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I went over to my parents' house and am trying to fix a combination switch. The old switch stopped functioning recently and my Dad had removed it. I do not know how it was wired previously but my Dad mentioned that it was non-standard. The old piece was a double toggle with two single pole switches similar to this product.

He tried installing a new switch but could not get both circuits to work.

I then attempted to diagnose the problem by reading voltages of the wires in the box. I first measured all wires in relation to the ground and had 4 at 60v and 1 at 0v. This didn't make sense so I started measuring the difference from each wire systematically. The annotated picture below is what I found: wires with voltages

I have a couple of questions now:

Why would there be 60v on the white neutral and the ground from the 3 cable romex?

How should I wire up a new switch? (It is the same product that was linked previously)

Thanks in advance.

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  • Harry, what we need is the other 1/2 of the puzzle. We could make a guess that the white black pair is part of a switch loop or that the real power is on the black of the 4 wire cable guessing is a way to see if your breaker panel is up to snuff or if things start melting. If we can get the connections at the light or wherever the cable goes we can help you. , just a note most meters have a very high internal resistance to not load the circuit and they pick up phantom voltages a voltage but no potential or current that’s what I would say about the 60v readings but not the 120 , but it could be
    – Ed Beal
    May 12 at 21:00
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    Ehere did you get your ground reference? I see only 1 ground and it says 60V. May 13 at 0:58
  • @Harper-ReinstateUkraine Duplicated from my comment below: So in the initial post I described that I measured voltages in relation to the ground. Starting from the bottom left white wire and moving around clockwise I measured: ~0v, ~60v, ~60v, ~60v, ~60v. This didn't make any sense to me until I realized that my ground must have a voltage near 60v. The voltages in the picture are measured in relation to the top right black wire. The voltages are the same when using the red wire as the reference.
    – Harry
    May 13 at 15:02
  • OK, so in your perspective, the upper right black wire is ground. You are a bird on a wire. The wire you are sitting on is ground. The planet is 120V. May 13 at 20:46

2 Answers 2

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So what you have here is almost certainly a switch loop and a three-way switch.

The top cable /2 white is the constant hot from the fixture and the top cable black is the switched hot. Those go on one switch and the white wire needs a marking to show that it’s not a neutral conductor. Wrap it in black electrical tape with the power off.

The bottom /3 cable is a bit more tricky because there’s a lot of options. It could be part of a three-way switch circuit or it could be intended to send constant power to more than one other fixture in addition to being switched. It could also carry a neutral. But it’s most likely a three-way switch.

As to the weird voltages — that’s almost certainly phantom voltages due to having constant power in the same cable as non-constant power. If your multi-meter has a mode that is labeled as “low impedance” then you can use that to get rid of the phantom voltage and get a more accurate reading.

Ask your dad if there’s another switch somewhere that also turned off and on the fixture of one of the switches you’re trying to replace, and that will point to a three-way circuit. I know you said that it’s two single pole switches but there may have been a three-way at one time.

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  • Thanks for the response. Your intuition is correct in that the bottom cable is part of a three-way switch circuit but the other switch hasn't been operable in longer than my Dad can remember (I can open that up and take a picture of what's going on there if that is helpful). We don't know why it was set up to work as a single pole circuit but I'm hesitant to tackle that problem before making it work as it did previously. I didn't mention it initially as it complicated the initial post and I wasn't sure if it was useful information.
    – Harry
    May 12 at 22:03
  • @Harry one thing you can do is use the multi-meter to see if the voltage change when the other switch is activated. It’s possible that the three way switch doesn’t work merely because of the use of the single-gang, two switch both single pole device that you linked. But I do know that it’s possible to buy a single-gang, two switch unit where one of the two switches is a three-way and the other is a single pole. So you may be able to get the three way switch working again if that’s what your dad would like. May 12 at 22:08
  • Unfortunately the multi-meter doesn't have a low impedance setting so I can't assuage my concerns about phantom/real voltage on the ground line. I took your advice and checked the voltage change when I toggle the other three-way switch. Doing so does change the voltage. The down position is depicted in the picture. The up position puts 60v on the orange line and 0v on the lower white line. All other wire voltages remain unchanged.
    – Harry
    May 12 at 22:53
  • @Harry that suggests to me that the white and red wires on the lower cable are the travelers and the black on the lower cable is the common. You can certainly find a single-gang dual switch product that has a three-way switch on one or both rockers. Check the usual brands. May 12 at 23:19
  • thanks for all your help. One last question, why would the two travelers be alternating between 0v and 60v rather than 0v and 120v? Should I check to see if the ground is touching something in either the fixture or the other switch?
    – Harry
    May 12 at 23:33
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I suspect you have two independent switch loops.

But let's put that to the test.

I think there are 4 pairs of wires that should be tested. (crossing cables is a mistake).

  • Black and white in the /2 cable
  • Black and white in the /3 cable
  • Black and red in the /3
  • White and red in the /3

As a testing procedure, I recommend cutting the main breaker then wiring a receptacle hot and neutral to the wires under test. (or you could lop the plug off a 2-wire extension cord and wire-nut its wires to the wires under test).

Once you have a socket, turn the main breaker on and plug in a big load like a hair dryer or heater. The reason for the load is to provide some resistance so we don't accidentally create a dead short. Under this condition, also go operate the other 3-way switch and see what happens.

  • If a light comes on, and the other 3-way has no effect, we can guess this is a single control for that light. These 2 wires go to a switch.

  • If operating the other 3-way causes a light to go on and off, then we have identified common and one of the travelers. Further testing will probably identify the other traveler.

  • If no lamp comes on, but the hair dryer runs, plug in a small load like a cell phone charger. If that works also, then we have identified regular old always-hot + neutral.

From there you can try to figure out what you have.

If both cables control lamps, then they are independent switches. They make "tandem 3-way switches" just like your old switch, but with 3-ways in one or both positions. Most of those have a "common side" with a tab or blade pull-out, since they are expected to be fed from a single hot wire. You will need to break that tab or pull-out so the switches are fully independent. Never cross wires from 2 different circuits.

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  • Thanks Harper, it is two independent circuits: a switch loop and a three way switch. I was able to get a product with the correct switches and hook everything up so that both three way switches operate the light. It followed this diagram with my picture represented by the box on the right. I never figured out the ground voltage issue but no one seemed to think it was a big deal so I guess I won't worry about it.
    – Harry
    May 16 at 15:46
  • @Harry there are lots of ways to wire things that will work, but then kill you. Indeed I think they are two independent switch loops, but it's important you separate the "tab" to isolate the two switches from each other. Otherwise you are creating a current loop that could cause big problems. May 16 at 19:21

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