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An animal shelter (in USA) where I volunteer has a new 240V Maytag electric dryer that will not operate (when "on" is pressed, the electronic panel gives a single brief flash of all LED's, that's all). The Maytag service tech says the power is defective because he measures 163V from hot leg to ground; he expects 120V (as one finds in residential split-phase 240V) and says the problem is in the power wiring, not the machine. We have no other power source to try.

The dryer has a three terminal panel with two hots and a ground (no independent neutral), to which a NEMA 10-30 cord/plug is attached and correctly wired. It is plugged into a NEMA 10-30 receptacle, also correctly wired. The voltages at the receptacle and inside the dryer are 240 hot to hot, and 163 from either hot to ground. The building is commercial and has three-phase service.

I think all this is as it should be, given that the building has 3-phase service and no neutral is employed, and the problem is in the dryer. Am I correct, or have I missed something? (I'm not looking for an explanation of three-phase delta/Y or center-tapped transformers, just whether the power wiring to the receptacle is correct and the voltages normal.)

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    A NEMA 10-30 should not be installed in any commercial building at this point in time. A NEMA 10-30 does not have any ground. It has two hots and a neutral. A NEMA6-30 has two hots and a ground, but most dryers in North America won't work on one. A NEMA 14-30 is what you need, usually two hots, neutral and ground. unless you actually have a full 240V dryer, in whcih case you need a NEMA6-30. Having a NEMA10-30 cord attached as donated suggests that a NEMA 6-30 is not the correct cord. Edit to provide a specific model number, but what you need is an electrician and the correct outlet.
    – Ecnerwal
    Feb 3 at 15:43
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    You really want an electrician than the lonely maytag repair man(yes I'm that old).
    – crip659
    Feb 3 at 15:50
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    Can you measure the hot-to-neutral voltage at all receptacles, and can you get us a photo of the electric meter, or do you know what sort of three-phase service you are receiving from your utility? Feb 3 at 18:19
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    What is the model number of this Maytag dryer? Feb 3 at 19:21
  • Picture of your electric panel? Feb 4 at 0:44

2 Answers 2

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The assertion that the NEMA10-30 is correctly wired is false.

Depending on the type of 3-phase, the hots should be 240 or 208V apart, and the hot to neutral (there is NO ground, which is why it should not be there at all) should be 120V in either case, for a "correctly wired" NEMA10-30.

For any dryer that used to work on a 10-30 cord, the correct receptacle is a NEMA14-30 with ground, and a NEMA14-30 cord, and following the instructions on the dryer or in its manual for correctly separating the ground and neutral on the dryer itself when attaching the NEMA14-30 cord.

For a very rare in North America full 240V dryer (motor and controls included) the correct receptacle is a NEMA6-30.

The NEMA10-30 was banned in the 1996 code cycle, because the things kept killing people, mostly children. While they are still sold, that's only to repair grandfathered (pre-existing) with like-for-like, and it's not a good idea to keep doing that. I'm uncertain if they were banned earlier in commercial occupancies (which are often held to a somewhat higher standard in code) rather than residences.

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  • Even worse, measuring 163V from a hot to neutral may indicate a "lost neutral" at the socket, which is exactly the scenario in which those old sockets are so dangerous! This is electriying the chassis of the dryer when it's turned on. If they're in 3-phase "Wye" 120/208V, it's fine to connect that. Feb 4 at 2:24
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The service tech is correct: a standard USA "240V" residential electric dryer should not be connected to an electric service that has hot legs more than 120V from neutral or ground. 163V is right out.

163V hot-to-neutral or ground is not a voltage you should see in any standard three-phase setup that is 120V or 240V phase-to-phase. Depending on whether the setup is open delta/high-leg or wye, you should have 240V or 208V leg-to-leg but always 120V to neutral or ground (except 208V to neutral or ground for the high leg in a high-leg setup). You should never see 163V hot-to-neutral or ground.

This means you probably have a wiring problem in the building that needs to be addressed by a licensed electrician (as opposed to having a service that is fundamentally incompatible). While they're at it, the obsolete, deadly 10-series receptacle (hot/hot/neutral) should be upgraded to the modern 14-series standard (hot/hot/neutral/ground).

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  • (or anyone), Do you know what specific wiring error could account for 163V hot-to-neutral on a 240V hot-to-hot receptacle? My research confirms the your description of what you should get in correctly wired 3-phase source receptacles, but I can find nothing that references wiring errors for 163V. I played around with SQRT(3) but nothing clicks.
    – MadMonty
    Feb 19 at 5:00

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