Technology and Society

Tracing Circuits

30 Comments

I have a switch in the house that I have no effing idea what it does. I’d have thought that it would be easy to buy a tone generator that can be detected through drywall to follow the wire, but after going to Home Depot, and searching on line, I’m coming up empty. What am I missing?

[TUesday-morning update]

I’m not looking for theories of what it might be, I’m looking for a tool that will tell me.

30 thoughts on “Tracing Circuits”

  1. If the switch isn’t known to do anything, remove it and no one will miss it?

    I have a mystery switch that operates a fan that ventilates the basement (Midwest, not Florida without basements). Have no idea what the fan is for. Maybe for radon abatement?

        1. Very similar to what I have. It should be under $50, Home Depot should also have them and any good local electrical supply will have a better version at more $. If there’s a good rental shop around then as mentioned below you may get a better unit.

          You should be able to trace the wire through the wall, the tone detector has a sensitivity setting. Might be wise to turn off the main breaker to reduce shock hazard and RF noise.

          1. I don’t get your need to trace in it the wall. You should be able to detect it at whatever other service point it is at, likely another outlet or a light fixture. If you can’t detect it at either, then… well… question answered right? And in that case just disconnect it and plaster it up or put on a cover plate over it if its gonna keep you up at night.

          2. Yes. It will work to a depth of several inches, will probably even work through concrete if it’s not full of rebar.
            The tone is extremely strong if the detector is touching the connected wire anywhere along it’s length, but it radiates RF so it works at a distance from that wire.

  2. I’ve had issues like this before.

    First off, where is the switch? Second, is it in a lone plate, or with something else? The reason I ask is if it’s, say, in a room, and shares a plate with a room light, it might very well be for a ceiling fan that’s no longer there (or never was).

    Another common cause is a switched plug where only one of the two outlets is switched. If that’s not the case, it may well be that someone replaced such an outlet but hooked it up as unswitched.

    Take the wall plate off and have a look to see if it’s even hooked up. A further clue would be to find out which fuse circuit the power feed comes from. If it’s something like “Master bedroom lighting” that can narrow it down, but remember that “lighting” circuits often include wall outlets.

    If all else fails, you need a good RF tracer, a cheap one won’t do the job in most cases. I rent one from an equipment rental store when I need one.

    1. “Another common cause is a switched plug where only one of the two outlets is switched.”

      I have two of those in my apartment–the outlet’s several feet from the switch, and only the top outlet is switched–the bottom one is always on, although I guess someone could wire ’em the other way. If you didn’t know about that, you could plug something into the bottom outlet and flip the switch all day and you’d never realize it did something.

    1. OK so less humor more help.

      I like this approach. To get it to work best, temporarily replace the switch with an outlet and then plug the transmitter into it. You can then use the receiver to trace it back at least to the circuit breaker it is on. If the electrician did a halfway decent job wiring the house he/she will have labeled what part of the house that breaker is on. Then you can use the receiver to go outlet hunting in that part of the house.

      1. I’ll spare you the details of hot vs neutral and the color scheme.
        If you don’t know these then best read up first before attempting to do this or hire an electrician.

    2. The two things I like about this device is 1) it will tell you if an outlet is wired properly and 2) it will work with a hot circuit. No need to shut power off in the house or disconnect wires. Well maybe you would prefer to do that while replacing the switch with an outlet. But then you can turn the house back on while you search and once you’ve found the breaker it’s on you won’t have to power down the whole house from then on. Of course you can do the two person trick and have one person at the switch looking at a voltmeter while you toggle each breaker individually, then once you’ve done that reverse roles and have person 2 toggle the switch while you use the voltmeter on various outlets and light fixtures your electrician hopefully clued you onto. I suspect Patricia won’t stand for this. So the RF tracer is worth the ~$50 to preserve marital bliss…. 😉

  5. Um, stupid question, but did you take the switch plate off to see if it is even wired up?

    Dr. House would want at least a blood workup and patient’s temperature taken…. 😉

  6. Reminded me of this Steven Wright joke:

    “In my house there’s this light switch that doesn’t do anything. Every so often I would flick it on and off just to check. Yesterday, I got a call from a woman in Madagascar. She said, ‘Cut it out.’”

  7. Then I’d go with the four most likely suspects:

    1) An interior single outlet, other one likely on always.
    2) An exterior single outlet, other one ditto.
    3) An interior ceiling light, burned out, possibly defunct or never installed (covered by plate).
    4) An exterior light that is burned out.

    Since you don’t own a house in the northeast I’d eliminate this possibility:

    5) A not-up-to-code switch to your oil burning furnace. (They are supposed to be marked as such). 🙂

  8. Or this one I haven’t seen since childhood:

    6) Overhead light to either your outhouse or chicken coop..

    🙂

  9. Here’s another common cause of mystery switches; many homes have switched outlets in the garage ceiling for the garage door opener. In many cases I’ve seen, only one of the two outlets is switched. If the opener is plugged into the other one, the switch won’t do anything noticeable.
    If this is the case, the switch is probably inside the house, but not all that far from the door to the garage, or maybe in the garage itself.

  10. Also could be a Christmas light receptacle in the soffit outside your front door … unless your house pre-dates the popularization of that feature.

  13. Were you flipping that switch on and off a lot on Monday? Because my ceiling fan kept starting, running for a little while, then stopping…

    P.S. There are a slew of tone and probe tracers on Amazon.

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