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Suppose someone has a 240V 50A plug at a rental house. It is wired to a 10-50 receptical. My understanding is that this is a deprecated standard, and my guess is that it is 6-50 wiring that was put into a 10-50 receptical.

Suppose they want to change the receptical to 6-50.

There are three wires which I will call (G)reen, (R)ed, and (B)lack. The heavy duty wire is in a metal sheath that is earthed (E)

Using a multimeter I detect: E-G: 0 E-R: ~120V E-B: ~120V G-R: ~120V G-B: ~120V R-B: ~240V

My read of this is that I can simply change the receptical to 6-50.

  1. Is there any specific risk to this? It's a relatively modern house.
  2. Are there any other tests that I can perform to determine whether this is 10-50 wiring or 6-50 wiring?
  3. Does this matter? The difference seems rather subtle.

Aside: It's not particularly relevant, but the wire I referred to as red was actually white. It's clearly live, so this is the "typical" I've-only-got-three-wires-and-don't-want-to-cap-a-white-wire-so-I-declare-white-equals-red. I have put black tape on it to make this clear.

I haven't opened the distribution box to look at the wiring, but am hoping that the multimeter can give me enough information.

As requested, here is a photo of the junction box internals, showing the white wire (with a bit of black tape on it), black wire (these are both hot and 180 degrees out of phase), and green wire (it is 0V from the earth and electrically connected to it presumably at the distribution board).

Junction Box Internals

Further thought: According to this discussion board which agrees with my other reading, 10-50 is hot-hot-neutral and 6-50 is hot-hot-earth. Since my green wire is in continuity with the box, my wires are hot-hot-earth.

  • Is this terminating in your main panel? Or a sub-panel? – Tyson May 30 '18 at 0:26
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    Main panel. It's a direct connection through the garage. – Dr Xorile May 30 '18 at 0:27
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    Can you post a photo of the inside of the junction box, and of the armor on said heavy duty cable? – ThreePhaseEel May 30 '18 at 0:41
  • What color are the wires natively? You discussed how the "red" is actually white. What color is the "green"? – Harper May 30 '18 at 0:45
  • I have added the photo. The green is green and is at 0V to earth. The black and white are both hot. – Dr Xorile May 30 '18 at 2:40
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Assuming that cable is in fact 6 AWG, that cable was always correct for a NEMA 6-50 receptacle. It was always wrong for a NEMA 10-50.

It really matters. Some people have seen the inside of a panel and see where neutrals and grounds land on the same bus. That's not quite what's happening. Neutral and ground serve very different purposes. One returns current normally, the other one is a safety net that saves lives. They are supposed to be kept totally separate, with equipotential bonding created between neutral and ground in one place - the main panel. However in a simple residence, there is only one panel, so they allow the shortcut of simply putting them all on one bus. That's all.

The only wire relabeling that is permitted is a white wire in a cable can be relabeled to be a hot, so what you have done is correct. This is the normal way of powering NEMA 6 circuits using common multiconductor cable.

Being a modern house, if this was a range circuit they would have already used /3 cable and your range receptacle would already be a NEMA 14-50. So I gather this is in a utility space and is intended for welders, kilns, EV chargers and the like. A great many of those work fine without neutral, so you may never need a neutral. If you did, you'd need to replace the entire cable. You cannot retask that green wire to be anything else. You also cannot use the MC shield as a ground path, so you do need the green wire.

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This is indeed the wiring to support a 6-50 receptacle

What you appear to have is a 6/2 MC cable where the neutral has been retagged to a second hot -- in the common flavor of type MC cable, the armor is grounded, but not a grounding means as it can't be guaranteed to carry current. At this point, you have two options (with your landlord's consent of course):

  1. Rerun the cable (if practical) -- this may be the easiest if it's an exposed run and you and your landlord are comfortable with this, or if the landlord's electrician feels most comfortable with this for that matter. A 6/3 MC cable can be used here to provide neutral and ground, converting it for future installation of a modern NEMA 14-50R in case an appliance that needs both 120 and 240V needs to be plugged in here.

  2. Convert this to a 6-50 only setup -- this is simply a matter of switching the 10-50R out for a 6-50R, and making sure the yoke contacts the box directly with no little cardboard squares in the way in order to ground the box.

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2 Are there any other tests that I can perform to determine whether this is 10-50 wiring or 6-50 wiring?

Use a wire gauge. This is a piece of stiff metal with holes that just fit 6 AWG etc. wire. They exist as standalone tools but are also commonly found drilled into the side of wire stripper and crimper combo tools. There’s a good chance you have one of these in your toolbox. Look for the numbered holes.

3 Does this matter? The difference seems rather subtle.

The terminations on the 6-50 receptacle are listed to handle certain wire sizes (and Al/Cu or Cu only). They are not tested for safety with other wire sizes.

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