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I have a separate garage that was built in 2001. It has a 100 amp sub panel that is fed from the main panel in the garage of my house. The panels are about 50' apart and the wire is in conduit. The sub was bonded between the ground and neutral bars but I have separated them and there is also a separate ground rod for it. There are only 3 wires between the two panels and not a 4th grounding wire.

I have a low voltage PLT license that I use for the trade I am in and have done quite a bit of high voltage wiring in houses that I have lived in.

A couple of weeks ago I was going to use a craftsman skill saw to cut a board and when I tried to start it, the saw didn't start just hummed like the brushes were bad. The strangest part is that the light on the garage door opener flashed on and off when I attempted to start the saw. Now all 7 120v circuits in my garage don't work. I have a 220v compressor that does work but none of the 120v circuits do including ceiling lights.

I have a digital multi meter and have checked voltage at the main breakers. It is consistent at 120v at both breakers. As I have investigated the situation I have found that 3 of the circuits when energized will cause the voltage to drop from 120v to 110v and the other leg goes up to 130v. Even after I have disconnected the wiring on those circuits I cannot get power to the other outlets or lights. With the power off, I have also checked with an ohm meter between the neutral and hot wire at the breakers and it shows open on all circuits. When I have the light switches on and plug in my irrigation controller the transformer hums loudly and the lights do seem to go on.

What am I missing? Where do I check next?

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  • Is the conduit metal or PVC? Jan 31, 2016 at 1:21
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    If the conduit is all-metal all the way, that is a large ground path that should be sufficient to trip any load, provided it's working right. Inspect the run to make sure none of the joints have separated and loosen-retighten each one in case of any oxide build-up. However, this has nothing to do with your problem, which sounds like your neutral wire getting loose or oxidized. Do not use the circuit for anything until you've given all 3 conductors a good once-over. Jan 31, 2016 at 3:59

2 Answers 2

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All of your symptoms sound like a faulty Neutral connection. Faulty neutral causes L1 to affect L2, and has no affect on 220 equipment just like you described.

Fix that quick. A faulty neutral can damage your equipment, and because you installed a second ground, it can harm YOU as well. Check that the neutral wire is firmly connected, and using Noalox:

noalox

ALL aluminum connections for that matter should be protected from corrosion and thermal expansion with Noalox.

You are lucky that you have a conduit! You have an EASY way to remove the entire length of the neutral conductor to inspect it for damage. You should also take advantage of that conduit to pull a ground conductor through it and use that to ground your sub panel VIA your main panel. Do not use a second ground rod at the sub. It is not worth the risk in your case.

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  • The garage is required to have it's own ground rod, although it's also required to be connected back to the main. Good catch on the neutral.
    – TFK
    Jan 31, 2016 at 1:54
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    I agree, bad neutral, given that the 2 legs total 240v but are unequal (130/110) on a lightly loaded sub-panel. I wouldn't bother pulling the wire out of the conduit until I'd given all the ends an inspection, cleanup and re-seat. Jan 31, 2016 at 4:03
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Sounds like a short or ground fault somewhere.

Since there is no ground wire connecting the sub to the main, any sort of fault will NOT trip a breaker. I'd suggest being cautious, but try testing voltage between your ground bar and neutral. If there is a voltage difference here, something isn't right.

You might try reconnecting the ground bar to the neutral. Right now it's completely useless and will not project anything. The reason you disconnect ground and neutral at a sub is so that there aren't two paths back to the main (on the ground wire and neutral) - this would put them in parallel and cause overvoltage to run on the ground and the neutral. Since you only have a single wire, I believe tying them back together will provide safety, but you truly need a ground wire ran back to the main.

I'm not sure that this is the issue, but currently you are unprotected and could get zapped yourself.

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  • He may have an EGC running sub-to-main here after all -- we'll need to know more about his setup to be sure. Jan 31, 2016 at 1:22
  • @ThreePhaseEel He said that it's 3 wires (L1, L2, neutral) and there isn't a ground.
    – TFK
    Jan 31, 2016 at 1:25
  • a metal conduit is-a-kind-of EGC. Jan 31, 2016 at 1:32
  • @ThreePhaseEel I would doubt it being metal conduit, but you are correct there.
    – TFK
    Jan 31, 2016 at 1:56
  • I would recommend pulling a conductor instead of relying on conduit. Jan 31, 2016 at 3:38

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