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I have a trio of: (2) three-way switches + (1) four-way switch connected to a trio of hall lights (controlled as a group), and am trying to replace one switch with a smart switch and still maintain four-way switching.

Is this possible?

I'm guessing I need to replace the panel side three-way switch with the smart switch, leave the four-way as-is, and rewire the load side switch (disconnect one traveler and bypass straight to the lights, and connect the other traveler and neutral to the switch)?

That would give me the following setup:

  • Smart switch on the panel side:

Line input

Switched and traveler outputs to four-way switch.

  • Four-way switch in the middle:

Switched and traveler inputs from smart switch

Two traveler outputs, one to lights, one to third switch

  • Three way (now acting as standard switch)

Traveler input from second switch

Neutral output

For reference: Panel side switch

enter image description here

Four way switch

enter image description here

Load side switch

enter image description here

Smart Switch

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  • What make/model of smart-switch are you trying to use for this, and can you post photos of the insides of the switch boxes involved please? – ThreePhaseEel Feb 18 at 12:45
  • I added the pictures and link to the smart switch to the post. @ThreePhaseEel – user130387 Feb 18 at 15:16
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    Backstabs: Kill With Fire™ (While you're in there, undo the backstabs, make a hook in each wire, and screw them down tight. The backstabs can come loose over time, causing things to not work some times, or worse, causing arcing and fires.) – FreeMan Feb 18 at 15:25
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    Already planning on it! @FreeMan – user130387 Feb 18 at 15:29
  • @user130387 -- is getting a different smart switch an option for you? – ThreePhaseEel Feb 19 at 0:07
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You can make it work, but the results aren't going to be the greatest

The good news is that since your smart-switch supports 3-way switching and uses the neutral for power return, you have a couple options for making this switch work in your situation. The bad news is they both come with limitations as to the results you'll be able to get.

Option one: traveler shenanigans for fun and for profit

Based on the possible wiring diagrams given in the manual, your smart-switch uses an injected-signal sensing scheme to determine if the remote switch is open or closed. This can be extended to four-way switching with the switch at either the panel (line) or light (load) ends of the complex, but may yield counterintuitive results based on how your switch interprets the traveler input and how you wire the two switches. Furthermore, you're constrained on how you connect the two switches based on the wires you have in the wall; unless you replace one of the inter-switch-box cables with a /4 (line or load, neutral, and two travelers), you're stuck with either an AND (series) or OR (parallel) connection for your remote switches, vs. an XOR (three-way) connection which will likely yield better-behaved results.

Option two: relays to the rescue

Your other alternative is to use something called a relay (basically, a solenoid-coil-operated-lightswitch) to "translate" the single-pole switching (ON/OFF) command from your smart-switch into something meaningful to the four-way switch complex. The upshot of this is that you will get somewhat more intuitive behavior from the remote switches; however, the behavior of the smart switch itself might not be quite as intuitive, depending on the full scope of functions it provides.

The challenge here is that you can't just buy any old relay and whack it in, as most relays aren't intended for direct application in a mains electrical circuit without further blessing by UL. However, there are some encapsulated or enclosed relays that are UL listed for field wiring use. In particular, the Functional Devices RIBU1C provides an attractive option as it's listed for energy management and lighting control service as well as general signaling use and also fully rated to switch up to 240W of tungsten load when used in a multi-way configuration. (If that's not big enough for you, you can use a RIB2401C instead, which will handle just about any lighting load you can come up with.)

However, it comes in its own enclosure, designed to mount to the outside of the junction box in question via its integral nipple and a conduit locknut. Since your panel-side box appears to be metal, punching out a KO in the box and mounting the RIBU1C to it isn't a big deal, as it can be safely buried in the wall by dint of being in a plenum-rated enclosure; however, that requires some drywall work. Given that that may not be an option for everyone, and given that lighting draws much less power in our LED age, the Air Products & Controls PAM-1 can be a suitable alternative; it's also UL listed, but not as extensively tested and rated as the RIBU1C (it has a "generic" 10A @ 120VAC contact rating), but has an advantage over the RIBU1C in that it's an encapsulated module designed to mount in an existing junction box.

Once you have your relay picked out and mounted to/in the panel (line) side box, it gets hooked up as follows:

  • Contact common (blue on the PAM-1, yellow on the RIBU1C) gets nutted in with the incoming hot wire and a black pigtail to the hot terminal on the smart switch
  • Contact NO (orange for either) gets nutted to the black traveler wire
  • Contact NC (yellow on the PAM-1, blue on the RIBU1C) gets nutted to the red traveler wire
  • Coil 120V hot (black on the PAM-1, white/black striped on the RIBU1C) connects to the LOAD terminal on the smart-switch
  • Coil 120V neutral (white on the PAM-1, white/yellow striped on the RIBU1C) connects into the existing neutral (white) bundle along with a white pigtail to the neutral
  • Coil LV hot (red on the PAM-1, white/blue striped on the RIBU1C) gets capped off with a wirenut
  • And the existing grounding pigtail gets connected to the grounding screw on the smart-switch

Note that the traveler terminal on the smart-switch is unused in this configuration as it is effectively switching a single-pole load (the relay coil) ON/OFF now, and neither the PAM-1 nor the RIBU1C has a ground terminal, as neither module has anything on it that would need to be grounded.

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  • Sounds perfectly reasonable! I don't know why using relays never dawned on me. Like you said, with a metal junction box and unfinished drywall in the panel-side garage box, installing the RIBU1C should be fairly straight-forward. To clarify, I would swap out the panel-side 3-way switch with the smart switch, install the relay, and leave the other two switches as-is? @ThreePhaseEel – user130387 Feb 19 at 5:56
  • @user130387 -- yeah, you'll have to do some drywall work when you install the relay but other than that, you have the right idea – ThreePhaseEel Feb 19 at 12:42
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We can't say. It will vary on smart-switch design. As you may know, 3/4-way switches operate by making one traveler hot, and the other not. Any given 3-way or 4-way exchanges which one that is.

3-way diagram

4-way diagram

It's hard to draw, but that 4-way is a reversing switch. It exchanges the two yellows, or not. By the way, I really like re-marking travelers with yellow tape or shrink tubing, it makes complicated boxes much simpler by eliminating confusion. As you can see, there is no need to distinguish travelers from each other.

Anyway... either the smart switch will want to sit at the position nearest the lamp and have 2 supply inputs that exchange... or, it will want to sit at the position nearest the power supply and exchange its outputs. I would think the last one would be easier to manufacture. Your particular 3/4-way layout is more like the above picture, with neutral in all locations, so you're all set. Lots of people have it more like the lower picture where neutral is only in certain boxes.

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  • A thought for 4-way switches: maybe an extra color - e.g., in this example, change one of the sets of yellows to blue. That way there isn't any confusion if you ever have to replace the 4-way switch. Of course, if each set is coming in via a cable then it would be easy enough to figure out. Just a suggestion. – manassehkatz-Moving 2 Codidact Feb 18 at 19:06
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    @manassehkatz-Moving2Codidact My rev 1.0 of this drawing did exactly that. I eliminated it because I felt cases where you couldn't distinguish the traveler pairs would be rare (and a codevio anyway, since you must identify groups). Actually I had a 4-way that was on a spur... of conduit! All 4 wires same color. I eliminated that (since I was eliminating the 4-way and turning it into independent 3-way strings). – Harper - Reinstate Monica Feb 18 at 19:47

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