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I have two cables going into a ceiling electrical box; how can I tell which is coming from the panel?

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  • Did exactly that so now light has been removed and wires are capped off. When breaker is on both blacks are hot, When breaker is off both blacks are not hot. I can turn one of the wire off by the wall switch but the other is not effected by a switch. Since I only need one set of wires ( black, white and ground ) can I just permanently cap off the second un-needed wires and use the one set that is controlled by the switch and just wire to the light?
    – lcm1947
    Commented Jul 27, 2020 at 11:29
  • I think your comment here is in response to manassehkatz's answer below - putting it down there would have made more sense. The one that's hot when the wall switch is off is the hot power coming in from the box. Cap (but do not cut) the unneeded wires (don't cut them, you never know when they may be needed again - if you cut them short, they'll be unuseable).
    – FreeMan
    Commented Jul 27, 2020 at 12:13

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  • Turn off the appropriate breaker
  • Make sure, with a non-contact tester, that all the wires are "dead"
  • Take pictures of the wires as currently installed, so you know what goes where
  • Disconnect the wires and cap them with wire nuts for safety
  • Turn on the breaker
  • Check with a non-contact tester to determine which wires have power

As others have noted, non-contact testers can give false positive readings. That is good from a safety standpoint, but sometimes gets in the way of figuring things out! If a non-contact tester shows both cables "live" even after they are disconnected from each other, then either one is a false positive or there is something really wrong - unusual but possible. The next test is with a multimeter to check voltage. That eliminates some, but not all, phantom readings. If a multimeter shows one of the cables with a low-but-not-zero voltage, that is typically a phantom voltage. But if it shows something close to the usual 120V, it could be real. The final test is to (carefully, doing any actual connecting/disconnecting of wires with the breaker safely off) put a load into the circuit. With a light fixture, the simplest thing is to connect only one of the cables and see what you get. If that shows "live" then disconnect it and connect the other cable and see what you get. Of course, a switch (if properly wired) complicates things too. Sometimes the solution is to think through the possibilities logically, carefully and methodically to figure out what is actually going on.

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  • Be aware that a non-contact tester will often tell you that both cables have power when in fact only one of them is live.
    – A. I. Breveleri
    Commented Jul 27, 2020 at 10:09
  • Thanks for confirming that non-contact testers are not 100% accurate like the contact testers are. How anyone can trust a non-contact tester is beyond me.
    – lcm1947
    Commented Jul 27, 2020 at 11:39
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    I trust them to give a warning, that's their purpose, they are not measurement tools.
    – Jasen
    Commented Jul 27, 2020 at 11:58
  • Since I mainly use them after flipping a breaker off to double check that the wiring or device I'm working on is indeed dead they don't cut it for me. The little two prong lighted tester is tons safer in my opinion.
    – lcm1947
    Commented Jul 27, 2020 at 12:47

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