I live in a condo built in the late 70s. I'm installing a ceiling fan in the master bedroom and need to run electricity to it. I went in the attic and there is an existing romex wire near the location of the fan that I thought I might tap into. I turned off the master bedroom circuit breaker to see if that wire was on the bedroom circuit, and the wire was still live. To make a long story short, I turned off and on each circuit in the house, and nothing killed the power in the wire! I finally find out that I had to turn off the master bedroom circuit breaker, and also the circuit breaker for the den next door in order to get the non-contact voltage tester to stop beeping. All the lights and outlets in the bedroom and den behave as expected (when I turn off the bedroom circuit breaker all the bedroom lights and outlets go off, and when I turn off the den circuit breaker all the den light and outlets go off).

So why is this wire in the attic controlled by two circuit breakers? Is this normal? I believe it's a typical 12-2 wire (all wires in house are 12 gauge). Are both wires (black and white) hot? Bad idea to tap into this wire (adding a junction box) to run electricity to my future ceiling fan?

  • Do you have a small incandescent night-light and a receptacle you can temporarily attach to those wires while they are dead, and then turn them back on again? – ThreePhaseEel Apr 16 '19 at 0:57
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    Is it a 3 wire cable (red, black, white, ground)? Sometimes two opposing voltage lines are run together and share the identified(white) conductor for a return path. If it's only a 2 wire cable(B,W,G), in all likelihood you have a cross between two circuits of the same phase. If the latter is the case it should be corrected as it could result in conductors carrying more than their rated current. – K H Apr 16 '19 at 1:26
  • Could be that in a switch box somewhere that has devices on multiple circuits the neutrals got mismatched, resulting in the neutral in this cable/circuit also functioning as a neutral in another circuit. Which is not a good thing. – manassehkatz-Moving 2 Codidact Apr 16 '19 at 1:51
  • Thanks for the responses. I'm sure it's a 2 wire cable (B,W,G), it's stamped on the wire. I had the whole panel replaced recently when I moved in the house as I had a Federated panel which was flagged by the home inspector. Sounds like I might have a wiring problem in the circuit panel. Is this something I could easily see if I take off the cover? – Danno Apr 17 '19 at 1:14
  • It's possible you're seeing phantom voltage. If so it will likely disappear if you plug a load into the circuit while it's disconnected. – Miles Budnek Apr 17 '19 at 2:54

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