Help decipher this wiring

Question

I am trying to put a flourescent light fixture into our pantry into an existing fixture/junction box. The previous light was taken down by some previous help I had and they had made a couple of comments about the wiring. They aren't here now, but I am ready to put up my new light. To the best of my knowledge, this fixture is just controlled with one light switch. This is an older house and during this kitchen remodel, we have found several old locations under the drywall where previous switches were. However, the wiring into this box all looks new from the time the house was gutted in 1997. But we have only ever used one switch to control the light.

Here is the description of what is coming in: Three cables are coming in. Each cable is two wire (white, black, ground). G1 + G2 + G3 are connected. B1 + B2 + W3 are connected. W1 + W2 are connected. B3 is by itself (capped).

The light switch just has one cable to it.

Just trying to sort out what might be happening up there. I'll probably need to get up in the attic and see if I can determine where the other cables go.

Answer 1

One of B1 or B2 carries live to the light fixture, and the other carries it to another fixture somewhere else. W3 carries live to the switch, and B3 carries switched live back to the light fixture. B3 is capped off temporarily because it could carry live, depending on the switch position.

To install your light fixture, you'll need to connect the light's live wire to B3, its neutral to W1 + W2 and ground to the other grounds. While you're working in there, wrap a piece of black electrical tape around the insulation at the end of W3 to indicate that it's carrying live, and do the same at the switch end too.

This is a circuit diagram, adapted from one I drew for a similar question:

I didn't show the ground wires because they would have made the diagram more cluttered and as long as they're all connected together, you're OK.