Breaker size


Old 06-13-09, 11:49 AM
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Breaker size

Hi All, I'm new here and have a question. I'm getting a 3 hp electric compressor that the manufacturer states uses 17 running amps. The electrician moved a 15 amp 110 circuit to free up space for a single 230 volt circuit for the plug which was installed just below the main panel. The breaker for that is 20 amp. I read that one must take the 17 amp figure and multiply that by 125% which comes out to 21.25. The compressors start up voltage being much higher than 17 amps will trip the breaker, won't it?
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Old 06-13-09, 11:58 AM
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Well, not an electrician...but it probably wouldn't trip on startup. Breakers have a tolerance or trip curve I think it was called.

As to the correct size of the breaker you need, what you stated seems right, but there are different rules for motors I think. One of the Electrical Pro's should be around soon.
Old 06-13-09, 06:22 PM
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If this is a true 3 HP not the 3 HP " specal" rating then you need 25 amp breaker however there is a specal loophole in the NEC for hardwired motors only { the cord and plug motors do not use this rules.}

You can use much larger breaker on motor load as stated in Art 430 NEC Code.

IMO if the 20 amp breaker trip during restarting cycle then go with 25 amp { yeah you can go big as 40 amp but only for last resort for hard to start motor loads }

Old 06-13-09, 09:48 PM
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Thanks for the replies, GUNGUY45 and Marc. Right after I posted, it occurred to me that I should try it first then ask questions. It is a constant speed compressor running at 1740 rpm and it is a plug and cord setup. The electrician seemed confident that other than lights dimming during startup I would be okay. I'll post back with an update.

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