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Edit: I know my initial question was a bit tricky to understand, so here are some pictures as requested.

enter image description here enter image description here

In the box I have two 3-way switches for different light circuits. I want to put a put a Shelly relay in there to control light B, but I need a hot wire to power it - and all I have are travelers. Is there any way I can achieve this?

Here is the relay for reference:

enter image description here

Normally I would connect the switch B common to the SW terminal on the relay, and the O terminal on the relay would go to the light (load). But I need a hot wire to power this relay (terminal L). So my original question was asking if there was some way to modify the circuit for light A such that I could get a hot wire in there. I do need the relay to be in this box because the O terminal needs to connect to the light B load directly.

Original post:

So I have a 3-way switch and in the switch box where the common terminal connects to the light I instead want that to be the hot wire and its sibling switch to connect to the light.

Why? Well because in that same switch box I have a another 3-way switch with the common terminal connecting to a different light. So in that switch box I have 2 3-way switches that are both connecting to their own lights via common and I don't have a dedicated hot wire coming in - but I want one for powering a smart relay (a Shelly) to control the opposite light I'm asking about. But I only have 2 sets of traveler wires in there.

I understand switches pretty well but less of what's happening behind the walls. So is this possible? I'm open to other solutions as well. Thanks.

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    Behind the walls, you'd need to route the cable from the light to the other switch box. That is generally difficult or at least messy.
    – Ecnerwal
    Jan 27 at 2:44
  • Four-way switches don't have "common".
    – nobody
    Jan 27 at 2:50
  • @nobody right I just mean that its part of a 4-way light which is irrelevant for my purposes. It's physically a 3-way switch. I'll see if I can edit my question.
    – Adam
    Jan 27 at 2:53
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    The three-way circuit is two SPDT switches wired back to back. One switch routes hot to one of two "traveller" wires, the other switch selects which of those travellers is then connected to the load. Same switch, used two different ways. I'm not sure what you mean by "reverse", though. How about describing what you are trying to achieve rather than how you are trying to achieve it?
    – keshlam
    Jan 27 at 3:28
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    Instead of trying to describe it, flip off the breaker(s) and pull all the switches out of the wall (without removing any wires) and get good, clear, focused, well lit pics showing what wires go where. They you can draw on one to show us what you're attempting to do. Maybe even draw in the Shelly (I use several, love 'em, make sure you get the UL listed versions!) and which wire(s) you're not sure of the connections for. Pictures are worth 1000 words, they say...
    – FreeMan
    Jan 27 at 4:15

2 Answers 2

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Yeah, I grasped your intent from the original post. The pics match my presumptions, except both switches here are the last switch in a power-switch-switch-lamp wiring arrangement, so there is no way to get always-hot power here, other than the "2 smart switches" plan I proposed.


So you want to steal neutral from the 4-way switch group and steal hot from a different 3-way switch group.

Nope, not possible. NEC 300.3 and 310.10 violation and it'll trip any GFCI involved.

This is not DC power, it does not work the same way.

Attack this in a different way. Look at the entire 3-way complex and convert it to smart switches, with a new wiring arrangement appropriate to those switches. Typically this involves re-tasking black to always-hot, white to neutral (which then brings power to both smart 3-ways), and red becomes either a data wire or the switched-hot to the lamp.

Now if that smart switch conversion provides the automation you are after, then you're done.

Otherwise implement any UL-listed solution according to the instructions (which UL approves as part of granting the UL listing, and on which the listing is conditional). Other NRTLs are fine.

Many Shelly devices are not UL/CSA/ETL listed and is not legal in NEC and CEC countries. It qualifies as 'random crud off Aliexpress'. My impression is that Shelly is a typical "nerdhaus", two guys in a garage who put up a Kickstarter and send stuff to China for manufacture, completely oblivious about codes and compliance... and Shelly eventually got clued in, and started making UL listed items, but continues to design and sell non-UL-listed products.

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  • I've edited the post with pictures
    – Adam
    Jan 27 at 15:43
  • Could you help me understand the "2 smart switches" concept? Are you suggesting to use 2 of these Shelly relays? One in the box where line comes in and also one where I normally would?
    – Adam
    Jan 28 at 3:10
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    I agree with your "don't buy from aliexpress/amazon" rants. However, many/most Shelly devices ARE UL certified, and more are being certified. It's important to be sure to pick the proper ones, but, since they only cost $1 or $2 more, it certainly makes sense. Please don't disparage the entire brand just because a couple of their devices (being phased out of their lineup) aren't certified. (Not a rep, just a fan.)
    – FreeMan
    Jan 30 at 15:11
  • @FreeMan alright. But they sure are slack about it. If I ran a typical nerdhaus (2 guys in a garage with a better idea who put up a kickstarter and start acquiring fans, and then go "OMG there are standards", I'd go fully UL, not push a few UL listed products into stores while continuing to design and sell non-listed products out the back door. Jan 31 at 19:49
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I was able to get this work by converting the other switch (not pictured) for light A to a SPST and re-purposing the traveler wires - one for dedicated hot and the other to just carry the load from the SPST. Then I removed the switch pictured for light A and just pass the load on to the light. This gave me a hot to power the relay for switch B as needed.

Obviously this isn't going to work for everyone, but I don't think I have ever used the switch I removed so it was an easy decision.

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    Have you checked to make sure that Code doesn't require a switch at that location? (usually an issue for stairs or hallways) Jan 30 at 4:40

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