I've read up on three way switches, and have replaced a few over the years. I recently installed a bathroom fan, and so had to replace two three way switches. Very simple setup - two switches, one fixture with four lights. As soon as I looked at the old wiring, I got confused. After removing the switches, I did some testing. Each switch box has a hot line coming in. By that, I mean that with no switches installed, each box has a hot pair into it. The only thing that I can figure is that it was a Carter system, as that is the only diagram that I have seen with hot/main into both switches. A little disturbing, as this house was build in 1970, and Carter hasn't been code for ages.

Is my assumption correct? Is Carter the only one with main/hot to both switches? If so, what do I do from here? I'd rather not wire up another Carter setup.

Thanks, Erick

Additional information: I've attached a picture with what I've discovered. My next task (tomorrow) is to open up the fixture and see how it is connected. It seems like 3 way switch in box 2 was wired in series with the rest of the circuit.

  1. I verified A is connected to B by turning off main, then checking continuity with a multimeter. Verified both are hot with main on with a multimeter.

  2. I assumed C by turning off power, then checking continuity with a multimeter. I don't know if there is a junction or anything else along this, but it does connect through.

  3. I have one line going to the fixture (D), but I haven't verified it yet. It could be some combination of C, D and E for all I know.

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I had to admit defeat on this one. I wired up the new switches in exactly the same way as the old. It works, but I still don't understand it. At some point I'll revisit and rewire. Thanks to everyone for their help!

  • are you sure that the wires weren't shorted at the either end? is it also both hot if you remove all the lights (you are sensing residue from the neutral through the lights) Jan 25, 2015 at 0:57
  • I'm not sure how to determine that - would I need to unwire the lights? (this house has had a whole host of wiring issues, so I can't really assume anything about how it's wired.)
    – Erick T
    Jan 25, 2015 at 1:17
  • Just because hot goes to both boxes is not necessarily a cause for panic. But if it is a normal three way switch, wouldn't there need to be two wires going from Box 1 to Box 2? Maybe it was Carter. But you want to wire it as a normal three-way, right? So you either need to find or add another wire between Box1 and Box2. Seems like maybe you could use wire C as a fish to pull a new 2-conductor cable from Box 1 to Box 2. Connect hot in Box 2 to common on switch. Box 1 doesn't need hot. Make sure to put tape on the white wire between Box1 and Box2 so future electricians know it might be hot.
    – mkeith
    Jan 25, 2015 at 8:53
  • Can you take photos of the box innards, or describe them in more detail? This could very well be a case where B goes to other things in Box 2 and to E, but is not connected to the 3-way switch in Box 2 or to C. Jan 26, 2015 at 23:08

3 Answers 3


It's possible that you are detecting residual voltage from the neutral through the lamps at one switch (in the order of 50V often), you can use a multimeter in voltage mode and see how much voltage you actually detect. Or take out the bulbs from the fixtures and check again.

Other wise you can (with the breaker off) connect 2 wires at one switch then check continuity of the wires at the other end (with a battery and little bulb for the poor-man's checker). If the non-lives are those that connect on both ends then it's fine.

  • I verified voltage, and it was the same. There was residual on another pair, but I discounted that. I added additional info that might help shed light (ba-da-bing!)
    – Erick T
    Jan 25, 2015 at 2:04

There are a few possibilities.

Perhaps the hot-and-neutral pair coming up cable B are merely passing through box 2 on their way to cable E, and they don't connect with the 3-way switch at all. In that case, cable C would be a 14/3 (or 12/3) containing 2 messengers and a common, in the usual style.

Perhaps you have a Carter 3-way system, which has been illegal since 1923, but some electrical "genius" might have "reinvented" it. This would require "hot" and "neutral" to be present at both 3-way switches, but would only require a single wire in cable C.

If I were you, I'd install a modern "smart" 3-way switch in box 1, with its power fed by cable A. I'd put a "smart switch" remote in box 2, with its power fed by cable B. Hence, A-D would be one circuit, and B-E would be the other. I'd shop carefully for smart switch/remote combos that didn't require a signal wire between them, or, if they did, were able to reside on different circuits and use wires in cable C for communication only.


In the 1960's/70's (in Canada at least) it was normal to run a three wire with two hot lines to save on wiring, even if you removed one of the hot lines, there was still another that was live.

Have you traced back both lines to the box? A three wire with this setup will have both the red, and the black line connected to separate breakers.

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