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I went to wire the replacement ceiling fan and discovered the colors of the house wiring to not be identifiable.

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They both look white. On the other hand they both look black. How can I determine which is the white common wire? I have access to a cheap multimeter if needed.

  • There's probably another wire in the box. Where does it go? – isherwood Jun 1 '17 at 17:30
  • That looks like a mounting bracket that attaches to the light box and to which the fan base attaches. If you remove those two screws in the picture and remove the bracket, you may see the wires further up have identifiable color. – user4302 Jun 1 '17 at 18:44
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If there is a ground in the box, or if its a metal box and its grounded, then measuring from the hot wire would show voltage and the neutral would not. Also, you could get a non-contact voltage tester and it may indicate the hot wire if they are separated by a small amount. Here's an example tester: http://www.homedepot.com/p/Klein-Tools-Non-Contact-Voltage-Tester-NCVT-1SEN/100661787

  • As chance would have it, a Licensed Electrician was working next door. I asked him about it. Oddly enough, he used the tool you linked to, or similar. I may purchase one of those testers, seems like a good thing for safety in an old house. – Paulb Jun 2 '17 at 12:20
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Plug a modern (polarity-enforced) extension cord into a nearby outlet. Set your multimeter to 200V AC (or something just above line voltage where you are). Touch the negative probe to the neutral (wide in the US) slot of your cord, and the positive probe to each of the wires in your light box. Only the hot should show voltage near line-level. (The neutral may show residual or induced voltage at a much lower level.)

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Looking at the picture it appears the two wire twisted together would be the neutral so that would probably make the single wire a switch leg. Is there a switch attached? If so you could check to see if the wire has voltage when the switch is on and no voltage when you turn the switch off.

Also looking at you wiring is the old rubber insulation with cloth covering last used around 1950. Be very careful handling it as the insulation will crack and short out. You might think about scheduling replacement it with a newer type system like NM. This could be over time. For example your replacing the ceiling fan. Why not look into replacing all of the wiring and appurtenances at least back to the switch or a junction box or the entire circuit. Then the next time you are fixing or replacing something. Do the same thing. This is just a suggestion.

  • You're right about the cloth covered wires. The house was built in 1937 and probably a third of the circuits are cloth wiring. I don't like fussing with them for fear of disturbing the insulation effectiveness. – Paulb Jun 2 '17 at 12:22
  • @Paulb - You are right, the worst thing you can do to old rubber insulation is to start moving and bending them around. Rubber becomes brittle over time and usually it is safe as long as no one tries to mess with it. But as soon as you try to work with it, you will begin to crack and break the insulation. So handle with care. – Retired Master Electrician Jun 2 '17 at 12:31

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