I am trying to wire for a 4 prong plug into a an old receptacle pre 1972 with only red and black wires with metal conduit into metal box.
It has been used since 1964 hard wired 220B with metal conduit all the way to the box.
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If all you have is a red and black with the conduit being used as the ground you CANNOT under any circumstances use this as-is for a 4-wire receptacle. Even using it as a dryer receptacle would be a MAJOR code violation and an unsafe installation. You'll need to pull in more wire to do it correctly and safely. Is there conduit the whole way to the panel? – Speedy Petey Aug 28 '15 at 0:23
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What size is the conduit? – WarLoki Aug 28 '15 at 0:32
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1@ChiefTwoPencils, if you have a typical electric dryer in the US it is NOT a "220V" circuit, it is a 120/240V appliance that requires a neutral. Using the conduit as a current carrying neutral is about as bad as it gets. Just because it has worked does NOT mean it's safe. – Speedy Petey Aug 28 '15 at 2:28
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@SpeedyPetey, I suppose mine's not typical then. Mine is exactly as I stated above. In fact, I actually don't recall ever having a dryer 120/. I don't disagree with the fact using conduit for a grounded conductor is horrible; my comment was based on a different definition or understanding of "typical". My concern is accusations or assumptions of "code violations". – ChiefTwoPencils Aug 28 '15 at 2:37
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1Just a note to anyone following this. It seems ChiefTwoPencils has an odd electric dryer that does not require a neutral. This is different than the original question where Tom is asking about a "4-wire" receptacle, which DOES require a neutral conductor. – Speedy Petey Aug 28 '15 at 11:07
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Warning: You'll be working around live electrical wires during this procedure. If you don't feel comfortable doing so, please contact a local licensed Electrician.
Okay, going off of what you have, you need at least a 3/4in conduit back to the panel. You are going to need to pull a white and a green from the panel to the receptacle box and install a ground stinger in the metal box to bring it up to code. The wire will need to be sized to the amp draw. If it is not a cost thing I would pull all new wire in using the old wire to pull it in.
You can turn of the main breaker to the panel, but the feeders from the meter will be hot.
Warning: You'll be working around live electrical wires during this procedure. If you don't feel comfortable doing so, please contact a local licensed Electrician.
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1How did you figure the conduit size? I didn't see any mention of amperage or conductor size, so I'm not sure what you're basing the size on. – Tester101 Aug 28 '15 at 1:35
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Was the first warning not enough? BTW, trying to find out why OP would need to pull a grounding (the green one) conductor back to the panel; didn't they say there's metal conduit feeding the box? – ChiefTwoPencils Aug 28 '15 at 2:14
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2@ChiefTwoPencils, because a wire is a MUCH better equipment grounding conductor than a 50 year old conduit run. – Speedy Petey Aug 28 '15 at 2:28
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@SpeedyPetey, I'd say it depends on the install; given a good electrician I'm indifferent to it. It's rare to find a new construction job that doesn't spec a separate conductor but if they don't the NEC has no problem with it. But more importantly I'm concerned with the assertion a grounding conductor is required to "bring it up to code"; that part is inaccurate. For the sake of the content I think it's more appropriate to give the factual code specification and interject opinions as side notes. Just MHO. – ChiefTwoPencils Aug 28 '15 at 2:44
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@SpeedyPetey, and this is now assuming since you didn't say something like - "because since 2015 it's required" - that things haven't changed :). – ChiefTwoPencils Aug 28 '15 at 2:47
