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Two rooms in our house have older electrical wiring: two-wire 12 gauge insulated wire (no ground wire) that looks like an older version of romex (no metallic sheathing). These wires terminate outside the electrical panel at a 15 AMP GFI. The GFI is connected to the panel.

I recently replaced three older outlets that had been painted over too many times with new ones. As expected, the outlets in the older part of the house all show an "open ground" on my plug tester.

However! After I replaced the outlets the very last outlet on the circuit now displays "correct" on the plug tester! This surprised me: no other outlets in those room display this reading and there is no metallic sheathing terminating at the metal boxes and I know the two-wire terminates at the GFI.

Am I experiencing some kind of "phantom" voltage in the box enough to fool the plug tester?

I took it apart again and used my multimeter and put one lead on the hot wire and one lead on the metal box and found a voltage reading of 120v. (For comparison, I tested another box around the circuit and got a voltage reading closer to 30 volts.)

I unhooked the metal clamp and freed the incoming wire (just to see if any was frayed or maybe touching the box) but even with the wire suspended in mid-air I am still getting voltage reading from the metal box.

image of open box

I am a little perplexed and don't have much experience with jumping/phantom voltage (if this is the case).

I am inclined to remove the metal box and put in an old work PVC box but I wonder is there something else I need to consider?

I have removed another box in a different part of the room and didn't find any secret grounding or bonding behind it and and suspect this is the same.

Appreciate your insight.

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    Is that a plaster wall? Is there metal lath behind the plaster? If so, the metal lath could be touching the metal box and also touching ground such as a water or gas pipe.
    – MTA
    Mar 25, 2023 at 14:27
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    Not sure why you feel that a grounded metal box is unsafe. Some of my best friends are grounded metal boxes. In the years after your house was built, all metal boxes were grounded in compliance with code. I would leave it alone, but if it worries you, feel free to ask a new question explaining how you think the box became grounded, mentioning the GFI device and asking explicitly if it's safe to leave it alone.
    – MTA
    Mar 25, 2023 at 15:54
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    If you have any reliably grounded 3-prong outlets in the house, plug an extension cord into it and take the other end to the vicinity of your suspect metal box. With your multimeter, measure AC volts between the metal box and the ground hole of the extension cord. It should read 0 volts, or nearly so. If it does, the metal box is not energized, it is grounded.
    – MTA
    Mar 25, 2023 at 17:02
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    @thornomad and by "much" MTA means a volt or two. Plastic boxes are never safer nor better. I have no idea why you think that. Plastic boxes are popular with builders because they're cheap and they allow faster wiring (the hokey cable entries, that barely pass as clamps, can be easily molded into the plastic). I'm presuming you are following the law and either GFCI-protecting the circuit or using 2-prong outlets. If not, all bets are off, but that's not the metal box's fault, and plastic improves nothing. Mar 25, 2023 at 21:34
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    @thornomad So it's confirmed that the boxes that show ground on the device that you call a plug tester are in fact grounded, not energized, so they are perfectly safe and should not be replaced. The grounding is by chance and not to be relied upon for safety as in a 3-prong outlet. The reading of 40 volts from hot on that one box is a phantom voltage that is due to the nature of digital voltmeters and the capacitance of objects. With one voltmeter probe on hot, anything conductive that you touch with the other probe will give a reading, e.g., the plug end of your unplugged extension cord.
    – MTA
    Mar 28, 2023 at 16:49

1 Answer 1

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In comments, you said that this is a plaster wall. If the wall has metal lath in it, the lath could be touching both the metal box and a water pipe or gas pipe or other ground. If so, that would explain the apparent grounding of the box.

In my opinion, this is not dangerous in any way, but the fact that the box seems grounded does not make it OK to install a 3-prong grounded outlet in this box because it's not an intentionally wired hard ground.

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  • It's also possible that someone was ... creative ... and attached the ground wire to the outside of the box...
    – keshlam
    Mar 25, 2023 at 19:00
  • @keshlam that would be unfortunate since it doesn't count if you can't see it. OP could just retrofit grounds per the 2014 rules for that, which are quite permissive. Mar 25, 2023 at 21:35
  • Simplest answer here seems to be to install GFCI outlets in all those locations, which is safety-equivalent to having a safety ground...?
    – keshlam
    Mar 25, 2023 at 22:33
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    @keshlam — the circuit is protected by a GFCI outlet downstream.
    – thornomad
    Mar 26, 2023 at 16:12

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