New to me problem

[billconner]

NM-B, 14 ga. I have a 3 (+ground) between a switch box and box for sconce. I'm trimming now but had temped one of the circuits - feed to one branch circuit. So in prep to trim out, my Klein no contact tester showed voltage on both wires. Verified neither end of that wire is terminated. Put a Amprobe vom on it and as expected hot was just over 120 volts, but surprisingly unterminate other conductor was just under 60 volts.

Is this what happens if there's a nail or screw through the NM-B? New to me.

 

[pattenp]

If using a digital multimeter you are most likely seeing phantom voltage. This is induced voltage from the magnetic field of the hot conductor.

 

[bronc076]

Yes I've seen voltage with a meter on the unused conductor before too. a screw through the cable would most likely give you line voltage. You could turn off the breaker, isolate the three conductors and check continuity between them to verify but this seems normal. The wires are in close proximity in the cable so this happens.

 

[micromind]

If using a digital multimeter you are most likely seeing phantom voltage. This is induced voltage from the magnetic field of the hot conductor.

Exactly.....happens all the time.

A solenoid type voltage tester will provide a low enough impedance to make the phantom voltage disappear, as will just about any other kind of load.

 

[billconner]

Thank you!

I thought I'd check for continuity between them but must not have, as there is non.

Last wiring I did was EMT and thhn. Is that less likely to induce a voltage in a conductor in same pipe than NM-B?

Just changed this circuit's breaker to a combo AFCI/GFCI - as I know local inspector requires - and I'll proceed.

Full panel is getting crowded with so many AFCI replacements. Hard to get to ground and neutral buss bars.

 

[micromind]

Phantom voltage is present in conduit too, it seems to be about the same to me.

 

[billconner]

Well, it's all working. A little mystified by previous owners wiring add ons. Wish there was a simple way to trace a circuit. Resorting to double breakers - cheaters - to divide some double tapped circuits, though with LED lighting on most of them, probably not a real problem.

Previous owner also ran a circuit in PVC conduit - but skipped the glue. Sure made it easy to relocate and reuse materials. Same guy also ran grounds to neutral buss in branch panel. Adding a ground buss to make separating easier. I think I've found them all.