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10 gauge wire on 20 amp circuit

5 sandpiper's picture

Have a friend who wired a 20 amp circuit in his garage for a midsize air compressor, 8.5 amp. Complained that the compressor sometimes trips the breaker. He's got a run of 20 feet max on 10-2 with a 20amp SquareD breaker and a 20 amp duplex
receptacle. He's a retired phone lineman-assuming he wired it right. Is the 10 gauge too big for that short run ?

iz dis a trick question?  (post #205603, reply #1 of 8)

iz dis a trick question?  i9gger the better except for cost and ease of install. 

Yeah, with a wire that big (post #205603, reply #2 of 8)

Yeah, with a wire that big the electrons don't have to work so hard and get sluggish.

(It most likely trips the breaker because the compressor is hard starting and draws 20-30 amps while starting.  This may be due to a defective unloading valve.)


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Small compressor (post #205603, reply #3 of 8)

If a compressor that draws 8.5 full load amps is tripping a 20 amp breaker, I would definitely be looking at the compressor, not the wiring. Have him try it on some other household outlets, he may get lucky and find that it works on them and that the new one in the garage is fussy and can be replaced (maybe just move them around in the panel).

The size of wire is definitely not the problem... (post #205603, reply #4 of 8)

Heavier wire is better, though it can be harder to work with...  The problem most likely is either with the compressor or the breaker.  I would start by replacing the breaker with another 20 amp breaker. It doesn't happen very often, but once in a while you get a breaker that trips easier than it should.  This is really cheap and simple. Then I would look at the compressor and see of there is something wrong.  Try the compressor on another circuit and ee if it blows that breaker as well. 

I am assuming that this is a regular breaker, not a GFCI or AFCI breaker.... 

If the FLA is 8.5a he can (post #205603, reply #5 of 8)

If the FLA is 8.5a he can legally bump the breaker up to 25a but it does sound like there is something wrong with the compressor

Greg

Of course, starting amps of (post #205603, reply #6 of 8)

Of course, starting amps of an induction motor can be 3-10 times the full load amps (though the design of units like this usually keeps that multiplier on the low end).  It's typical that the motor will draw, say, 25-30 amps for a second or so -- the breaker should tolerate that.  If the motor is hard starting, though, the 25 amps could be drawn for 5-10 seconds, long enough to trip the breaker.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

10-2.... (post #205603, reply #7 of 8)

with a 20 A breaker should work just fine.

You said it "sometimes" trips the 20 A breaker. Compressor in garage.......

Would that "sometimes" be when the weather is cold? Or, at least more frequently when it is cold?Don't know where you are located, may not be a factor at all.  "Cold" = less than 30.

Clamp on ammeter during start-up might answer all of your questions.

Jim

Never underestimate the value of a sharp pencil or good light.

Thanks for the help (post #205603, reply #8 of 8)

Thanks for all the responses, talked to him again, turns out he had to rewire the compressor after he bought it (new) because wiring was backward. Not sure about that or his knowledge. The compressor is a Craftsman, never quite trusted Craftsman electric tools, some are great, others not great, some are junk.