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I have a fairly standard setup here. A lamp controlled by 3 switches, the middle of which is a 4-way switch (but I think that's fairly irrelevant). I would like to add an extra lamp, controlled by the same 3 switches. It would be no problem if it were near the first lamp, but it is actually right next to the switch named S2 below. I can't run another wire back from S3 to S2. How can I wire things?

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I was thinking of adding an SPST relay near S2 as shown below. Would that work? Is there a better solution?

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  • Be aware that any voltage drop across the relay coil will make LS1 correspondingly dimmer, so choose the correct type of relay. For this series-wired application you want a current-actuated relay rather than a voltage-actuated type. – A. I. Breveleri Jul 9 '17 at 20:10
  • If I read your question correctly you don't need the relay. Just connect the new light as shown in the second diagram but without the relay. As @A.I.Breveleri points out wiring a relay coil as shown will dim the original lamp. – Tyson Jul 9 '17 at 20:33
  • That's not at all what I pointed out. What I said was that OP needs to choose the kind of relay that will introduce a negligible voltage drop when wired in series with the original lamp. Such relays are available. - OTOH if OP wires a second lamp in series with the first per your suggestion, he would need to choose a kind of lamp that will introduce a negligible voltage drop when wired in series with the original lamp. No such lamps are available. – A. I. Breveleri Jul 9 '17 at 21:12
  • One disadvantage with this circuit is that if lamp LS1 burns out or is unscrewed, lamp LS2 cannot be switched on. - Nevertheless I believe you have devised the best solution for the constraints as described. – A. I. Breveleri Jul 9 '17 at 21:50
  • @A.I.Breveleri Thanks. Indeed, the voltage drop from the relay needs to be minimal. I don't mind LS2 not being able to be turned on if LS1 is out, but good point anyway indeed! Turn that into an answer and I'll accept it :) – Jean-Philippe Pellet Jul 10 '17 at 9:35
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Be aware that any voltage drop across the relay coil will make LS1 correspondingly dimmer, so choose the correct type of relay. For this series-wired application you want a current-actuated relay rather than a voltage-actuated type.

A simple current-actuated relay has only a few turns in its coil, which is wound with wire sized to handle a lot of current. For your application the winding might have one to three turns, and the wire must be at least as large as your household wiring.

When you shop for the relay, you may find it easier to buy an electronic device than a simple relay. This would probably operate on the same principles as a GFCI. Its voltage drop and power consumption will be too small to measure. Such a device may require its own constant-hot and neutral connections, which fortunately you have available at S2.

One disadvantage with this circuit is that if lamp LS1 burns out or is unscrewed, lamp LS2 cannot be switched on. - Nevertheless I believe you have devised the best solution for the constraints as described.

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If you wan the same three switches to control the second light, the. You need to parallel the first light with the second light.

So, you have to connect the second light to the same two wires that the first light is connected to.

OK after thinking about that again, your relay would be in series with the first light. Which may not be a big problem but it will reduce the voltage at the light. It depends on the voltage drop on the relay coil.

So, back to my first suggestion.

  • You'll basically need a current sense relay for the relay solution -- these things exist, but involve a bit more jiggery-pokery than depicted – ThreePhaseEel Jul 9 '17 at 21:15
  • The amount of jiggery-pokery depicted in @Jean-Philippe's second diagram is sufficient for a simple current-actuated relay. If he chooses a more modern electronic device that requires its own neutral, then of course he will need to add a bit more jiggery to the circuit, but if he doesn't use any backstab connections then the amount of pokery should remain unchanged. – A. I. Breveleri Jul 9 '17 at 21:40

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