Undersizing Motor OCPD

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Br4nd0n said:
Code allows a 70A-3P breaker at 250% FLA for a compressor I'm installing. How likely am I to experience startup problems if I put it on a 60A-3P breaker?
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Very hard to predict, but I will say that if it is a reciprocating compressor they are notoriously hard to start. They have a large mass and high resistance to overcome.

Do you understand that you can use the 70 amp breaker even if the conductors are not rated 70 amps?
 
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The breakers is for short circuit and ground fault. The overloads are for overcurrent protection.

If your in a pinch, you could put in a 100A if you have that, as long as you have O/L set properly. The motor isn't going to be damaged. I assume your talking about a 30Hp compressor?

Just make sure you change it out later, but at least your working?

Code, Schmode..... rules are made to be broken. :angel:
 
iwire said:
Very hard to predict, but I will say that if it is a reciprocating compressor they are notoriously hard to start. They have a large mass and high resistance to overcome.

Do you understand that you can use the 70 amp breaker even if the conductors are not rated 70 amps?
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. If this is a thermal magnetic breaker it trips either thermally or magnetically. If the current that is required to start the motor is not long enough in duration to trip the motor thermally you will not have an issue with the breaker tripping because of an overload. This current would start with the motor's LRA which often times is about 7x the FLA..
Since a 60a breaker will has a magnetic (instantaneous) trip setting of 10x the breaker's rating it would trip at 600a. And since the magnetic calibration is customarily +-20% and it is customarily the calibration to be on the high side you can expect the actual magnetic trip calibration to be up to 800A.
The, you have to consider the magnetizing current to energize the motor which can be 13x the FLA. As such take the motors FLA x 13 and see if that conflicts with the mag. trip rating for the breaker of 600-800a.
 
Br4nd0n said:
Great response! Just what I was looking for! FLA is 28A x 13 = 364A. So it looks like a 60A breaker "should" hold. The customer's compressor guy never came - maybe today...
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Great, glad I coud be of some help. LRA should be close to 200a or so also which should not be an issue either which should not cause a thermal tripping issue if the motor starts normally. The accepted method for field testing a breaker themally is that it should trip in less than 140-160 seconds at 300% of its rating, 60x3=180a. That motor certainly isn't going to take 2 minutes to get up to full speed either.
 
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