I have a groundless outlet that I'm attempting to add a ground to. The breaker that this circuit is tied to is a one pole AFCI, and it seems this is the only outlet on the circuit for the time being. (Unsure where the cut wires lead, but we'll get to that later)

It would appear the outer jacket has some sort of bare copper in contact directly with the box, but attempting to tone the bare copper or metal junction box does not seem to produce a tone at other outlet ground receptacles in the room (for functioning outlets). Although I'm unsure if you can tone grounds in general.

When the receptacle is in place, my voltage proximity tester gives a very pronounced signal from about a foot away, but I'm unsure if this is just a standard reading for an ungrounded outlet. My multimeter shows ~40vac between the box and hot when the receptacle is hot, so I'm just generally confused here at what's going on.

image of a junction box with an ungrounded outlet protruding zoomed image of a junction box, with details of the strange bare copper more visible

I toned the cut wires and tested another location in the room where inactive floor outlets are, but no signal. And testing their voltage while the circuit is hot produces 0vac. So I can only assume these are buried somewhere due to an old renovation, especially as this box is on a wall shared with another unit.

So it seems the box isn't grounded, but if so, why is the bare copper touching the box, and what does 40vac between the box and the hot terminal mean? I'd guess the next step would be to take the panel face off and check for this wire's ground, but I still feel a little too confused to proceed.

  • 1
    The bare copper you think is wrapped the cable is the ground wire from inside the cable. The extra that was left when the cable was stripped. The ground wire is probably making a bad/poor connection to the box which is why you get ~40v instead of ~120v. Imagine if you unwrap the ground and connected to a newer outlet, you would have a grounded outlet. Should strip those other wires and put wire nuts on the ends.
    – crip659
    Aug 30, 2022 at 21:49
  • If that's the case, why didn't the bare copper wire seem to "tone" when tested on a shared ground? Do toners not work on ground wires?
    – brubsby
    Aug 30, 2022 at 22:21
  • 1
    Might not be connected at the other end either. What do you see at the panel when you turn off the power for that outlet?
    – crip659
    Aug 30, 2022 at 22:46
  • the 40V means the box is not grounded
    – Traveler
    Aug 30, 2022 at 22:49
  • There seems to be a tiny bit of bare copper visible at the bottom of the box behind the little pile of cruft. What is that?
    – FreeMan
    Aug 31, 2022 at 14:06

1 Answer 1


It appears to be what I'd call "transitional" NM type cable. Looks like plastic wire insulation, has a ground wire, but the jacket appears to be cloth.

Older stuff all the insulation is cloth and there's no ground wire, newer stuff all the insulation is plastic.

The presence of a ground wire does not mean the ground wire was properly connected at either end, much less both ends. But the presence of a ground wire does mean that could be corrected.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.