Identify ceiling fixture wiring

September 5, 2009 3:03 PM

Replacing ceiling fixture. Removed old fixture. Help me identify the wiring so I can wire the new one?

Very old house, very old wiring. Third floor ceiling (attic is above). Removed old fixture, there was no box, so I tore out a little ceiling to find a joist to nail a new box to.

Now to wire the new fixture. Hanging down are three wires, with one of them actually jacketing two separate wires (individually insulated, but in the same outer insulation). These 2 were twisted together as one wire and connected via wire nut to the old fixture.

So if you call that one wire, then there's that, and then two separate wires. They all have an outer cloth kind of insulation, then inside that the actual wire(s) are insulated with something that was very brittle and crumbled where I touched it. So I'm going to cover everything with heatshrink tubing before I start manipulating it & stripping it and connecting it and folding it into the box, but the question:

how do I figure out which of these 3 (4) wires goes to black on the new fixture, and which to white? Could one be a ground? Do I have to buy a tester thing?

The fixture is controlled by one switch only.

posted by stupidsexyFlanders to home & garden (11 comments total)

With all due respect, I don't think this is a do-it-yourself project. You get this wrong and you could start a fire, or you could kill someone.

I think you"figure this out" by hiring a qualified electrician to rewire the box.
posted by Chocolate Pickle at 3:29 PM on September 5

Yeah, sounds complicated. My guess is that the other two wires used to be connected together as well (with an old lighting fixture, there's likely no ground). But I wouldn't bet my house on it without some serious investigation. Did you look at how the wires were connected to the old fixture?

FWIW, it sounds like you're working with old knob and tube wiring. AIUI, one big problem with that style of wiring is that it's easy to mess it up (knock wires off of their knobs) by accident when working with part of it.
posted by hattifattener at 3:44 PM on September 5

Buy a "tester thing" aka a voltage detector. These are a couple of bucks for the basic at HD or Loews. Then you will know which is the hot wire and which the neutral and wire accordingly once you've determined that the insulation is sound.
posted by Neiltupper at 4:12 PM on September 5

They all have an outer cloth kind of insulation, then inside that the actual wire(s) are insulated with something that was very brittle and crumbled where I touched it.

This alone screams "get an electrician". And be prepared to pay for at least that circuit, it not a lot more, to be recabled. Crumbling insulation, particularly when disturbed = short circuit = fire risk.
posted by Pinback at 4:26 PM on September 5 [1 favorite]

Sometimes armored cable can deteriorate to the point that you can't tell that one of the two sides is white.

Also, "there was no box".. SCARY! Even if you do a 100% safe job on this box, how many other boxless, deteriorating connections are there elsewhere in the house?
posted by blenderfish at 4:53 PM on September 5

Yeah, I think I may just pay an electrician to get up in that attic and rewire all the ceiling fixtures on the third floor, as they're probably all in the same condition, I'll want to change out those old fixtures at some point, and everything's easy to get at up there.

Thanks all!
posted by stupidsexyFlanders at 5:06 PM on September 5

The knob-and-tube style didn't require boxes behind every fixture like the modern romex style does, aiui.
posted by hattifattener at 5:54 PM on September 5

It does sound like Knob and Tube, if your electrician can not replace all the wiring. Please consider having him install a Combination Arc Fault Circuit Breaker in your panel.
This will protect against some problems that a standard circuit breaker will not.
posted by Haujobbin at 5:57 PM on September 5

It is definitely knob and tube. Why does that not require a box?
posted by stupidsexyFlanders at 7:28 PM on September 5

It's not that it doesn't require a box, it's just that they often didn't use them back in the palaeolithic. I had a knob and tube house once (clearly wired in Edison's day and repeatedly patched since then by Donald Duck) and couldn't remotely afford a full re-wire, but I certainly put in a heavy plastic box whenever I had to repair a fixture.

As for which wire was which, my best friend was a continuity tester. Not being eager to fry myself, I disconnected the entire fucking house at the main panel and worked by flashlight. And when I couldn't tell the difference between the filthy grey BLACK crumbling fabric insulation and the filthy grey WHITE crumbling fabric insulation where I was working (which was always) I traced the wires back to where I could tell the difference. Most of the time that was all the way back to the panel. Three different times it also involved ripping a hole in the plaster to find out exactly what Mr. Duck had done in the wall.
posted by jfuller at 8:03 PM on September 5 [1 favorite]

Something similar happened to a friend recently. He had a flood in his basement, when they started pulling out the bottom third portion of the drywall, they discovered extremely sub-standard wiring. (No insulation whatsoever)

The insurance company then stopped all flood-related activities until the wiring was brought up to code - otherwise a short/burn could take out the entire house.
posted by jkaczor at 10:33 AM on September 6