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I am doing a kitchen remodel that involves some rewiring. I am wiring a 3-way light switch, and my wiring diagram looks like the following drawing:

enter image description here

However, the light does not turn on in all 4 possible orientations. I have tried multiple combinations, and cannot figure out the error. Note, "Switch 1" is a dimmer switch.

I have correctly identified the incoming line (new wiring) but cannot find which of the travelers/neutral is which on switch 2 (old wiring without any flags or coloring.) Using a voltmeter, I can get a 120V reading when connecting two of the wires when switch 1 is up, and 120V for another combination when switch 1 is down. Obviously, one wire is present in the 120V reading for both orientations of switch one.

Voltmeter reading 120 depending on switch 1 orientation, unknown (uncolored) wires at switch 2

My question here is, with the information I have from the voltmeter, how can I tell which line is which at switch 2? And how should I wire this to get a functioning 3-way switch?

enter image description here

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  • Are you sure you have a 3-way dimmer? Most modern dimmers need a neutral, so the switch would require 4 terminal plus ground (5 wires) Hot, neutral, two travelers and a ground, or a special 3-way as slave that the dimmer has ground, neutral, hot, hot between switches, and single communication/signal wire between switches. Feb 1 at 17:02
  • Yes, it is a Lutron Toggler Dimmer. On the box it says, "Single pole / 3-way" For a light(s) controlled from 1 or 2 locations. Feb 1 at 17:02

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Assuming you have a functioning ground in each box, which is not 100% guaranteed, but should be the case for any 1960s or later house, and many earlier ones as well, and

assuming your 3-way switch setup is as drawn: Panel -> Switch 1 -> Switch 2 -> Fixture

  • Turn off the breaker.
  • Check each wire for continuity with ground (ground wire or metal box).

When all wires are connected properly you should have exactly one wire in each cable that is continuous (or extremely low resistance) with ground. That is your neutral wire. The other 2 wires in each cable are then, by definition, travelers.

However, while the first assumption (ground present everywhere) is a pretty reasonable one, the second assumption (simplest 3-way switch configuration) is NOT a safe assumption to make without a lot more details. In addition to some other more complicated scenarios and some non-code-compliant scenarios, there are two very common 3-way switch configurations which do not match your diagram:

  • Power -> Fixture -> Switch 1 -> Switch 2

In this case, neutral connects from the incoming power line at the fixture and hot and switched hot go down to Switch 1 and the /3 cable could be hot/travelers or switched hot/travelers.

  • Power -> Switch 1 -> both Switch 2 and Fixture

In this case, the Switch 1 box sends neutral directly from the incoming power line to the fixture and the /3 cable could be hot/travelers or switched hot/travelers.

A lot can be figured out by:

  • Identify which cable is incoming hot (turn off the breaker, mark and disconnect all wires from switch 1 and switch 2, turn on the breaker and check each wire with a non-contact voltage tester - the one hot wire is the incoming hot and should be paired with the incoming neutral)
  • The number of cables in each box will determine the rest. /2 + /3 in switch 1 and /2 + /3 in switch 2 will match Power -> Switch 1 -> Switch 2 -> Fixture. /2 + /3 + /2 in Switch 1, /3 in Switch 2 will match Power -> Switch 1 -> Switch 2 and Fixture, etc.
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    Thanks for a quality answer. One other observation is that my voltmeter reads ~40V when testing between the hot and one traveler on switch 1 and the load to the light and one traveler on switch 2. These readings do not change when the light is turned on or off. Feb 1 at 17:59
  • ~40V sounds like "phantom voltage" Feb 1 at 18:03
  • That's what I gathered from the other things I read. What's weird though is that it reads 40V no matter the position of the switch, meanwhile the other traveler that reads either 0V or 120V depending on if the light is on or off. Feb 1 at 18:07
  • That is interesting. That implies that the wire is actually not attached properly and is getting an induced 40V. But there are other possibilities. Feb 1 at 18:13
  • I'm wondering if it is a loose connection then. I didn't include this in the diagram, but I ran new 3-way wiring from switch 1 to a junction box in the attic above where it is attached to the old 3-way wiring (without colors or flags) which then runs to switch 2. There were reasons for me doing it this way, rather than running all new wiring. I could've sworn when I first wired this it was working correctly, and one day it started having trouble. Now I'm wondering if a connection is just loose. Feb 1 at 18:22

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