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TL;DR

I've got high resistance between the ground pin on two different outlets near each other, how should I diagnose and fix this?

Background

In my garage, I have two outlets right next to each other, a 240V NEMA 14-50 and a typical 120V NEMA 5-15. The 240V is a pretty new run. I had someone do it about 5 years ago. The 120V outlet is part of a circuit for the whole garage. It was here when we moved in. It was likely done by a previous owner in the 1970s or 80s.

This last weekend, I ran a ground wire from a metal network rack to an existing bare copper ground wire that is clamped to pipes near my water heater. The manual says the rack should be grounded and has a stud for this. When I was doing this, I took some measurements with my multimeter and I got some confusing results.

240V (NEMA 14-50) outlet is as expected

With power on I measured ~249VAC between L1 & L2, ~124V between N and either L1/L2, and a few millivolts between G and N. With power off, Resistance between G and N is ~0.1Ω. Resistance from G on this outlet to the water heater pipe is the same. This is new work, so I guess N and G are bonded at the panel, and that is connected to the same ground system as the pipes.

120V (NEMA 5-15) is confusing me

The 120V outlet is confusing me. With power on, I measure ~124VAC between L and N. 22V between G and N and 108V between G and L. With the power off, this outlet has about 1MΩ (megaohm!) between G and either L or N. N to G is s 1MΩ. Just under 1MΩ from this outlet's G to the pipe.

With both breakers off, I got just under 1MΩ between G on both outlets. The neutrals between the outlets appear well connected, 0.8Ω which I think is expected. I've read that at given panel all neutrals share a bus bar and then each circuit's L runs through the breaker.

Looking at the wiring in the box, the 120V is in the middle of the run and has the expected L, N, and bare copper in the cables entering and leaving the box behind the outlet. 240V is on it's own circuit and has the expected three large conductors and a bare copper ground.

What's going on? What to do?

Any idea what is going on? The 120V circuit does not appear to have N and G bonded at the panel. here does appear to be a connection from the 120V circuit's G to the "house ground", but it is ~1MΩ.

This feels like a thing that should be fixed, right? Would it be safe to connect a ground wire between the two outlets inside the wall? They're in the same stud cavity, so I don't think it would be hard for me to do. If this isn't right can/should I connect those grounds in some other way?

kbyrd
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3 Answers3

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As you probably already know, the "ghost" voltage on the 120 VAC receptacle putative ground is picked up capacitively from proximity to L. (Consider a conductor halfway between plates of a capacitor; one expects half the AC between it and each side.) Your DC measurement of ~1 MΩ to true ground is further evidence of a serious safety issue, particularly in a garage, where one might step on bare concrete or rest against a pipe or other grounded object.

The 120 VAC ground wire should be grounded at the mains entrance panel. Check at the main breaker panel that it is indeed grounded, and if not, an electrician should make repairs!

However, most likely, the 120 VAC line to the garage makes a few stops at junction boxes, perhaps with electrical outlets, and the ground wire was not connected inside one of those boxes. Perhaps it was inserted in a "stab" connector and has loosened, or broken inside a wire nut, or was wound around a non-grounded screw. That can easily be fixed:

  • Confirm that the main breaker panel is indeed grounded - check resistance to a metal water pipe or gas line.

  • Assuming it is (since the 240 VAC ground is good), shut the breaker for the 120 VAC to the garage, and tag it, lest it be turned on.

  • Check each outlet on that circuit between the breaker and the garage, and others in the garage.

    • Confirm power is off because the breaker was shut.
    • Unscrew and pull out the outlet and check continuity in the ground wire(s) entering and leaving. You can also check resistance referencing a (long) wire to a pipe.

  • Hopefully, you'll find the break, and repairs might be as simple as using a ground screw, rather than stab connector, or putting on a new wire nut.

DrMoishe Pippik
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Follow the wire back towards the panel until you find a place where the resistance between N and G is low, like less than 10 ohms on your meter. It should be much less than 10 ohms but many meters don't real low resistances accurately)

When you find that place check that the ground connections are good by pulling on them etc. Check the connections both ends of the wire that runs from "good" to "bad".

Jasen
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Use lights, radios, fans, and a buddy to map every light and outlet in your house. One room at a time, plug stuff in to every outlet and turn off breakers to see what's what.

Then, knowing all the switches and outlets related to the bad circuit, leave it off and measure N-G resistance everywhere. For light switches without neutral you'll need to use a bit of wire to bring neutral from the nearest outlet on the circuit.

Then, knowing all the junction boxes on the circuit and which ones are good or bad, you can vaguely map them from the breaker panel outwards, but don't rely on that too much, and narrow the problem down to hopefully just 3 or 4 junction boxes. And finally, inspect those for problems.

jay613
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