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My house was built in 1960. The socket shown used to be a switched outlet, with one permanently live outlet and one controlled by a wall switch. I had ceiling lights installed about 15 years ago and, as part of that work, the contractor converted the socket so that both outlets were permanently live. I wanted to convert the socket to a GFCI as my circuit test showed that there is actually no ground connection. There isn't a GFCI that is controlling this socket, so it seems wrong to have a three-prong socket here anyway. enter image description here

There is a cable on the left and one on the right. The blacks are joined as are the whites and the red is connected to the hot side of the socket. I believe that the red and black on the left cable are actually joined at the box containing the wall switch, although I have not been able to confirm this. As connected, there is 120V between the red and white, 120V between black and white, and 0V between red and black.

Disconnecting the socket, I see this: enter image description here

(I added the Wago connector as there was a nick in the red wire insulation)

There is 120V between the two black wires, 120 volts between the white and black on the right, and 0V between the black and red on the left. I totally forgot to measure the voltage between the black and white on the left! With the socket not wired up, the ceiling lights do not work even when the breaker is energized.

Can anyone explain how this wiring works and whether it would be possible to convert the socket to a GFCI. I can do some continuity testing on the wiring between the socket and wall switch if needed.

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  • note: the white wire is wrapped around the screw in the wrong direction ... the way it is now, the hook on the wire opens up when the screw is tightened
    – jsotola
    Commented Feb 12, 2023 at 3:44
  • It looks like a raised switch/receptacle cover on a shallow pancake box, so doubtful if a GFCI will fit in there, and even if it did, would violate electrical code limits on box fill with all those wires, which look to be 12 AWG. You could place a suitable Wiremold surface box on top to add room. It's also possible you could abandon that red wire (cut and cap or tape at both ends).
    – kreemoweet
    Commented Feb 12, 2023 at 7:54
  • The 4 wires don't matter, you'll be connecting all hot wires to the LINE brass terminal and all neutrals to LINE silver terminal. You don't use LOAD for a single receptacle. Read the instructions for how to put 2 wires on 1 screw. Commented Feb 12, 2023 at 9:18

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TL;DR Yes, this is a good use-case for GFCI

Your original wiring was most likely:

Cable from panel:

  • Black - hot - connected to receptacle ("on" half) and to black going to switch
  • White - neutral - connected to receptacle and to white going to switch

Cable to switch:

  • Black - hot to switch
  • Red - switched hot from switch - connected to receptacle ("switched" half)
  • White - neutral to switch

The updated configuration is:

Cable from panel:

  • Black - hot - going to switch
  • White - neutral - connected to receptacle and to white going to switch

Cable to switch:

  • Black - hot to switch
  • Red - hot from switch box (but not connected to the switch itself) - connected to receptacle
  • White - neutral to switch

What changed?

The contractor likely swapped an old ungrounded receptacle for a grounded receptacle since they wouldn't bother to stock ungrounded receptacles any more. Red to hot (as before), white to neutral (as before) and removed a pigtail that was going to the hot bundle. The contractor also changed the switch wiring, connecting red and black together to provide hot to the receptacle, as well as installing the ceiling light fixture and cable.

What they did wrong:

They didn't double-check the ground, or they checked it but didn't worry about it not working. Often (at least in my house) old ungrounded receptacles in metal boxes actually have ground attached, you just can't see it. But in your case apparently there is no ground. There is a small possibility that you have ground but also have a cheap receptacle that does not ground through the metal yoke. Better quality receptacles do that, but only if you have a metal box that is itself properly grounded. You can double-check this yourself: Use a multimeter to check either voltage from black or red to the metal box (should be ~ 120V) or resistance between white and the metal box (should be close to 0 Ohm). If you have ground then you likely just need a better quality receptacle (look for "self grounding" in the specifications).

But if you don't have an available ground then GFCI is a legitimate and safe alternative. Install the GFCI as normal (in this case, red to "line hot", white to "line neutral"), test to make sure it works properly (TEST/RESET on the GFCI itself - the TEST button on a plug-in tester will likely not work without ground) and then label it "NO EQUIPMENT GROUND".

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    That's a small junction box... get the most low profile gfci you possibly can. If even that won't fit, it might be possible to put a gfci somewhere between that outlet and the panel. (And if you want 2 prong receptacles, they do exist... just not at a big orange store.) Commented Feb 12, 2023 at 3:09
  • Thanks for this answer. I did have an existing GFCI outlet and wired it as suggested and it did work. However, as noted a couple of times, the box is small and so I need to find a low-profile GFCI and remove the red wire.
    – masonas
    Commented Feb 12, 2023 at 19:34

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