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I had a single phase 115v ac compressor connected to a 20A circuit. It worked in that location for many years.

Recently, I moved the compressor to another outlet on the same circuit. Now it trips the breaker whenever the compressor comes on - but only if I'm not watching it... if I turn the compressor on and off manually the breaker never trips.

The breaker itself is fairly new. All wiring is 12 ga nonmetallic cable. The new outlet location is the first one on the circuit. In other words the previous compressor's location was downstream of that outlet, in fact it was the last one on the circuit so it should have suffered from more voltage drop than the new location.

If I move the compressor back to where it used to be, it works fine and the breaker never trips.

I couldn't find anything wrong with the receptacle, but I replaced it with a brand new 20A commercial grade duplex receptacle anyway. I couldn't find anything wrong with the wire nuts but I replaced them too. All wires are wrapped nice and tight.

The compressor motor nameplate indicates 15A, with a locked motor current of 98A. Obviously its starting current is a lot more than 20A and I know I'm pushing the limits of a 20A circuit. Starting current might be around 60 to 80A, possibly more, but that misses the point that the compressor worked and continues to work in its old location.

There is nothing else on that circuit except for a very small battery maintainer that doesn't draw much current. And remember, all I have to do to make things work again is move the compressor to its old location, all the way at the end of that same circuit. Which by the way used a residential grade 15A duplex receptacle.

I don't have any way of actually measuring current that big, but have no reason to doubt it's really drawing too much for that breaker.

What the heck is going on?

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    What make/model are your circuit breakers?
    – ThreePhaseEel
    Commented May 16, 2021 at 4:44
  • 2
    "previous compressor's location was downstream" so the problematic outlet is closer to the breaker? If so, the resistance of the wire to the far outlet is helping limit the start-up current. Sometimes using an extension cable accomplishes the same benefit.
    – P2000
    Commented May 16, 2021 at 5:27
  • Is the breaker double width? Does the breaker have a TEST button?
    – Harper - Reinstate Monica
    Commented May 16, 2021 at 6:19
  • You moved the compressor. For a reason. To do a different job. Are you using different hoses? One that might be plugged and would cause the compressor to trip (although it shouldn't). Why did you move the compressor? What is the new job?
    – Steve Wellens
    Commented May 16, 2021 at 13:54
  • @P2000 confirms what I suspect, which is the longer circuit cannot deliver the starting current the motor is capable of drawing. Moving the compressor to the new location closer to the breaker allows for greater inrush current, which trips the breaker. Using an extension cord is not a solution I am comfortable with, although I may try it to confirm this suspicion. The eventual solution may be to change that circuit to a 240v one, and rewire the motor.
    – Mike Galatean
    Commented May 16, 2021 at 15:27

3 Answers 3

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You said down stream: if this receptacle further away from the panel than the original I would suspect a bad connection in the wiring or this receptacle is loose creating a voltage drop.

With reduced voltage the amperage to start the motor will be higher and that could be why the breaker trips, now and did not in the original location.

Yes your motor can draw 3-5x the rated current but only a high end meter could actually see the real peak value even a small reduction in voltage 1% could increase the current the motor draws creating the occasional tripped breaker.

I would be going back and verifying proper pigtails are snug or wraps around screws are snug (you said 20 amp so there should be no back stabs in use but they are a common source on 15 amp circuits however some contractors use stab blocks instead of wire nuts I have only seen these a few times in my area).

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  • No back stabs. The receptacle has only screw terminals. If the receptacle had back stabs I would not use them anyway. The problematic receptacle is the first one on the circuit, connected directly to the breaker with no intervening connections between it and the breaker. Its entire length can be visually inspected. All other receptacles (most notably, including the one where the compressor works) are connected to the receptacle's other terminals.
    – Mike Galatean
    Commented May 16, 2021 at 18:48
  • The next thing I would do is check the receptacle with a withdrawal gauge. In some occupancies they are required to test the withdrawal force or how tight the connection from the receptacle to the plug is a loose connection there will again cause a low voltage and high current connection anc in some cases will create a warmer receptacle. Or change out the receptacle to a new spec grade that only costs a few dollars.
    – Ed Beal
    Commented May 16, 2021 at 23:27
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Since the problematic outlet is closer to the breaker (""previous compressor's location was downstream") the resistance of the wire to the far outlet is helping limit the start-up current. This prevented the tripping in the former location.

Sometimes using an extension cable accomplishes the same benefit, which you could use as a temporary workaround. I don't think it's necessarily a dangerous trick to prevent occasional start-up tripping.

If you provide the details of the current breaker (see comment by ThreePhaseEel), you may get some answers for substitute breakers with a trip curve more suitable for hight start-up currents.

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  • Further away would reduce the voltage further increasing the start up current so that would make less sense to me brown outs or low voltage is why small motors are required to have internal overloads.
    – Ed Beal
    Commented May 16, 2021 at 17:25
  • @EdBeal I think that's correct for brown-outs on machines that do not provide enough back-EMF to keep te voltage afloat, but for minor current induced drops in the supply, the current itself will be limited.
    – P2000
    Commented May 16, 2021 at 18:10
  • @P2000 I am nearly certain using an extension cord would mitigate the problem, it's just that I wouldn't consider it a permanent (or even temporary) solution. It would however confirm my suspicion. Unsurprisingly, the compressor manufacturer provides ample warnings against using extension cords. Can't blame them because who knows what junk consumers are going to use. Since ThreePhaseEel inspired me to investigate the breaker's model, I am going to try a "high magnetic" version of the same thing. I am just as certain it will prove to be the solution I need.
    – Mike Galatean
    Commented May 16, 2021 at 18:49
  • Back emf is caused from a collapsing magnetic field. Low voltage is low voltage causing higher current, the higher voltage the stronger the lines of force and the lower the current that’s how motors work.
    – Ed Beal
    Commented May 16, 2021 at 18:51
  • @MikeGalatean agree 100%
    – P2000
    Commented May 16, 2021 at 19:52
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Thanks to @ThreePhaseEel I dare say the problem is fixed.

Asking the right question - in this case "what make/model are your circuit breakers" is what led to the answer - a standard breaker was inappropriate for such a large inductive load. After replacing it with a "high magnetic" breaker it hasn't tripped once. I did not know such breakers existed. If @ThreePhaseEel hadn't asked I would not have investigated.

Since I have no way to accurately measure current that large the reason the previous breaker did not trip when the compressor was farther away from it can only be assumed: I surmise the motor can draw more current from the shorter run than it could from the longer run; a difference of perhaps twenty feet. The problem was that the breaker wouldn't let it.

Total cost to fix: $10 for the right breaker.

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