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I have two lights on two separate circuits connected to separate switches in the same box. The lights control overhead lighting in the same room and there's no reason (code/etc) they shouldn't have been on the same circuit in the first place. I don't want to rewire the ceiling and would like one circuit to be completed only if current is flowing through the other circuit. Is there a type of switch that does this or, alternatively, a type of switch that toggles two circuits simultaneously?

Edit: A simple double pole switch won't work, because one of the two switches is 3 way.

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    Are you looking for both of these to still have separate switches, but one is subordinate to the other? In other words, switch A is primary and must be on in order for switch B to work? Or would you prefer that all the lights are switched together by switch A and switch B is simply eliminated? Also, we'll be able to give more specific suggestions if you clarify whether the 3-way is to be the primary control, the subordinate, or the one to be eliminated.
    – Greg Hill
    Commented Jun 25, 2020 at 20:39
  • @GregHill I want switch A to control everything and switch B to be eliminated. Switch A is the 3-way. Unfortunatelly because of the way the circuit is wired, I cannot simply take the light connected to switch B and connect it in parallel with the light on switch A, it needs to remain powered by a separate circuit.
    – Michael Jarret
    Commented Jun 25, 2020 at 21:37
  • Can you post photos of the boxes involved please?
    – ThreePhaseEel
    Commented Jun 26, 2020 at 0:08
  • One reason for them to be on different circuits (particularly depending what else is on each circuit) is so that the room is not left in the dark in the case of a breaker trip. I prefer to keep lights and outlets on completely separate circuits, which makes the odds of a lighting circuit trip very low.
    – Ecnerwal
    Commented Jun 26, 2020 at 1:16
  • @Ecnerwal I assure you, there's no reason. The previous owners (or whomever they hired) did some crazy things with the wiring in general and I'm slowly undoing them.
    – Michael Jarret
    Commented Jun 26, 2020 at 18:32

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A relay could be added to the circuit so that switch A, the three-way switch, controls the power on the relay's input. The relay's normally open (N/O) and common terminals can connect in place of switch B. When switch A is powered on the relay will close and this will allow current in the second circuit to flow to the lights formerly controlled by switch B.

There's a caveat: you'll need access to the switched output of the 3-way controls to make this work. You'll have to unpack the two switches out of that box to find out whether the A-switched output is available there.

Relays have three important ratings: the control voltage, the load voltage, and the load current. Load voltage and current ratings vary depending on the kind of load: motor, fluorescent lamp, tungsten lamp, etc. You'll have to search for one that meets your requirements. They can sometimes be found as a convenient UL-listed module packaged with pigtail leads like the one below.

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