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Okay, so it's like this: my workshop has surface mount metal electrical boxes linked with metal conduit throughout. The switches (a single 2-gang box that housed two single pole switches side by side) that controlled the overhead lights were located in an asinine location (in the MIDDLE of the pitch-dark room filled with sharp stuff).

I decided I'd like to use the snazzy smart switch (with timer and occupancy sensor with built in override, et al) to manage the lot of em, so I figured I'd cut out both switches, join em together into one. It was only after I'd cut and spliced everything to right next to the main door I realized: there are two CIRCUITS running into that box. Two separate breakers, each controlling half the lights.

How CAN I wire things up such that the single smart switch acts as the power on/off for ALL the lights?

I have to assume there's some kind of switch that draws power from neutral (or, in this case, the load wire from the smart switch) that functionally is in an "on" stare when energized and off when not. I don't want to screw with trying to rewire the whole ceiling, nor do I want a sub panel. I acknowledge it's safer to keep the load shared across two 15A breakers, too.

But, because this fancy schmancy switch does all this sensor-driven hoopla, simply stitching in another fancy one won't work (there'll be edge case inconsistencies), and stitching a vanilla single-pole alongside it defeats the POINT of said schmanciness.

Really, what I need is some means to complete Circuit B when this switch governing Circuit A has Circuit A energized. Are there 15A relay switches rated for home electrical system use? Everything on both circuits is done on 12/2 (1971).

What kind of switch do I seek?

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  • To be clear: this is not a "can I" question. It's a "can I without violating code and/or burning my house down" question. I'm not sure what kind of switch or relay is best suited for safely carrying out such a task. I'm looking at 120v, 20A ONOO relays, but I'm not seeing anything that looks intended for home wiring (eg a.co/d/egQV2mk). I'm not asking for brand recommendations (though I'd not complain), simply what FLAVOR of device can I legally and safely use for this?
    – NerdyDeeds
    Apr 20, 2023 at 3:50
  • The right answer will depend on whether 0, 1 or 2 of the existing switches are switch loops - i.e., breaker -> switch -> light vs. breaker -> light -> switch. Can't tell without seeing the switch connections. Upload pictures showing all existing wires/cables going to each switch. Apr 20, 2023 at 3:50
  • 2
    While you are talking about relays, etc., if the only stuff controlled by the two switches is a bunch of lights, and the lights are all now (or can be changed to) fluorescent or, even better (in terms of total power required) LED, they should all fit on one 15A circuit with plenty of room to spare, so the simple solution may be to simply merge everything into one switch, which you then make "smart". But can't tell without details (pictures) of the existing setup. May be super-easy. May be incredibly hard. Though since you have conduit, running new wires if needed is possible. Apr 20, 2023 at 3:52
  • What make and model is your smart-switch? Apr 20, 2023 at 4:32
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    ONOO is the wrong approach, you want a relay from an electrical manufacturer like Siemens, Square-D, or Eaton.
    – Jasen
    Apr 21, 2023 at 0:29

6 Answers 6

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One very viable way to solve this problem very nicely is to simply replace both of the existing switches with the Lutron Caseta series smart switches. Lutron also has the PICO remote module which is a small remote which can then be paired with both of the smart switches so that one control operates both switches even though they are on separate circuits. The PICO remote can be wall mounted in the Lutron wall mount adapter plate so that it looks much like any of the other Caseta switches. If you wanted to have two PICO remotes in the garage at opposite entries the Caseta switches can be paired with multiple PICO units. The PICO units operate from a replaceable coin cell that lasts 10 years so no AC power is need at the locations where you place their wall mounts.

I have no affiliation with Lutron but have used their products throughout my house for almost five years now and have really liked the product and its reliability. In fact I had a similar problem to yours in my garage where there were added lights on multiple circuits. In some cases the added lights were plugged directly into outlets on the ceiling (originally being switched via pull chains). I added the Lutron plug-in lamp modules for three of those type lights and added two Caseta smart switches for the other lights. Finally two wall mount PICO remotes are paired with the two switches and the three plug in lamp modules and allow lights on/off control for all the lights from two locations.

Overall this solution uses agency listed components throughout and not requiring relays and contactor gadgets that never work very well with conventional house wiring setups.

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  • My initial reaction was that this could be generalized to any smart switches and controller, EG Z-wave switches and a Flic switch or Alexa. But .... and TIL this ... the Pico remotes work with no hub, no Wifi, no "account". I'll look a these next time I'm installing lights. BTW you could think of smart switches as "relays and contactor gadgets" that do work well in residential.
    – jay613
    Apr 20, 2023 at 13:09
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Two easy ways (hopefully easy, depending on details)

  1. With a relay/contactor to switch the "other" circuit
  2. Combine the two circuits or at least the lighting parts of them onto one breaker.
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Another approach: Put the smart switch on one circuit. Put another smart switch on the other circuit. Use home automation programming so turning on or off one triggers an action to turn on or off the other -- linking them in radio/software rather than wiring/hardware.

I presume you already have a hub of some sort which can monitor and trigger these events, since you have the first smart switch.

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  • OP's "smart switch" isn't that kind of smart switch. It just has a motion detector.
    – Ben Voigt
    Apr 20, 2023 at 16:11
  • Ah, missed that. So they might have to replace both.
    – keshlam
    Apr 20, 2023 at 18:10
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Add relays to your two different circuits And then you can control Both two relays simultaneously from one switch

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I already see answers of "use a relay", but your question is how to do it within regulations, and I suspect that hooking up an industrial relay won't cut it.

Looking at a trade supplier I do see relays or contactors (big relay) that are intended to be mounted in a consumer unit, by substituting for one or more MCBs or by starting with an empty unit and filling with blanks as required. Unfortunately I see you are on 120V and the parts I see are 230V type but hopefully the principle still stands.

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You are looking for a Double Pole switch. This will allow you to connect two different circuits to the same switch without bridging the circuits.

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