Knob and Tube shared neutral

planes

We have a "new" arch fault breaker installed when we upgraded an electrical panel. We are required to use this new breaker according to city codes. The old breaker worked for 20 years, however, the arch fault flips off. We have traced it to a light that has knob and tube wiring. The hot comes in at the ceiling light. The neutral comes from a junction box with a separate circuit and it "shares" a neutral which runs to the light switch and up to the ceiling light. So the light has a hot and neutral from different locations. We don't know where the hot wire in the light box comes from. Every time we turn on the switch, the breaker flips. Any suggestions on how to fix this without tearing out the ceiling?

Jim Port

You might be dealing with a switched neutral that was common with K&T,

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Answers based on the National Electrical Code. Local amendments may apply. Check with your local building officials.

stickboy1375

But that wouldn't trip an arc fault breaker, a shared neutral would.

AllanJ

You may not feed a light or other load with the hot wire from one branch circuit and use the neutral of a different branch circuit.

Some knob and tube wiring satisfies this requirement even though the hot and neutral do not follow the same path back to the panel. Some does not.

It was even possible for K&T to form a multiwire branch circuit, legitimate in its heyday, where the hot from one side of the 120/240 volt line ran along the north side of the house, the hot from the other side ran along the south side, and the shared neutral ran down the middle. An MWBC of any vintage will not work with a single pole GFCI or AFCI breaker but they do make double pole GFCI breakers (AFCI breakers too?) for this putpose.

If some jury-rigging or bootjacking (bootlegging? jacklegging?) of wiring was done and no neutral was handy and someone tied the new light to "any old neutral" then you will definitely have problems with ground fault interrupters and arc fault interrupters.

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The disadvantages of crab apple trees. In summer, the apples are too sour to pick and eat. In winter the birds come and leave dropping all over the place.

planes

Thanks for the information. What can I do about it? How do I fix something like that?

k_buz

I would MAKE sure an AFCI is needed when a service change is done. Its not required here, and I can't remember anyone saying it was required of them in this application.

Call the electrical inspector and ask if AFCI breakers need to be installed when a panel is changed/upgraded.

It would help if you stated your location, someone here might know something about your community.

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stickboy1375

One thing you could do is locate the circuit in the panel that feeds the light tripping the afci and combine that circuit to the existing afci breaker that is tripping... Of course, this could result in other undesired results as an overloaded circuit.

AllanJ

This would be "overloading" only in the sense of planning what you use so as not to trip the breaker. Not overloading and overheating the wiring and causing a hazardous situation.

(You may not use one 20 amp let alone one 30 amp breaker to serve what used to be two 15 amp circuits with 14 gauge wiring.)

When two wires want to go under one screw, cut a short length (pigtail) of the same kind of wire and wrap one end around the screw. Connect the two wires in question to the other end of the pigtail. (A screw with a capping washer or a grooved bed that will hold two wire ends without wrapping around and without the possibility of slipping out slightly before the screw is tightened may be used to hold two wires of the same size.)

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The disadvantages of crab apple trees. In summer, the apples are too sour to pick and eat. In winter the birds come and leave dropping all over the place.

stickboy1375

Agreed, safe to do as I proposed, you just may find the circuit is inadequate for your needs, or it will work flawlessly. But either way, its perfectly safe.