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We just moved into our house and the previous owner remodeled the master bathroom. There is a GFCI outlet in the master bathroom and it was ungrounded and I took it out and discovered that the ground wire was wired in with the neutral wire.

With my multimeter I measured 125.9V from hot to ground (this is good), 4.92V from hot to neutral (it should be around 120V), and 123.8V from neutral to ground (it should be around 0). I think these readings indicate an open neutral between the GFCI and the breaker panel.

This was a remodel job, and the GFCI wasn't there before the remodeling, and no new outlets were added between the master bathroom and the garage, which is where the breaker panel is located. There is a utility room between the master bathroom and the garage and there is nothing new in the utility room; no new outlets, and everything in there works when the two master bathroom breakers are turned off. There are no new outlets in the garage, either. I'm praying that there is not a disconnected neutral wire in the walls or ceiling.

And so my question is, "How do I check for the open neutral in the breaker panel? Do I look for a loose white wire or screw at the 2 master bathroom breaker switches? Do I measure with my multimeter? Where do I put the red and black probes? Would I get the same reading as I did at the GFCI outlet? Should I leave the power on? Is that safe as long as I don't touch the wires?"

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  • Are your wires in cables(all wires together covered in plastic) or conduit (Separate wires in tubes)? Would turn off main breaker(for safety) and remove the cover. Find the breakers for the problem circuit/s and find where the black(or red) wire enters the panel, should be a white wire with it. Check connections of white wire, then you probably need to check the whole circuit at outlets/lights/ and switches for broken neutral connection.
    – crip659
    Dec 30, 2021 at 18:21
  • Yes, all the wires are in sheathing and are non-metallic (NM-B). I'll turn off the main breaker, etc.
    – David Watson
    Dec 30, 2021 at 18:38
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    I take it the other outlets in the house test correct? My initial guess is that there's an issue with a shared neutral that someone was having trouble resolving. Maybe installed a GFCI on the bathroom circuit that was shared on the same neutral with that bedroom outlet, and everytime someone used that outlet, it tripped the bathroom gfci. Does the breaker that turns off the bedroom outlet have a wire that leads to a cable with three conductors (a red, black, and white)?
    – Edwin
    Dec 30, 2021 at 19:48
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    I'm not able to check the breaker panel as of yet but when I do I'll see if the breaker that turns off the bedroom outlet has a wire that leads to a cable with three conductors (a red, black, and white).
    – David Watson
    Dec 30, 2021 at 20:26
  • @DavidWatson - Could you take a picture while you're there - with the cover off?
    – Edwin
    Dec 31, 2021 at 0:27

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Hopefully this is a simple problem that was not handled correctly by the last person who worked on that GFCI outlet.

If the GFCI is the only outlet on its circuit, then you might have a loose wire in the panelboard. However, if there is any other outlet (receptacle, light, appliance) on the same circuit, it will be necessary to first determine if the other outlets have the same problem or not by testing all of them with the multimeter.

So the first step is to check which circuit breaker turns off the hot wire to the GFCI.

Second, check which other lights and receptacles stop working when that circuit breaker is off.

Third, turn the circuit back on and check the hot-to-neutral voltage at every outlet.

Fourth, if you find some outlets are working and some are not, you will likely need to open up every outlet on that circuit and look for loose neutral wires. Likewise, if none of the outlets are correct, you might need to figure out which outlet has the homerun cable and figure out which end of that cable (outlet or panelboard) isn't connected to the neutral bus.

Fifth, you indicated "123.8V from neutral to ground" at the GFCI. This could mean something else on the circuit was turned on and connected to the same open neutral when you measured the GFCI.

If you remove the panelboard cover for any reason: Wear gloves, wear safety glasses, and don't touch anything you haven't verified as 0 V to ground.

Additionally, avoid using any outlet that reads less than 110 V hot to neutral. The extra voltage naturally wants to go anywhere it can, creating unsafe conditions for fires and personnel.

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  • There are two 20 amp circuits that serve the master bath, and in addition to the GFCI there is a NuHeat SOLO floor heater and a towel rack warmer that looks hard wired and never worked. After I put the ground wire in its place on the GFCI and turned on the power the NuHeat didn't come on anymore and neither did the electric toilet. Wouldn't that indicate they're on the same circuit? The NuHeat doesn't have a ground wire; grounding is done via the grounding wire in the heating apparatus. I don't know which components are on which of the 2 breakers that serve the master bath.
    – David Watson
    Jan 1, 2022 at 12:37
  • Don't panic. Just turn off one of the two circuits. Check the hot wire at the GFCI. If still hot, try switching off the other circuit. If still hot after that, you need to keep looking for the correct switch. If you can do the same with your bathroom appliances that might help. And you will need to follow the other steps from my answer if you want to diagnose this by yourself. It's better to check the outlet connections before assuming the problem is in a service panel.
    – Robert Chapin
    Jan 2, 2022 at 21:46
  • I tested the GCFI outlet and the towel warmer outlet; they only have the house wires in them now. Black, white, and ground are all continuous between these two outlets and I still have 0V hot to neutral and 120V hot to ground in both of them. The only outlet left in the bathroom is the floor heater panel. I read that if there is continuity between neutral and ground on a receptacle the problem (i.e. disconnected neutral) is in that receptacle or the one next to it. Do you agree? The floor heater has the ground in the heating apparatus and it may be on a different breaker. What next?
    – David Watson
    Jan 3, 2022 at 23:08
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    Checking continuity is just going to slow you down. If you can turn off the circuit and determine which outlets are no longer hot then you will be closer to finding the homerun cable for testing. If the GFCI is at the far end of the circuit and the towel warmer is somewhere in the middle, then is there another outlet involved? Have you determined that all of the neutral wires in that circuit are abnormal?
    – Robert Chapin
    Jan 4, 2022 at 3:38
  • Both breakers are off. There are 3 outlets in the bathroom. The GFCI has no outgoing wires, and the towel warmer outlet has no outgoing wires. How can that be? They are on the same breaker and are both at the end of the circuit and the towel warmer has the same abnormal voltage as the GFCI outlet. The 3rd outlet has a screen on it and is for the floor heater and it turned off when I rewired the GCFI correctly. It's off now when both breakers are on. I took the screen off; there are 2 red wires and 2 black wires to the back of the panel that I can't pull out.
    – David Watson
    Jan 4, 2022 at 12:30

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