Forum Title: Motor amperage lower than name plate
I checked out a sewage pump today with questions of why the breaker was tripping every once in while (actually only twice in about 4 to 5 days) I think I may have found the problem in a wire that had a "nick" in it that fed the timeclock that operates the pump that was laying up against the rough edge of the time clock. I'm thinking that the vibration of the motor was possibly causing the wire to short out, but don't know for sure since I didn't see any burn't marks on the timeclock, (that is until I started to get a amp reading and it blew up in my face:(But laying this thought aside, I took amp readings on the motor phases. The voltage is around 213 volts on each leg.The nameplate at 200 volts is showing 15.4 amps. The actual amperage is only around 7.7 amps. I don't understand why the amperage is so low. I disconnected the motor wires from the rest of the circuit and checked to see if one was shorted out to ground and they were not. I checked the motor numerous times and the breaker never did trip. Any ideas why I would get so low amperage when the nameplate is calling for twice what it was pulling??When I left I had the pump coming on and off with the timeclock like it was suppose to. The customer is suppose to check it later and give me a call. I hope it is fixed but the amperage puzzles me. Thanks for the input.. Steve
Category: General Electrical Discussion Post By: George Moore (Ranchitos Del Norte, TX), 03/23/2017
It's like lifting weights ... lift more, work harder.The amperage on the pump is based upon the maximum load under normal conditions. If you're not lifting the water that high, it will draw fewer amps.You'll still need to allow for the full current inruxh when the pump starts, though ... that's likely when the breaker is tripping. I'd see if I could go up a breaker size if the existing breaker is less than 5x the full load amps on the nameplate.If you can take the pupm out of service, I'd try to spin the thing by hand. It ought to spin freely in BOTH directions- and you ought to be able to feel if a bearing is binding.

- Anita Mendez (New Lisbon, NY), 03/25/2017

Reno, I was just getting ready to post about the breaker size. Where do you get the 5x at? I am using an old code book, but I go to the table that tell the Maximum rating of the motor branch circuit short circuit device and it tells me I can go 250% of the full load amps, which in this case250% of 15.4 amps is 38.5 amps, and if I figured it right I should be able to go to a 40 amp breaker. Tell me if I'm wrong, it's been a while since I been to code class and I don't work with motors all that much. The motor is only on a 30 amp 3 phase breaker now. I did notice at one point the in rush current was 44 amps and this could be the reason of the breaker tripping. Most other checks I did it was 33 amps which still might cause the breaker to trip. Of course this set up is a old set up and has been there for years. Question is why has it started to cause problems now, unless of course the 30 amp breaker is weaker. It does show signs of age.. Thanks again

- Gary K (Laton, CA), 03/25/2017

At a certain point you have to just pick up the Easter egg and believe it was from a bunny. If the breaker is not tripping any more you fixed another one.As for the breaker. 250% is the max. Most motors will start with a lot less. Think of all of those 12a 120v motors on 15 and 20a breakers.

- Dustin Delgado (Strasburg, IL), 03/25/2017

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