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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?

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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. Sep 8 '20 at 14:00
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    Does this answer your question? Unexpected voltage with switch turned off Sep 8 '20 at 14:17
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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 8 '20 at 14:52
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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 :-) Sep 8 '20 at 15:11
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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 8 '20 at 22:38
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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

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