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I have a modern external light fixture that needs to be attached to an existing exterior electrical 'box'. The older parts of the house have knob & tube wiring and there is evidence of gas plumbing for lighting before the knob & tube was installed. Here, i think but am not certain, it appears as if the knob & tube comes into a 'box' around an old gas line (see photo) but I may be mistaken.

The instructions say to attach:

  • white to white
  • black to black
  • exposed wire to ground

but all i have are two old wires wrapped in what appears to be cloth. the cloth has become quite brittle.

So, how does one go about, safely, installing the new fixture?

external electrical box fixture installation pg3

Edit: More pictures as requested:

enter image description here enter image description here

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Is your box connected to armored cable? (Metal sheath wrapped around the two cloth wires) –  The Evil Greebo Jun 1 '12 at 18:42
    
Can you post a pic of a more direct shot up into the box? –  The Evil Greebo Jun 1 '12 at 18:50
    
OK, i added 2 photos. best i could get. –  Meltemi Jun 1 '12 at 23:04
    
If the insulation is so far gone that you can't find the stripe, are you sure that moving the wires around isn't going to break it at or behind the box? You don't want to create any dangerous situations here. –  Michael Kohne Jun 2 '12 at 18:59
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FYI, the pipe-like threaded thing at the center of this electrical box is probably not a gas line, just an old-style mounting point for a fixture. –  Shimon Rura Jun 3 '12 at 3:35
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3 Answers

up vote 5 down vote accepted

The correct answer is: Redo the wiring all the way back to the panel.

The workable answer is: The wire with the black stripe on the insulation is BLACK. The other one is white. The junction box is probably not grounded, but I'd attach the ground wire of the light to the junction box anyways.

Safety wise, the ground isn't entirely necessary on a ceiling light as it is pretty unlikely that anyone will touch it.

But seriously, consider replacing the wire right back to the panel. It can be a pain to pull it, but modern wiring is much safer.

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thanks! i don't think i'll find a "black stripe" on what's left of that brittle insulation. Any other (safe) ways to figure out which one is "black"? –  Meltemi Jun 1 '12 at 17:42
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@Meltemi The safest way is to call an Electrician. –  Tester101 Jun 1 '12 at 18:05
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@Tester101 - of course, but then I wouldn't be posting on a "DIY" website, would I? –  Meltemi Jun 1 '12 at 18:13
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@Meltemi I'm not trying to be mean here, I'm just trying to keep you safe. It's very dangerous to mess with electricity if you don't have proper training. –  Tester101 Jun 1 '12 at 18:25
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@Meltemi Some times the best answer to a DIY question is to call a professional. –  aphoria Jun 2 '12 at 11:31
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Use a NCVT (non-contact voltage tester) and see which of the two wires alarms when the switch is on. That will be your black/hot wire. If both of them alarm, stop, do not pass go, something else is wrong.Non-contact votage tester

For the ground, you don't have one. The safest thing to do is run a whole new wire back to the panel. Anything else is "less than safest".

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I got a NCVT ($17), thanks! Good advice! –  Meltemi Jun 2 '12 at 14:14
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In this case I would carefully remove any cotton or cloth covering on the existing wires, after shutting off the branch circuit. Hopefully this leaves you with a rubber or plastic layer, but either way it will be ok. Now push those wires back into the ceiling as much as you can.

Mechanically rip that entire metal electrical box off the building. A non-contact sensor can help you figure out which wire is hot (black) and which is neutral (white). Ignore the ground wire for a fixture nobody can touch.

Install a new plastic box. Slip loam over the old wire, or wrap well with friction tape (use white and black). Thread the wires through the new plastic box and screw it in place. Install your light fixture per the directions.

That wire that's in your ceiling is probably Knob & Tube: soldered and really bulletproof, it does not really even need insulation because of the knobs and tubes. By replacing the junction box you remove the one weak point.

And for bonus points: find an electrical supply house than can sell you some "loam" for Knob & Tube wiring. Use this instead of friction tape.

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