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how do I replace my standard 3-way switch with a Meross 3-way smart switch MSS550

  • must I replace the old switch with the live wire and not the one with load wire? -the 3-way switch is the one on the left of the photo, with a wire bundle (red, white, black) connected to the switch. In the bundle I assume the black is the live?
  • is the red to be connected to Traveller T1?
  • Which wire should be connected to Traveller T2? Should it be the black of the black/white bundle adjacent to 3-wire bundle? Is so, I should just connect T2 to the cap capping all the black?
  • The white from the smart switch should be just added to white bundle at the back of the switch?

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There are many different ways that 3-way switches can be wired. There is no standard color-coding.

Unfortunately, the Meross manual is nearly useless. Beyond the usual safety warnings, it really only says two things:

  • Neutral required
  • Open the Meross app, it will guide you through the wiring process.

As far as I am concerned, "open the Meross app" is 100% a show-stopper. If they said "we recommend you use the Meross app, but here are actual instructions in case you are an old-timer who doesn't want to mess around with a smart phone while wiring up a switch and/or would rather figure out things in advance because there are so many different possible 3-way configurations, some of which may not even be compatible with this new switch", I would be fine with that. But the instructions don't say that and don't have the necessary information.

My hunch is that the switch will work best with "live" available, but there is a 3-way/4-way diagram on the Meross web site that says it can work in place of either of the 3-way switches (just not in place of a 4-way switch) which implies that it doesn't matter which switch you replace, in general.

However, neutral is a requirement. That is actually quite common for smart switches, for very good reasons. And for those same reasons, more recent NEC versions require neutral in most switch boxes. However, that neutral must be the correct neutral for your setup or it is useless.

In particular, it appears that your 3-way switch is connected to black/red/white all in one cable. Assuming that is the case, since those 3 wires are in one cable, they must be two travelers plus either hot (hot from other box returning to the other switch through one of the travelers) or switched hot (hot from other switch via one of the travelers, returning to the other box through switched hot). It can't be neutral. Which means (as expected) you don't have neutral already on the switch (that doesn't happen with a proper mechanical 3-way switch configuration). And you can't use the other neutrals in the box because they are not in the same cable as either your hot, switched hot or travelers (depending on configuration, neutral needs to match up with one of those to keep the currents balanced).

Clarification based on additional images:

  • Switch 1 (box with Wago for grounds) has black come in from elsewhere and goes to multiple locations, including the black wire of the /3 cable. Which means the black wire to Switch 1 must be switched hot and the red and white wires are travelers. So in Switch 2 (left switch of original picture), black is hot, red and white are travelers. You have neutral in that box but it is not matched to that 3-way circuit, so you can't use it for a smart switch.

As a result, it is likely that the Meross switch needs to be installed at the other box to replace the other 3-way switch. But there might be a way to make it work here. I just can't say for sure because I am not installing the Meross app to find out.

Based on the additional pictures, what will almost certainly work is to do the following in what I call the "Switch 1 box" - the box with the Wago connecting grounds (nothing special about that, just identification):

  • Black from 3-way switch to Meross L
  • Red and white from 3-way switch to Meross T1 and T2
  • Meross N connects to white neutral bundle above the Wago connector
  • Meross ground connects to box and/or other grounds.

Since you have a nice metal box, really all the grounds should go to the box and not to a wire nut or Wago. Switches with metal yokes can ground directly to the box, though if they have an attached ground wire like the Meross (rather than just a ground screw) then that should be grounded to the box. But since this is a "neutral required" switch, that is for safety (important!) but not for functionality.

If it were me, I would return this and get a different brand after checking the instructions first to make sure they actually included useful wiring information.

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    +1 for "paper documentation"!
    – FreeMan
    Jan 12 at 16:28
  • It’s impossible that I don’t have a neutral as my house was built in the early oughts by reputable developer. I have added another photo of the 3-way switch at the other end (right hand side), where the black wire connected to the switch is from the adjacent black/white bundle
    – Raffi
    Jan 12 at 16:38
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    You might not have a neutral for this part of the circuit in this particular box @Raffi. What would be impossible would be for the light to have ever worked without a neutral. ;)
    – FreeMan
    Jan 12 at 16:43
  • My house was built around 2013. . There is neutral for sure. The current 3-way switches have been working with no issue
    – Raffi
    Jan 12 at 16:48
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    Carefully reread the 3rd paragraph from the end. Nobody is claiming that there isn't a neutral in the circuit. Just not a usable one at this location for this switch.
    – FreeMan
    Jan 12 at 16:49

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