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On one circuit breaker I have a manual switch. On the second circuit breaker I have a motion sensing switch (using a mechanical relay). I have a number of lights wired in parallel as the load. The lights should turn on as a result of motion or manually turning them on. Unfortunately I can't just wire the two switches in parallel on the same breaker because of physical constraints; I'd have to perform a major renovation to rewire.

It seems I need a sort of DPDT relay like this, which is normally open, has two magnetic coils, and keeps both circuits separate:

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Does a relay like this exist? Is this the correct way to solve this problem?

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    You might want to try this question over in Electrical Engineering. Don't get me wrong, there's a couple guys here that will be happy to answer your question in 6-8 long paragraphs of mindless dribble, but I bet you get a quicker response from the EE's than the DIYers.
    – NPM
    Sep 3 '17 at 9:45
  • If your house burns down because of something related to this, best of luck with insurance claims. I wouldn't screw around with trying to design something this non-standard yourself (especially if you're asking for help here) and would rather pay a licensed electrician to rewire it or do whatever else may be necessary protect the larger investment in your home and family's safety. Sep 3 '17 at 11:20
  • You need to adjust the load (lights) always being on one circuit or the other. After that you might be able to use a mechanical relay to add switch from a device on another circuit. There are also a variety of smart devices that may help you achieve your end goal, but you can't safefuly feed a load from two circuits, you're more likely to cause a fire.
    – Tyson
    Sep 3 '17 at 11:43
  • This question Is a classic X-Y problem question. This can be solved, likely with a relay but not as you ha e drawn it. What you need to tell us about is how the circuit is run, and where the wiring joins (apparently it comes to the same point someplace. Tell us where the wiring in the circuit can't be changed, and where it might be possible to change. Then someone can suggest and appropruate way to switch something on a different circuit, which is really what you are asking how to do.
    – Tyson
    Sep 3 '17 at 14:06
  • What make/model is your motion sensor? Sep 3 '17 at 15:47
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Use a simpler relay

A better plan would be to use a UL-listed SPST relay with a 120V coil driven from the motion sensor, with its contacts in parallel with the lightswitch. (A Functional Devices RIBU1C or RIBU1S will do the trick, provided your lighting load does not exceed 600W.) That way, all your lighting is powered from one breaker, and you don't have to do a bunch of rewiring.

Of course, this requires that your motion sensor power itself through the neutral, or be replaced with one that does, instead of "cheating" and trickling power through the load.

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You can't use 2 supplies for the same load.

You cannot entangle two supplies like this, it will create a death trap.

Aside from that, Mrs. Lincoln, the motion sensor will be a very unhappy puppy if its supply/load relationship is interrupted. Mainly, it will "forget" what the terrain looks like, tend to malfunction as a detector, while the light sensor that keeps it from coming on during the day will also forget what day and night look like. (Since that perspective varies depending on exposure and ambient light). Two better plans:

Smart switch

This is a cakewalk with a smart switches. You power the light off only one of them, the master. The other remote/slave smart switch has no wired connection to the light whatsoever. It uses radio waves or power-line signaling to communicate with the master smart switch. Shop carefully for products which do this.

Relay on one switch

Power the light from the motion sensor switch location. There, install a 12/24V relay with transformer (they make combo relay/transformers).

Use plain old low-voltage doorbell wire to connect the two switch locations, this is only carrying 12/24V so the rules are much relaxed. At the other switch location, install a surface-mount switch with no mains wiring in its junction box... and have that switch short the two doorbell wires. This operates the relay, whose contacts are wired in parallel with the motion sensor. If you wanted, you could use 3-4 wire doorbell/thermostat cable and put 12V motion sensors there.

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First off, why are you trying to operate a load from two different breakers? You don't need to do this. All you need to do is parallel the switches. Then either switch can operate the lights and if both switches are on at the same time nothing bad happens.

You may have L1 and L2 on two different legs of the service and are getting a dead short of 240 volts. That's bad.

You don't need any fancy relays or wiring scheme.

You didn't specify exactly what your goal was but from your drawings this is what I surmised.

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  • I need to operate the load from two different breakers because of the way the existing house wiring is done; it's physically impossible - at least without major reconstruction - to rewire so that both switches are on a single breaker. You're assuming that I can rewire, which I can't. If I could, I wouldn't be asking this question.
    – rcampbell
    Sep 3 '17 at 10:40
  • I did specify my goal, "The lights should turn on as a result of motion or manually turning them on."
    – rcampbell
    Sep 3 '17 at 10:41
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I think this is a perfect application for some simple home automation. You can replace the manual switch with a smart switch, and install a motion sensor that links to the manual switch. It avoids all the complications of two separate circuits.

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