Inrush Mystery

[templdl]

Everyone,

OP here. I have read through the forum, and come to the conclusion that more than likely, this behavior I am witnessing isn't an anomaly; I just haven't gathered field data from installations enough times to recognize that these numbers are actually quite normal, especially for brand new energy efficient motors, which these are. By looking at the trip curve for a 15A breaker, it is plain to see that the starting current will likely engage the instantaneous portion of the curve, and that's ok.

NEC article 430.52 states that for an inverse time circuit breaker, the maximum permissible size of the breaker should be 250% of the rated FLA, upsized to the nearest standard size. Our FLA is 8.3A, x 2.5 = 20.75A, upsized to the nearest standard size, this would bring our circuit breaker to 25A. A 25A CB will not instantly trip at the measured currents, so we are probably OK by simply upsizing the breakers. That's what the client has elected to do instead of further analysis.

This matter has likely been solved, but I'll post more if further issues arise. Thank you all for your valuable input; it has been educational.

Thanks! That is what I was getting at in my previous post that is identifying what event is causing the trip and is the device the correct one for the application.
It is great that you have zeroed in on the real issue. But I must admit that some of the responses for the cause of your problem were intriguing.

 

[Sahib]

As a crosscheck, the inrush current of a transformer having a KVA capacity equivalent to the sum of HP's or any one HP of the three induction motors may be considered. That would perhaps serve as a limit for any normal inrush of the induction motors.