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I have a recessed light that is being controlled by two switches. Pics in red and green show the two switches. The green switch is nearest to the door and ideally that is where I want to install a motion detector 3 way switch.

Can I install this lutron 3 way motion sensor to the green switch? What do I need to consider before I start this project? Thank you.

https://www.lowes.com/pd/Lutron-Maestro-Single-Pole-3-Way-White-5-Amp-Occupancy-Vacancy-Sensor/3531524

while google searching and reading up on 3 way motion sensor I came across this and I am now even more confused.

Replace one of the 3way switch to sensor switch in the hallway

The crux of the problem is that in a normal 3-way switch set up, neither switch really "knows" if the light is on or off, flipping it just reverses the state (for example you can have two switches up and the light is still off) so with a motion sensor you really need it to be in total control of the light, otherwise it might just turn it off after sensing motion when the light is already on per the other switch (not what you want). Most motion sensing switches have an accommodation for this but it is specific to the model.

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  • Can you post photos of the insides of the boxes involved please? Jan 16, 2023 at 19:06

3 Answers 3

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The answer is (essentially) in the instructions (PDF), which are linked on the product page.

There are two main ways that most modern smart switches (motion, timer, etc.) get power:

  • Neutral
  • Ground

Neutral is really the preferred method. But a lot of older switch boxes don't include neutral. In addition, 3-way switches in particular very often don't have neutral and hot in the same box, which sometimes results in a lot of complicated (and sometimes unsuccessful) diagnostics and trouble-shooting to figure it all out.

Ground has the advantage that most US houses built in the last several decades (not sure exactly, let's say 1950+, based on my limited experience) have ground available in most switch boxes. Ground has a couple of disadvantages:

  • Since ground is not supposed to normally carry power, NEC/UL/ETL/etc. will only allow ground to be used instead of neutral for very low-power devices.
  • If the switches/light are on a GFCI-protected circuit then it may trip the GFCI either right away or at random times. Lighting is not usually GFCI-protected, but sometimes lights are on the same circuit as receptacles that need to be GFCI-protected (e.g., garage, outdoors, etc.) and if the GFCI protection is provided at the breaker then the lighting is protected as well, whether you like it or not.

Follow the instructions carefully and you should be OK. As far as I can tell, there are 3 unusual features of this setup:

  • The switch is supposed to be grounded using both the green wire and bare wire. This is extremely unusual. I believe the green wire (being insulated) is for "power" and the bare wire is the "ground for traditional purposes". With an ordinary switch (smart or not, but that doesn't get power via ground) you normally have just one ground connection and that ground can be provided through the yoke with a metal box. But here ground is critical for regular functioning, not just for fault situations. If you have metal conduit and metal boxes and no ground wires then you will have to install a ground screw and connect that to the ground wires (or rather, connect a short bare or green wire to the screw and the other end to the switch ground wires).
  • The switch does not care about line vs. load. (Dumb switches never care about that, motion sensor switches often do care about it.) Actually, it cares a lot. But it has smarts inside that figure it out so you don't have to. Which is really important with 3-way switches as it can be tricky to figure it out manually.
  • One traveler is used as a signal wire and the other as power (either line or load, again doesn't care).

Note that the 2nd switch does not have to be a 3-way switch! It can actually be a standard switch. But the usual setup is "retrofit an existing circuit", so leave the 3-way switch in place but rewire it as directed. But that means that if at some point in the future you wanted to replace the switch with a different one (typical is toggle -> Decora, but you already have Decora) you could actually use a regular switch instead of a 3-way switch.

The directions are confusing. But if you follow them very carefully, including tagging wires as described, it should work.

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    very good explanation. thank you
    – Sam B
    Jan 16, 2023 at 16:59
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You can find the installation instructions here: https://www.manualslib.com/manual/1457075/Lutron-Electronics-Maestro-Ms-Ops5m.html?page=2#manual

The device needs to receive power in order to operate the sensor. So it is not wired in the "usual" way for a 3-way switch. You will have to modify the wiring of both boxes, and convert (or replace) the "other" switch to a 1-way switch.

It does not matter where the motion sensor is located - it can be at either end, so long as you meet the wiring diagram.

Before you do anything else, you should open BOTH boxes (just remove the faceplate) and check to see how much "slack" wire is available and how much free space there is in the box. I expect that the motion sensor is a "fat" switch and probably wants every available inch of space in the box, plus it appears to want wire nuts for connection. If you have a small box in the wall it may simply not be big enough to hold the new switch -- check that!

Make sure you have a voltage sensor or multimeter available for testing. Make sure you know how to use it!

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The motion switch appears to be a standard Decora size, so fitment should be no problem. The specifications fit your situation. (The manufacturer states it is 3way compatible.) Get the wiring correct according to the manufacturers directions and you will be all set. You are simply replacing a 3way switch with a 3way switch that has an extra built in function.

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  • can you answer in bit more detail? about wiring considerations? I have also updated my question with another gotach from another user
    – Sam B
    Jan 16, 2023 at 14:45
  • The wiring MUST be done to the directions that come with the switch. I cannot comment on how to wire...that is in the instructions. The product is labeled to work in a 3way switch set up, so assuming it won't would be conjecture.
    – RMDman
    Jan 16, 2023 at 14:54

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