When replacing a circuit breaker in the service panel, how can I determine which neutral is associated with the hot wire?

Question

In my service panel I would like to upgrade two standard circuit breakers with AFCI breakers.

AFCI circuit breakers differ from standard circuit breakers in that you attach the hot an neutral to the breaker, and then attach the AFCI's coiled white wire to the neutral bus bar. With a standard circuit breaker, the hot wire is attached to the breaker, and the white wire is attached to the neutral bus bar. Here is a diagram of an AFCI breaker:

I shut down the power to the panel, remove the cover and now I need to find the neutral (white wire) associated with each circuit. I see a dozen white wires.

I know which wires are the hot wires for each circuit, but how can I determine which neutral wire is part of that same circuit? I can make some guesses based on the style and markings on the insulation.

And for the record, all of these circuits were installed by licensed electricians. And yes, they used blue wire on some circuits, which is a bit unusual but not unheard of.

Answer 1

Turn off all loads on the circuits in question.

Turn off the main for this panel. Possibly overcautious, but that's part of what's let me get old.

Either: short the dead circuit, or turn on an incandescent bulb on it (main off, circuit dead, still dead - just the bulb/fixture/lamp turned on.)

Pull neutrals and look for one with low resistance from the hot in question. There should be only one. Unshort the circuit or turn off the lamp and make sure the resistance changes (if nothing else is turned on, should be infinite, but if there are things that are not able to be switched off, may be non-infinite. But you should see a difference when you switch the incandescent lamp off or remove the short - it's a form of verification.

I think you have enough neutrals not to worry too much about this, but: beware of "Multi-Wire-Branch Circuits" where one neutral serves two hots.

As for the wire colors, only white and green (with or without yellow stripe) are reserved (and IIRC, you can retag white to another color, but you can't retag green - and you can't retag anything else to white.) If your wiring is run in conduit, using the full range of available colors makes figuring out (what is connected to) the hot side of circuits much, much easier. Most household wiring is not run in conduit, so it's not often seen at home.

Answer 2

With all power off, use a low-voltage tone generator to trace the neutral back from the load side.