So I recently bought a house, it was built in the 80s and the previous owner had installed X10 switches (before X10 was a thing, when it was called DHC or Decora Home Controls). Anyway, the garage has a circuit which controls two fluorescent light ballasts; one switch by the interior-to-garage door and one switch by the exterior-to-garage door. The interior-to-garage door switch box has six wires coming out of it!

I pulled the switch out away from the wall and unplugged all the wires. I used a touchless probe to determine which one was the HOT, and I used my DMM in continuity mode to determine which ones were the Travelers based on continuity checks with the switch box wires across the garage.

interior-to-garage-door-switchbox exterior-to-garage-door-switchbox

I have two standard 3-way switches that I want to hook up to these boxes but I cannot figure it out. I'm not sure what the 'A', 'B', and 'C' lines are nor what to do with them.

Can someone help me determine how to hook up the 3-way so both switches can control both garage light ballasts?

Because I need to use the light in my garage, I currently have a standard 2-way switch installed in the interior-to-door switchbox with the 'B' wire tied to the neutral bundle and the switch connects/disconnects the wires labeled Hot and 'A'. However, this doesn't work perfectly because it only illuminates one of the two ballasts.

Trying to understand more about it, I tried flipping the 'A' and 'B' in my standard 2-way switch hookup but it still only lights up one ballast and it's the same one. I'm really confused at how to get this working again.


1 Answer 1


My guess is (and this is only a guess pending more information):

The 3 wire cable containing 'A' goes to the light(s). 'A's associated black wire is unused and its white wire 'B' provides neutral for the light(s). The travelers' associated black wire is wirenuted to the feed wire and connects to the other 3-way sw (you labeled it 'C') on its common screw. The travelers and 'A' connect to the 3-way sw with 'A' on the common screw. The wires 'B' and Neutral wirenut together to provide neutral to the light(s). And finally because a conventional sw doesn't need power, get rid of the pigtail you labeled as 'Hot', while keeping the 2 black wires wirenuted together.

The more information I mentioned is this: It looks like there're 2 3-wire cables coming into the top of the box and 1 2-wire cable coming into the bottom of the box. label the wires, including that black wire that looks like it is shoved into the box unused, as to which cable they're associated with, e.g. cable #1, cable #2 and cable #3. Because cables and wires have two ends, knowing where the other end of a wire is can tell you more than what a tester or meter can.

  • You were right the third black wire running with the travelers was just tucked back behind the other wires. I took your suggestion and hooked it up the way you described and both switches work like a 3-way should. However, only one of the ballasts lit up. I wondered if it was because one of the fluorescent lights was burnt out, so I pulled both bulbs out and swapped them. When I switch the light on one bulb lit up really bright, made a loud pop noise then fizzled and went dark. The other ballast still works fine but the close ballast doesn't light up. I tried it again and it did the same thing
    – Greg G.
    Jun 1, 2017 at 0:47
  • I think what's going on with the lights is a separate issue. So before we move on to the lights lets confirm that the switch circuit is right. I thought the unused black wire was in the same cable with wires A & B and you said in your comment that the unused black wire is in the same cable with the travelers. Which is it?
    – Rand
    Jun 1, 2017 at 21:07
  • Actually, the top black wire coming out of the yellow cap goes with the A & B wires (all in one cable), the travelers go with the previously unused black wire. You can see the unused black wire to the left of the sheathing for the travelers, it goes left, up and over the whole bundle.
    – Greg G.
    Jun 2, 2017 at 13:21
  • The more I think about it, I think what's going on is that the A and B wires might be going to the two electrodes one one side of the ballast and that might be why the light bulb is blowing. It only blows on one end, I bet we're putting 110V across the two terminals... still not sure how to fix that though.
    – Greg G.
    Jun 2, 2017 at 13:22
  • In your second picture are all 3 wires, black, red & white in the same cable?
    – Rand
    Jun 3, 2017 at 16:16

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