1

I noticed that the ground prong of the only 5-15 receptacle in my laundry room appears to be floating.

Findings:

  • The ground wire behind the receptacle measures 0 ohm (i.e. continuous) with the ground prong in the receptacle.
  • The ground wire/prong measure 0L ohm (i.e. not continuous) with the neutral wire/prong in the same receptacle, 0L ohm (i.e. not continuous) with ground prongs from other receptacles.
  • The ground wire/prong measures 60V with hot in the same receptacle, 60V with neutral in the same receptacle, 60V with ground prongs from other receptacles.
  • The ground wire behind the receptacle appears to come from a 12/2 cable.
  • There is a 1-pole 20amp breaker in the subpanel that serves this receptacle. I believe this receptacle is the only thing that the breaker serves.
  • In the subpanel, there is a corresponding 12/2 cable.
  • In the subpanel, the ground wire from the 12/2 cable measures as expected. ("as expected": 0 ohm with neutral, 0 ohm with other ground, 120V with hot, 0V with neutral.)
  • I checked all other receptacles throughout my home. All of them measures as expected. ("as expected": same as above)
  • I opened up every blank wall plate that I could find in areas between the subpanel and the outlet, and used a non-contact voltage detector to see if it's from the same breaker.
    • I found one blank plate that opens into a hole in the wall (instead of a box). In that hole, I can see 3 cables. One of them is from the breaker of interest. The cables do not appear to be protected (definitely not in a conduit).
    • I did not find any box with wires from the breaker of interest.

Background:

  • The receptacle is a 5-15 GFCI receptacle.
  • I recently had a contractor replace my subpanel. The subpanel serves everything in my home, except the air conditioner.
  • I believe the grounding of the subpanel is good:
    • After installation (and fixing a wiring error behind an electric oven), I verified that ground and neutral are not continuous when neutral feed is disconnected. I also verified that ground and neutral are continuous when neutral feed is connected.
  • A UPS (a small one for network and storage devices) is plugged into the outlet of interest, and its "building wire fault" light is on.
    • I do not know how long the light has been on.
  • I found the issue because I was replacing converting a nearby 10-30 into a 14-30, and used the problematic to sanity check the previously unused ground wire behind the 10-30. (On hindsight, I could have used the neutral of the 10-30, but anyways.)

What are possible causes? How do I diagnose it? (The first thing that popped into my mind is that the ground wire is severed. But is it likely that one wire in a 12/2 is severed, but not the other two?)

(I have a workaround. I found a box 1 ft (30 cm) from the problematic outlet in the same room, which holds wires appropriate for a 5-15 outlet. That box was fed from a different breaker, and its ground appears to behave as intended. But I'm still interested in understanding what's going on)

  • "measures ~60V with hot/neutral" Do you mean 60v between hot and ground, and independently 60v between neutral and ground? – A. I. Breveleri Aug 26 at 19:12
  • 1
    @A. Yes, and that probably doesn't mean much because the ground is floating. – Haozhun Aug 26 at 19:13
  • What is the voltage between hot and ground, and neutral and ground, of that particular cable, at the panel? – A. I. Breveleri Aug 26 at 19:14
  • If the ground was truly floating, you wouldn't be able to measure any voltage between it and anything else. To demonstrate this, go to any other outlet circuit and disconnect the ground at the panel, then measure the hot-ground and neutral-ground voltages at the outlets. – A. I. Breveleri Aug 26 at 19:19
  • 1
    Yeah, that 60V is just phantom voltage I'm sure. Where is the next outlet closer to the service panel? – Harper - Reinstate Monica Aug 26 at 20:33

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.