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I got a used air compressor rated at 15A (120V). When plugging it in to either of two 15A circuits I have in my basement, it will work for 8-10 minutes, then the breaker will trip. You'd think the most common time to trip might be when the compressor motor cuts in, but it actually trips mid-way through the 2nd or 3rd cycle. To my ear, it sounds absolutely normal and then just quits when the breaker trips.

There is nothing else on the circuits in question. The breakers are clearly 15A. One of the circuits is only about 10'/3m (wire distance) from the panel. My clamp multimeter is terrible (updates very slowly), so I can't see transients very well, but the compressor does appear to draw around 14.5A when the motor is running. The breakers were installed before I bought this house, and have seen minimal use (previous occupant was a renter who had a small fridge plugged in to one circuit, and probably never used the other. The guy sure as hell never used a vacuum cleaner, anyway). Finally, I am not doing anything dumb like using an extension cord--the 6'/2m cord and NEMA 5-15 plug are clearly factory and in good condition. I've cleaned both the outlet and compressor plug, and ensured they make a good connection.

Do I need to run a new circuit and 20A breaker? It'd only be 10'/3m of 12AWG and there's space in the panel. Or do I need to look for a fault in the compressor? Better yet, is this a good time to convince my wife I need a better clamp meter?

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  • Can you shoot a photo of the nameplate on the motor?
    – Harper - Reinstate Monica
    Aug 31, 2019 at 22:04
  • @Harper The motor itself is covered by a plastic shroud that seems impossible to remove without damaging it. The specs I gave in the OP are from the plate on the tank. The compressor itself is a Craftsman model 919.721920. Is there something else you need to know? I may be able to find out.
    – type_outcast
    Sep 1, 2019 at 12:08

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I think you need a bit of “headroom” so a new circuit with the 20A breaker should do it.

For both breakers to run the same amount of time seems to suggest both are functioning correctly. The regs I usually consult gives times for breakers to act if the load is close to the rated value so it sounds like those two 15A breakers are just being run close to the edge and tripping due to an extended time.

I would definitely test it with a 20A breaker - safely though...

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  • Alright, I was wondering if running so close to tolerance was an issue, and it sounds like it may well be. Thanks. Good thought to test it first, although I'll probably just upgrade one of the circuits anyway, as that has been the plan all along.
    – type_outcast
    Sep 1, 2019 at 12:32
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14.5 amps is a little high for an appliance that shipped with a NEMA 5-15 plug. I would expect less than 12. Is this item a cheap Chinese, or did the previous owner hack on this plug, is it actually sold retail in the US or Canada and has a UL, CSA, ETL etc. endorsement?

If it is the latter, then look at the capacitor, especially if the unit is >5 years old.

A delayed trip on a slight overload (or near overload) is perfectly normal. You're not supposed to run breakers at limit.

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    Retail Sears compressor (Canada) Craftsman model 919.721920. The cord and plug are factory and in good condition. And it definitely draws 14.3 ~ 14.8A. I'll see if I can get at the cap easily... that's an easy test.
    – type_outcast
    Sep 1, 2019 at 12:11
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I've seen this occur before. Breakers can do this when you put them "on edge" for extended periods of time. An Amp probe would definitely let you know what's going on... But I know you don't have one, know what it is or how to use it.

If your wiring is #14 going to this air compressor, you can not put any larger breaker on this circuit than a 15 amp. Otherwise, youre heating the wire due to the amount of current that's traveling through it, as compared to what the equipment needs to run properly and safely.

If by chance your wiring from the breaker to this air compressor is #12, you can replace the 15 amp with a 20 amp...

The other thing that this could be is there's a problem at the motor connection junction box, on the motor itself, informally known as the " pecker head ". Open it up ( with the power off ) and take a look at the connections and also see if it looks or smells " charred ".

Good luck sir👍

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  • I did measure the draw with my cheapo clamp meter and it averaged very close to 14.5A, never went over 14.8. And it doesn't even trip when cutting in, just mid-cycle. The existing circuits are 14AWG, but I have a spool of #12, and like I mentioned, it's about 10' wire length to the breaker box, so not a difficult task to upgrade a circuit, if it makes sense to. Good reminder to check the motor wiring itself, though. I've so far avoided that since the motor wiring will be a little difficult to access on this unit without destroying the plastic shroud. Thanks!
    – type_outcast
    Sep 1, 2019 at 12:22
  • Those cheapo ones give you an estimate, but they're not exactly accurate. You're compressor is stressing that breaker out and making it trip. Another possible solution is a threefold thing. 1) Look on the nameplate ot the motor... If it is running on 120 volts, and the name plate of the motor says it can be wired for 240 volts AND 2) You know this circuit only feeds this air compressor, then rewire the motor and 3) You've got an extra space in your panel for a 240 / 2 pole breaker, then do 1)-2) & 3) and your air compressor won't trip the breaker anymore. Hire someone if you can't do it.
    – Retired Electrician
    Sep 1, 2019 at 13:37

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