Neutral and ground in outlet *slightly* hot when either of two other breakers switched on

Question

Our house was built in 1996, and everything that I've encountered electrically in it so far seems to have been done correctly.

Today, I was replacing a worn out 15A 3-prong receptacle.

I used my normal 3-prong plug in outlet tester, and my son watched it over radio coms while I was down in the basement, switching what seemed to be the correct breaker, #12. My son reported that the lights on the outlet tester turned off.

I came back up and confirmed, and as one last sanity check, I used my non-contact voltage tester to make sure nothing was hot, but it fired on the outlet. Not just on the hot side either, but also on the neutral and ground ports.

We then went through what felt like a ridiculous comedy of errors over radio as we tried to figure out where this voltage was coming from.

We eventually narrowed it down to breakers #22 and #25. While #12 would make the 3-prong tester lights turn off, having either #22 or #25 on would cause both neutral and ground on the outlet to have some voltage, but no other breakers in the house would cause voltage.

With all three breakers turned off, I pulled the outlet out of the wall so I could inspect the wiring. It looked totally fine, with a single strand of white Romex entering the junction box, and nothing leaving, and the outlet connected correctly.

Now that I could see the wiring, my son and I did some more radio-linked testing, while I probed each wire individually. #12 would produce voltage only on the black wire. But either #22 or #25 or (both together) would produce voltage on the white and bare/ground wires.

The outlet tester isn't complaining about the outlet, and various appliances have always worked okay using that outlet.

I wasn't quite sure how to test this mysterious voltage with my multimeter (since I'd usually put red in hot and black in neutral to measure 120V AC), but with either #22 or #25 on, I seemed to be measuring a volt or two between neutral/white and bare/ground.

Furthermore, measuring between black/hot and white/neutral showed 119.2V AC whether or not #22 or #25 was on (as long as #12 was on).

Finally, my non-contact voltage probe does not fire when inserted into the neutral or ground ports of other outlets elsewhere in the house, even with all breakers on.

Answer 1

Summarizing the helpful comments from Mazura, crip659, and others:

Both non-contact voltage testers and digital multimeters can sometimes read phantom voltages in wires that don't actually have dangerous voltages in them. This can be caused via induction between the wire in question and other hot wires that run close to the wire for long stretches (for example, if two wires from different circuits travel together in the same conduit).

The suggested "real test equipment" is a Wiggy---which is the genericized name taken from the original inventor/manufacturer---also known as a solenoid voltage detector.

These are battery-free devices that move a magnet with a coil based on the amount of voltage present, resulting in a bar moving on a physical meter. For AC voltage, they also vibrate and make a noise. By holding the device while testing, you can actually feel the presence of voltage, without even looking at the device. Because substantial current must flow through the device for this to happen, they won't react to phantom voltages, which can't drive much current.

Doing additional research online, it seems like the best solenoid voltage detectors available are the Knopp K-60 and the Ideal Vol-Test or Vol-Con, which are both still being manufactured, and apparently still made in the USA.

These are relatively simple and primitive-looking devices, but apparently, many professional electricians still use them on a daily basis for "go/no-go" testing before handling AC wires.