by Dave L.
(Becket MA)

Compressor runs fine plugged into one (GFCI) outlet in the garage. But that wasn’t convenient, because of the outlet location. I installed an outlet off of another circuit, a circuit that’s used for lighting in and outside of the garage.

When the compressor is plugged in there, it hums but generally won’t start up. It’s essentially the same problem as when I use a long extension cord, that is to say it’s not getting enough current, I think.

My question is how can this be on a circuit with no extension cord? Wouldn’t it pull as much current as it needs? Wouldn’t it throw the breaker if it couldn’t get enough? Instead, it just doesn’t seem to be getting enough juice. How can that be?

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Compressor hums
by: Bill

If the start capacitor is degrading, and there is slightly less current to one socket versus another, that could explain in.

Does the problem occur in the new location if you try and start the compressor with the tank empty of pressure? Please advise.

Also, see how to check your capacitors here and run a check, will you? What did you find?

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Acts the same when trying to start on empty tank
by: Anonymous

Yes — with empty tank it acts the same.

I wonder if the fact that the power is switched via a three-way configuration (two light bulbs and the compressor via two three-way light switches) is the problem. Would such switches limit the available current? I doubt the bulbs matter, since they’re low-wattage CFL’s.

To be clear: I energize the circuit by flipping one of the three-way switches, THEN I turn on the compressor, so it’s not competing for “start-up juice” with the bulbs’ ballasts.

Can light switches passively limit current throughput?

I wouldn’t have wired the outlet that way, except that there’s no full-time power at the three-way switch box that I tapped into for the new outlet.

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Hmmmmm (pun intended)
by: Doug in s.d.ca

You don’t ID your compressor, so that complicates the answer, but I can say this:

Light circuits are usually wired for 15Amps and use 14 gauge wire. You should find (or install a breaker for) a 20Amp circuit, with 12 gauge wire.