Three-way switch wiring question

I live in America, so the electricity coming out of the wall is 120 VAC. Let’s say that I have a light fixture connected to a standard-issue three-way circuit. Power from the main circuit box feeds into a switch via L, N wires plus a ground wire. That switch is then connected via L, N, traveller, and ground wires to a second switch box. The second switch then feeds into the light itself.

So let’s say I want to convert this to a smart switch. Can I install something like a Sonoff Touch with into the switch location fed by mains and then use the traveller wire and the AC ground to use a GPIO pin and the AC ground to connect a switch in the second location (with the L and N wires in the second switch box just connected to bypass the switch entirely)?

Basically, can I do this:

(Use an AC circuit’s ground wire as the ground in a low-current 3.3VDC circuit?)

I don’t think that would ultimately meet the intent of the electrical code.

You have to remember what the purpose of the ground wire actually is… It provides a safe path to ground in the case of a line side (hot wire) short to the case of the equipment. Basically it causes the breaker to trip if the hot wire touches any part of the equipment that YOU could be in contact with instead of using YOU as the path to ground. But the path to ground thru YOU won’t provide enough current to cause the breaker to trip. The word that we use to describe that situation is “electrocution”!

There is no built in “ground” leg in a sonoff to provide the connection back to your system ground (unless the POW is different than the other versions I’ve seen which only provide a L & N terminal).

You could technically do what you want but I would disconnect the ground (brown) wire going from the second switch to the light. But then you lose the safety aspect of that ground wire. so you would need to run another wire to act as a ground from the main power cable, bypassing the sonoff and the two switches, directly to the light fixture.

Pardon the quick crappy paint drawing…

If your current light fixture doesn’t have a ground wire connection (and some don’t) then you should be OK doing what you want. I have used sonoff basics with the existing light switches in some situations to do what you want but not all installations provide that capability without pulling new wires.

watch DR zzzz he has some alsum youtube vidoes on this

Dr ZZZ’s video will work if you can get power to the three sonoff’s. You’ll still have to figure out, based on your existing installed wiring, if you can get the power to all of them without having to pull wires.

But my original post still stands. Don’t re-purpose or eliminate the ground wire if it’s actually being used as a safety ground.

OK, so can the ground wire be used as the AC ground and the ground in a DC circuit simultaneously?

No. That sounds most unsafe.

this is the best way to do it. @pbcrunch , go with @finity 's design.

also, just for the record this is called 2 way switching. 3 way uses 3 switches :wink:

As daft as it is… not where he comes from :stuck_out_tongue_winking_eye:

Yes, Americans like everything bigger, so they call a 2 way switch a 3 way switch. And they use miles, gallons, pounds. Their power supply is some weird voltage at a frequency no one else uses. Their TV is NTSC (Never Twice the Same Colour, oops Color) and their spelling is weird.

I wouldn’t advise what your attempting for several reasons, the chief being safety. The second code and law.
Although “ground” is “ground”, the paths are important. Im not an electrical engineer by any stretch but I believe what your proposing has the potential to be not so safe for your DC devices.
What I am in a similar situations was convert the 3 way to single pole, put a z-wave switch on one end, rewired to send 120 to the remote and am putting in a HA Switch Plate

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Xiaomi has both 2 wires (no Neutral needed) and 3 wires switches for its Zigbee line of products (look to have only L, L1/L2). Thus no rewiring is needed even without Neutral at switch. It should be noted that maximum load for the 2 wires switch is lower than for the 3 wires version (around 200 W) but for led lightning is more than enough. In HA 3 way switching can be done by combining any other type of wireless switch with the wired one.

Xiaomi requires their Zigbee gateway (there are alternative Zigbee project like zha but this switch is not yet supported) that does the nasty home calling stuff but it can be blocked in the router to stay on lan only (requires router with custom firmware like dd-wrt or Tomato but Asus routers can do it with official one too).

Make your life easier and go with a quality tested device for your 3 way situation. Here’s what I stuck to for my 3 ways and even single pole switches:

I used to like the GE switch but it only supports single pole while the Zooz can do both with ease.

I use the Sonoff solution, as in the first wonderful diagram using another wire (which I had to wire through the walls) without the need for a GND line (the sonoff case is plastic, no no risk of discharge there, and the light fixture is wooden…). All in all, I can use my existing design light switches and just add one 5$ sonoff that will also work without network support (advantage over Dr zZZz´s approach). That´s unbeatable (although the last wall switch is not that expensive, but there´s no european version).


This is how I did it. Works perfectly. It relies on having the AC input power and the output to the light being in the same box.

In your diagram, you only have 1 traveler wire running to the second switch? I’ve never seen that before. If all you have running to the second switch is a single red traveler wire, and ground I’d be leery as others have said about re-purposing the ground wire.