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First time posting on the DIY of StackExchange. I currently have a staircase with a light on the 2nd floor controlled by 2 3-way switches on both floors. I would like to add an existing light to the currently-covered junction box on the 1st floor. However, when I looked at the existing wires in the first floor junction box, I was unable to find a combination of wires to which I could connect a 120V light that would function properly.

On the right part of the image I have diagrammed the 3 different terminals/twists (?) (each has at least two wires connecting) available. When I flip one of the 3-way switches to turn the 2nd floor light off (as pictured in top right), those are the voltages across the 3 terminals. Ditto for when light is on (pictured bottom right).

I also diagrammed the downstairs 3-way switch in case wiring there may need adjustment (ground not pictured) using a current detector to determine flow depending on state.

Addendum: 3-way switch on 2nd flr is wired in the same fashion. However, when 2nd Flr light is off, wire leading to either 1 OR 2 is hot only (!) (the two will reverse if the 1st floor switch is used to turn on the light). When 2nd Flr light is on, in all cases, none of the wires leading to 1, 2, or 3 are hot.

Can someone help elucidate what my current set-up looks like and what changes may be required?

enter image description here Outline/Potential across terminals

I am limited in number of links I can share, so I posted on a separate imgur thread: https://i.stack.imgur.com/kpDOb.jpg

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  • Can you post photos of the interiors of all junction boxes involved?
    – ThreePhaseEel
    Jan 5, 2017 at 2:34
  • There's just one for the 1st floor where I'm looking to install the light (photo added).
    – palffy
    Jan 5, 2017 at 3:03
  • We'll need photos of the insides of the other boxes as well
    – ThreePhaseEel
    Jan 5, 2017 at 3:08
  • Are you referring to the inside of the 2 existing 3-way switches as well as how the 2nd floor light is connected to the ceiling thru its junction box?
    – palffy
    Jan 5, 2017 at 3:25
  • Yes, I am -- this is quite the puzzle, and we'll need all the information we can to decipher it
    – ThreePhaseEel
    Jan 5, 2017 at 3:25

2 Answers 2

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I ultimately was not able to resolve the wiring problem given the current set-up and the wiring in place.

However, I did end up purchasing two sets of https://www.amazon.com/RunLessWire-Simple-Wireless-Switch-Kit from RunLessWire directly ($99.99+s/h each), linked both wireless switches to both relays, removed the existing switches and shorted the appropriate wires, and 2 hours later, I had a working set-up!

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It's hard to make out that rat's nest of the 1st-floor junction box. But it just looks like a circuit distribution box, to me...feeding power from the panel to various parts of the circuit. Your light would be one path that stems from this box. That would mean there are no controlled Hots in this box.

Installing a controlled light in this box would require running a cable from the second three-way switch (which feeds the light) to this box. You would have to determine which switch it is, but it's likely the one on the second floor.

It's a bit moot since you've solved it with other means, but it sounds like you implemented the easiest solution.

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