Voltage on disconnected neutral wire of GFCI outlet

Question

I’m changing the switches around the house and some of the boxes are mixed light switch / GFCI outlet. After turning off the power, opening one of the boxes and pulling out the switches and outlet, I made sure no wires are touching and turned the power back on to be able to identify the line coming from the panel vs. all the other wires.

Originally, there was a bundle of white neutral wires all connected together and going off to various directions. Before turning on the power to find the line from the panel, I disconnected the neutral wire bundle, i.e. I now had three disconnected white wires going into the wall and one only connected to the GFCI outlet.

After turning on the power, I expected all white wires to have no voltage on them — as they are supposed to be neutral, right? But the situation turned out different. Two of the whites going into the wall now have voltage, as well as the white wire that’s only connected to the GFCI outlet and nothing else.

So my questions are:

1. How come the white wire only connected to the GFCI outlet now has voltage on it? Is this due to the electronics in the GFCI outlet working as a load and passing voltage through?
2. How come there is voltage on the white wires going into the wall? Could they be carrying return voltage from other GFCI outlets? Or would this be to some other load connected down the line and voltage coming back without a way to go back to the panel?

Answer 1

They're only neutral if they're connected.

If they're not connected, they are going to be hot, because power is coming off a hot wire, going through a load, trying to come back via neutral, and reaching the wire break and getting stuck there. The "Dead" side of the neutral wire is now floated up to 120V. And that's why we put insulation on neutral wires.

Generally speaking, you should be hooking up the new switches exactly the way you found them, mindful of broken tabs and that the function of the screws is decided by their markings and screw color, not physical position of the terminals. Those are randomized on every model of switch.

We have an FAQ question on replacing switches.

You have a certain confusion between voltage (potential) and current (flow). If the GFCI is connected to a hot wire, and not tripped, there will be voltage potential on its socket hot pins, as well as on its "LOAD" terminal hot pins and anything connected to them. That is normal.

If you connect only hot to a GFCI, and the GFCI is not tripped, then all terminals on the GFCI should ring out as hot. Line Neutral because of the GFCI's internal electronics, and Load because it is connected to Line. (except for ground of course, which is not connected to the GFCI in any way whatsoever. On GFCI receptacles, ground goes only to the round holes on the sockets.)

Oh, one more thing. How do you think GFCIs work?

• The GFCI receptacle provides protection only at the receptacle, and if this GFCI trips it was caused by something plugged into it. No hunting all over the house.
• The GFCI protects its own sockets and also any downline sockets fed from the "Load" terminals, and so I'm cleverly mapping my circuits so I only need one GFCI device per circuit, and using Load terminals accordingly.

EITHER ANSWER IS CORRECT, because, each method has its advantages. However, if you are doing the first one, then do not use the "Load" terminals: pigtail all wires to "Line". For what it's worth, the instructions will say how to attach 2 wires to a Line screw.