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Summarize the problem

I am trying to replace an existing 3 way switch with a smart light switch (Kasa Smart Wi-Fi Light Switch, 3-Way | HS210). The existing switch has wiring I can't quite figure out, but it works.

The first switch has /2 coming into the bottom and /3 at the top of the box. /3 red+white are attached to the brass screws; /2 black is attached to the black screw; /2 white is crimped to /3 black inside a nut (had to cut these to remove the nut).

The second switch has /3 coming into the top. /3 red+white are attached to the brass screws; /3 black is attached to the black screw.

When flipping either switch it will toggle the top outlet on/off for 5 outlets around the room.

Here's a diagram with the existing wiring. existing 3-way switch diagram

Describe what you’ve tried

When replacing the first switch for the smart switch, I followed the same wiring except I attached the smart switch neutral to the 3/ black and 2/ white nut. The smart switch is receiving power (the wifi indicator lights up orange/green); however, when I turn it on, the power cycles for all of the outlets and the switch clicks. When I turn off the original 3-way on the other side of the room, the lights stop cycling, but I notice a very low flicker on one of the lamps.

I then tried to re-wire assuming the /2 were power source (black line, white neutral); attaching the /2 black to the first switch's black screw, and putting /2 white, /3 white, and switch white into a nut; finally /3 red+black onto brass to travel. Unfortunately, that didn't work either.

Finally, I tested the /2 lines. Turns out the white is hot and the black is neutral. So I tried to rewire again using my second configuration, but flipping the black and white. The switch continues to cycle on and off.

Provide details and any research

I did a bit of digging on why white and black lines would be joined together, and it lead me to some information on a switch loop. I think it's possible that's what's going on here, but I thought I would start asking for help rather than going deeper into the rabbit hole. My home was built in 1959, from what I've read switch loops were common around the time. As mentioned in the comments, neutrals were not common when the house was built, however there are a bunch of boxes around the home with neutral wires and many without. We live in the United States.

Here are some additional photos from the first switch before removing the existing hardware. Thank you in advance!

switch connected inside box, switch removed

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    First, a white connected to black or a non smart switch is not neutral. Neutral has only been required to be in switch boxes by code since about 2008. Before that, you were lucky to find neutral with switch loops or three ways. There are a few smart switches made to use ground/or no neutral for this case or will need to rewire.
    – crip659
    Commented Feb 21 at 22:41
  • @crip659 - yeah, I didn't have much hope for either line being neutral given the way they are attached, but I do have a bunch of neutrals throughout the house (will update the post to include). Thanks for the comment, I'm pulling out my hair trying to get this thing figured out.
    – mtully
    Commented Feb 21 at 23:17
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    Neutrals must be matched with a hot(in a cable/conduit) unlike grounds that are allowed to be separate.
    – crip659
    Commented Feb 21 at 23:31
  • Neutrals are needed for lights/receptacles to work. You might mean grounds are not common, since grounds are only good for your safety, not for the circuit to work. Grounds are bare or green. Neutrals must be white(or grey)/but white does not mean neutral.
    – crip659
    Commented Feb 22 at 22:14

1 Answer 1

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Frame Challenge

This is NOT a good idea, even if you can make it work.

While code allows for switched receptacles as an alternative to switched hardwired light fixtures, there are some serious power issues. A typical toggle switch is rated for a full 15A. A typical electronic switch - dimmer, motion sensor, smart, etc. - is not. The linked example says:

15 A General Use, 600 W Incandescent/ LED/CFL

I am a bit dubious about the 15A statement, because if it can really handle 15A then it should be able to handle 1,440W (15A * 120V * 0.8) of incandescent. In reality, the lighting load isn't much of an issue - most people use LED or CFL these days and you can light a whole house with 600W of LED lighting.

But I have seen far too many people who don't understand switched receptacles and plug other stuff like computers into switched receptacles. With your current usage that obviously isn't planned. But what if:

  • The HVAC system is having problems.
  • Somebody plugs in a space heater, while the lights (plugged in to other receptacles around the room) are on so they aren't thinking about which receptacles are switched.

Will the switch do OK with 1500W for long periods of time? Maybe. Maybe not.

The usual excuse for not hardwiring lights is to avoid the work of electrical wiring. You are doing the work anyway to install switches. Do a little more work to install wall or ceiling fixtures (sconces, chandelier, etc.) and you will avoid this problem and have all your receptacles available for other things.

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