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I just moved into a new house where there are two 240 volt 10-30R outlets in the garage, each with a 30-amp breaker and 10 gauge wire. Voltage across the hot terminals is 247 and, of course, approximately 124 volts from each hot terminal to neutral. So far, so good. However, on both outlets, the voltage across the metal face plate to either of the hot terminals is 14-18 volts. Why the heck is that? There is a bare ground wire in each of the boxes, but there is no ground terminal on the 10-30R receptacle, so the ground wires are just pushed to the back of the boxes. Where the heck is that voltage on the face plates coming from? And is it a safety problem?

MikeB
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    Is the box metal or plastic? If plastic and the ground isn't connected to the metal plate of the outlet, what you are reading is leaking voltage. Digital volt meters are extremely sensitive and even the tiniest leak can register some voltage. – George Anderson Sep 08 '20 at 14:00
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    Does this answer your question? Unexpected voltage with switch turned off – Daniel Griscom Sep 08 '20 at 14:17
  • Thanks for your response. The boxes are plastic. As a former electronics tech, I would have expected to see "noise" in the range of a volt or less, but not 14-18. I just measured the voltage with another DVM, and it's around 20 volts. Wish I still had an analog multimeter! I assume it's OK though, since there is no path for current. Edit: Is that leakage voltage actually through the plastic receptacle body? – MikeB Sep 08 '20 at 14:40
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    You have grounds in the boxes. So. Buy two 14-30Rs, remove the 10-30Rs, connect the 14-30Rs, Smash the 10-30Rs with a hammer so they don't get re-used, place in trash or recycling. – Ecnerwal Sep 08 '20 at 14:52
  • @MikeB You're probably picking it up from the neutral in the outlet to the metal yoke on the outlet which in turn is screwed to the metal face plate... – JACK Sep 08 '20 at 15:09
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    @Ecnerwal Make that an answer. Quote one of Harper's famous "NEMA 10 is evil" answers. Unless Harper beats you to it :-) – manassehkatz-Moving 2 Codidact Sep 08 '20 at 15:11
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    @Ecnerwal Bordering on a RANT.. lol – JACK Sep 08 '20 at 15:12
  • MikeB, the leak is probably being dissipated by the plastic body of the box. It may also be an "induced" voltage just due to the proximity to the hots. I don't know where else you'd get such a low voltage reading. If you want to continue testing, try connecting a wire between a known good ground (like a metal water pipe) to the metal plate. If you have a meter with an "amp clamp" use that and see if you get any current. If you do, you have a problem. But I sincerely doubt it. It's current (amps) that kills, not voltage. – George Anderson Sep 08 '20 at 15:40
  • Actually "NEMA 10 is evil" is a Googlenope. But without quotes the top result includes one of Harper's famous answers: https://diy.stackexchange.com/questions/144475/outlet-looks-like-a-nema-10-50-but-is-only-stamped-at-30 – manassehkatz-Moving 2 Codidact Sep 08 '20 at 15:45
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    Answer made as requested. CW becasue, well, it's the community, ain't it? Harper can feel free to change the link if there's a better one... – Ecnerwal Sep 08 '20 at 16:02
  • Thanks for all the comments. I do have a current clamp and a ground at a 120v outlet nearby, so will try Jack's suggestion. As for the 10-40R outlet, the power cords for both my devices (MIG welder and air compressor) have only 3 wires, so I would have to use a 4-wire cord like a dryer uses to get protection out to the devices. – MikeB Sep 08 '20 at 19:28
  • Measured zero current between metal face plate and ground at a nearby 120v outlet using a current clamp. Followed up with a Fluke DVM on AC 10A, and then MA range. Still nothing. The voltage is probably just induced, but with zero current potential as mentioned above. – MikeB Sep 08 '20 at 21:36
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    Your "three-wire devices" (not being clothes dryers) are almost certainly set up for hot, hot, ground. So use an outlet that supports that (an extra unused neutral is not nearly the problem that using neutral as ground is.) L6-30R and L6-30P are a nice way to do it without the neutral, if having 4 wires on the receptacle bothers you. Cap the neutral and connect the ground. – Ecnerwal Sep 08 '20 at 22:38
  • Can you post photos of the insides of the boxes involved please? – ThreePhaseEel Sep 09 '20 at 00:00

1 Answers1

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You have grounds in the boxes.

So.

  • Buy two 14-30Rs
  • remove the 10-30Rs
  • connect the 14-30Rs
  • Smash the 10-30Rs with a hammer so they don't get re-used
  • place in trash or recycling.

Per comment about "3-wire compressor & welder": alternatively L6-30R Plus L6-30P for the cords, and cap the neutral while connecting the ground.

Outlet looks like a NEMA 10-50 but is only stamped at 30

Ecnerwal
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