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This outlet has two copper (ground) wires twisted together and it also had a black cap on it. This had a two prong plug originally and we are looking to switch it to a three prong. Do I untwist and only use the ground wire that’s around the back screw to ground to the new three prong outlet?

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  • You might want to check that the ground actually goes somewhere... a multimeter should show nearly 0 ohms between ground and neutral. (Turn off the main breaker before checking!)
    – Aloysius Defenestrate
    Jul 16, 2022 at 17:46
  • The two bare ground wires look like they might not be securely twisted together. Just to make things simpler, get a premade green 12 gauge short grounding "pigtail" wire and use a red size wire nut to attach the two bare ground wire ends and your pigtail wire bare end together securely. Then, just attach the other (sheperd's crook looped) end of the green pigtail wire to the green grounding screw on your new outlet. Be sure to watch a youtube video on how to use a wire nut, and on how to replace a 2 prong outlet with a 3 prong outlet.
    – Armand
    Jul 16, 2022 at 18:15

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Since it is a metal box which is grounded, installing a 3 prong (grounded) outlet will be grounded via the mounting screws/tabs. No need to connect the ground to the outlet.

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    Technically the ground wire should have been wrapped around a 10-32 screw placed into an appropriately threaded hole, not the cable clamp screw, but I don't see any reason it wouldn't work as is. It just might not pass a picky inspector.
    – FreeMan
    Jul 16, 2022 at 18:02
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    The new outlet might not have a self-grounding tab either.
    – Armand
    Jul 16, 2022 at 18:17
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    @FreeMan Yeah, I noticed that too. It's in my category of safe-it works, but clearly not code compliant. To Armand, most outlets are self grounding if in a metal, grounded box. The OP could undo the ground, pig tail to a proper screw in the box and to the outlet to make it code compliant. But it's functional, if not code compliant as it is.
    – George Anderson
    Jul 16, 2022 at 18:39
  • @FreeMan another comment, risking on getting conversational here. I doubt an inspector would be that thorough. When I wired my new house and the inspector did the rough in inspection he only caught a few things like I didn't have all the ARC fault breakers covering the required areas (had to add 3 more). But I was waiting for final before getting the POCO to hook up permanent power, I finally called them (L&I in washington counties), and they just said: If you can do the rough in we figure you can install fixtures and outlets, no followup, good to go.
    – George Anderson
    Jul 16, 2022 at 21:45
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That's a metal box. The ground wires coming in need to ground to the metal box first. So leave them wire-nutted to a pigtail that goes to the ground screw in the back of the box.

If you want to ground any receptacle, you can add another ground pigtail that wire nut, and run that to the receptacle.

However if you use a better receptacle that is marked "Self-Grounding", the receptacle will automagically pick up ground from the metal box via its mounting screws. Most better ($3+) receptacles have this feature, which is a brush or wiper that contacts the mounting screw.

Switches can self-ground this way, regardless of being marked "Self-grounding", because they're not serving ground to anything but themselves.

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