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At some spot (call it, say, S1) I have a 3-way switch with two incoming travellers from another 3-way switch downstairs (say, S2) and for some reason I cannot easily rewire things between S1 and S2.

I would need a (full) hot wire at S1 and there is none. So I thought: in theory, at any moment in time, exactly one of the two travellers is hot. Is there some device that I could wire at the end of the two travellers at S1 that would recover a permanent hot wire, switching automatically from one traveller or the other?

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This is S1. The two black wires are the travelers, the two white wires go to two separate lamps (and are bridged).

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  • I just described the process of a 3-way switch for @derNincompoop. You might take a look at that. Otherwise we would need a little more information as to what is in the S1 box and what is in S2 box in the manner of conductors. – Retired Master Electrician Jun 11 '17 at 14:06
  • The two 3-way switches work fine. I have no problem with them. I'm just trying to recreate a hot wire from two travellers. – Jean-Philippe Pellet Jun 11 '17 at 14:18
  • Will you ever want to install another switch in the middle in the future? It's possible to rewire your existing 3-way setup to let you do this without any extra gadgets, but that forecloses the use of 4-way switching. Can you also provide photos of the insides of the boxes please? And where on this planet are you? – ThreePhaseEel Jun 11 '17 at 14:47
  • I added some (some kind of primitive) schema. I'm in Switzerland. – Jean-Philippe Pellet Jun 11 '17 at 15:50
  • OK, the 4-way switch in the middle complicates things -- now you will need a gadget if you can't run more wires... – ThreePhaseEel Jun 11 '17 at 15:54
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This can be done with a relay. Some people would delight in a technical solution, while others would not want a non-standard lashup in their house.

Here is one way, using a mains-voltage SPDT relay that is a common industrial catalog item.

monostable SPDT relay

The activating coil is connected between one of the travelers and the neutral. The normally-open terminal is connected to that same traveler, while the normally-closed terminal is connected to the other traveler. The always-hot may then be drawn from the relay common terminal.

Unfortunately this kind of relay wastes a small amount of power (the holding current) when one of the travelers is hot.

Added in response to comments: A bistable relay does not require a holding current. These are harder to find, and probably will not fit into a residential junction box. If you can find one, you will probably also find instructions for wiring it for your application.

But just in case you find one at a garage sale and the instructions are missing, here is a way to use a bistable DPDT to do what you want.

bistable DPDT relay

  • Very nice. Would a two-coil bistable (Latching)?) relay solve the consumption problem? – Jean-Philippe Pellet Jun 11 '17 at 20:17
  • And are these things small enough that I could hide them behind my switch? The ones I find online seem to be pretty large... – Jean-Philippe Pellet Jun 11 '17 at 20:29
  • @Jean-PhilippePellet -- the consumption issue is pretty insignificant (it's a few tens of mA at 120V if even that) – ThreePhaseEel Jun 11 '17 at 20:40
  • Good to know. But that doesn’t answer the question: do latching relays also have this holding current issue? – Jean-Philippe Pellet Jun 11 '17 at 21:08
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    A mechanically latching two-position relay would not require any holding current. - But I could not find a latching relay in any catalog that would fit into a residential wiring junction box. – A. I. Breveleri Jun 11 '17 at 22:14

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