic breaker)

The breaker looks very small compared to the breaker in the pump panel......

I assume the small size has to do with its AIC rating which is 18ka.

There is no way to adjust the mag trip on this breaker, am I using the wrong style of breaker?

Thanks for any help
 
A thermal magnetic breaker can be sized up to 250% of flc. At 480 volts you're looking at a breaker rated 300 amps or so. You might have problems with nuisance tripping with a 150 amp breaker.

Wire is not protected from overload by this breaker, just from short circuit damage.

Jim T
 
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My first reaction is a question about the existing situation. I didn't see a voltage, but from the TED breaker I would assume 480v. If so, based on 430.250 and 430.22,the #2 THHN is way undersized.

Overlooking that opinion and addressing your question, I would think 430.62 would allow you to use a 300 amp OCP on the feeder and prevent nuisance tripping as jtester noted.
 
Yeah, a 150A thermal-magnetic does sound small for feeding a 100Hp motor. Recommend getting the trip curves for the 150A thermal-magnetic and see if the motor starting current curve falls inside the trip curve. You might also check if the mag-only setting is outside of the 150A CB trip curve. I doubt it is. You will likely have the mag-only set somewhere between 1000A and 1200A

Thermal-Mag, molded case CBs without separate programmable trip units are not adjustable - at least I've never seen one that was.

Generally, any molded case with trip adjustment dial(s) is a mag-only.

carl
 
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