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Old 09-20-2011, 07:29 PM   #1
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confirm wiring for 3-prong nema 10-50r plug?


I think the previous owner of my house is trying to kill me.

I'm trying to hardwire in a new four-wire cooktop to wiring from an existing three-wire receptacle (running new four-wire isn't really an option as the existing conduit has no more room and is cast in our floor - a monolithic slab foundation).

I carefully opened up the receptacle and found three 6 gauge wires - all black. One of the wires had a piece of cellophane tape with "RED" on it, another had "BLACK" on it, and one was unlabeled.

Using the typical receptacle pin labels W, X, and Y (W is the unslanted pin, then X and Y are the other pins, clockwise), the "BLACK" was connected to W, the "RED" went to Y, and the unlabeled wire went to X.

Originally from Canada but now living in Florida, my (limited) understanding of these old connectors is that W is the neutral, X is -120 and Y is +120 (by which I mean Y provides current for any dual-voltage needs).

If I had ignored the previous wiring of the receptacle, I would have chugged on and connected black->black, red->red and white+ground->unlabeled.

Am I correct that this labeling is incorrect?

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Old 09-20-2011, 07:53 PM   #2
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confirm wiring for 3-prong nema 10-50r plug?


with a typical NEMA 10 series receptacle, you connect the Hots(black/red etc) to X and Y, and neutral only to W. W most of the time will have a different color screw from X and Y(should be silver, hots brass).

the ground wire if you have it should be connected to the metal box if present, or capped.


if you need the ground wire to the cooktop you'll need a NEMA 14 series receptacle, (NEMA 14-50R)


if done correctly you should get 120V from X or Y to W, and 240V between X and Y

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Old 09-20-2011, 11:50 PM   #3
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confirm wiring for 3-prong nema 10-50r plug?


Considering that the wire colors are all the same and it's unclear if it's wired right, I'd say to ignore the labels and confirm the functions of each wire by testing. Two of the wires should have 120V to ground and 240V between them. They are the hots, "red" and "black" - or just both "black". The third wire should have zero volts (and continuity) to ground, and it's the neutral. You mentioned that this circuit is in conduit. If it's metal conduit that goes all the way back to the panel, then you can use it for the grounding connection. There's no need to run a fourth conductor.

If the receptacle were wired as you described and the original stove's plug were wired normally, the stove would not work properly and it's case would be electrified with 120V.
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Old 09-21-2011, 10:28 PM   #4
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confirm wiring for 3-prong nema 10-50r plug?


Thanks much, took the fluke to the wires and, as you anticipated, the connections in the outlet were just fine. Labeled to kill, but functional.
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