120vac circuits go off when 240vac compressor is off

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Old 08-28-19, 03:10 PM
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120vac circuits go off when 240vac compressor is off

Most, but not all, of the 120 vac 15 & 20 amp circuits in the garage go off when the 240 vac compressor that is not working is turned off, unplugged or the 20 amp circuit breaker is turned off. Turn on the compressor, the motor hums, does not spin, and all circuits work. Manually turned the compressor flywheel to rotate the motor in case motor was stuck in a bad spot. No luck.

My guess is a short in the compressor motor is causing the problem but why should an unplugged compressor cause a problem?

I wired the garage, inspected by a master electrician, 10 to 15 years ago. Everything has been working fine all these years.

Garage sub panel is fed from 50 amp circuit breaker in the house. Garage is protected by 40 amp circuit breaker in the sub panel and the compressor is protected by a 20 amp circuit breaker.

Can answer any questions if needed.
 
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Old 08-28-19, 03:31 PM
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You have lost leg of your service. One of the 120 supply lines is dead. Turning on the 240 compressor is back feeding the good leg onto the bad side. The compressor won't run because it not getting the full 240 volts.
 
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Old 08-28-19, 03:33 PM
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Check in panel that each feeder leg is providing 120V to neutral and also that you have 240V across the two legs.
 
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Old 08-28-19, 05:36 PM
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OK, thanks. My guess is 1/2 of the 20 amp circuit breaker is still open when engaged.

Will do that and let you know.
 
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Old 08-28-19, 06:23 PM
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OK, just measured the voltage, 120 on one side of the 20 amp circuit breaker and nothing on the other, just as predicted.
My guess is a faulty circuit breaker. will replace it in the morning and post results here
 
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Old 08-28-19, 07:32 PM
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If this is the 20 amp breaker to the compressor that is not your problem. Measure the voltages feeding the panel. One of the panel feeds is out.
 
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Old 08-29-19, 06:20 AM
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The two most common candidates would be a bad feeder breaker in the main panel or a break in the underground cable.
 
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Old 08-29-19, 10:47 AM
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OK, 240 v at the house panel feed, 240 v at the circuit breaker that feeds the garage sub-panel but only 120 v at the sub-panel buss bars. That must mean there is a break in one line that feeds the sub-panel but tell me, how do you find a break in 90 - 100 feet of 6 gauge wire?

Would finding which wire is open with a continuity test, feed power to only that wire, and use a AC sensor to see where no AC is detected work?

Wire is not underground. It runs from the house, up inside an interior wall and across the inside of the ceiling of an attached mudroom to the attached garage.
 
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Old 08-29-19, 11:34 AM
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Hi, the sub panel in the garage has a 40 Amp breaker there that appears to be back fed from the house panel, correct? Now you say you are only getting 120 VAC to the buss in the sub panel, correct? sounds like the breaker in the garage is bad.
Geo
 
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Old 08-29-19, 01:41 PM
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It is not likely with your setup that the wire has just broken in the middle of the cable.
You either have a loose connection somewhere or some rodent chewed it. I would start with the connections at both ends and then look for any possible junction boxes.

You say only 120 at the bus bars. What about at the connection point where cable comes into the box? If it comes into a breaker, the breaker itself or the breaker connection to the bus bar could be faulty.
Try turning the breaker off and on. Maybe one side tripped for some reason and didn't take the other side.
 
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Old 08-29-19, 04:46 PM
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Thanks. I do all my own household electrical work but am not an electrician and don't know some of the jargon.
I said the sub-panel had 120 at the buss bar to distinguish having 120 at the sub-panel main breaker. That could indicate a faulty circuit breaker but I removed and tested all breakers in question and all are closed/open when they should be.

I thought of mice chewing the wire but can they actually chew through a 6 gauge wire? My experience is mice chew the insulation. If they chew through the insulation on two wires a short can happen but an open would mean they chewed through the 6 gauge wire. If that is the case I don't need a mouse trap, a big game rifle would be more appropriate.

At any rate a local electrician is coming over in the morning. Will post what he finds.
 
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Old 08-29-19, 05:20 PM
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Testing a breaker with an ohmmeter does not always work. I had a double pole last year that tested perfect with an ohmmeter yet I could only get 120 volts out of it. After replacing it I busted it open to find one set of contact burnt right off. It contacted enough for the milliamps of the ohmmeter but not for the amps of 120 volts.
 
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Old 08-31-19, 08:23 AM
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Problem Solved! And thank you all for your help. The problem was a circuit breaker but not in either panel.

I didn't mention I have an emergency generator manual transfer switch I installed years ago connected to the main panel because I didn't think it was a factor. Turns out it was THE factor.

A breaker on one leg of one 240 circuit broke. The 240 I measured at the breaker that feeds the sub-panel actually feeds the transfer switch first. The electrician ($75) found the problem when he measured the output from the switch. I ordered two push to reset breakers and will install them when they come in.

I know if I had mentioned the transfer switch you guys would have found the problem right away.
 
 

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