Zooz Zen21 3-way Wiring - Please look over my shoulder


I’m requesting wiring assistance with my new Zooz SEN21 on/off switch. I’ve included two pictures of the two boxes involved with this 3-way setup. The attached pictures show both boxes involved with the 3-way setup for my kitchen overhead lights. I’ve done many (10-ish) 2-way GE switches without much difficulty. The one 3-way I’ve already done was more difficult, but I got it done after some trial and error. This will be my first Zooz switch and only my second 3-way and I wasn’t happy with the trial and error method . :grin:

The pictures of the two boxes involved are here:

  • The one on the left has (3) 3-way switches, The left-most switch is the 1st switch for the kitchen overhead lights. That switch has it’s wires going out of the box with none of them going to anything else in the box.
  • The right picture shows the 2nd switch for the kitchen overhead lights with an outlet in the same box. There are two bundles in the back of this box. One is all white wires. The other one (pulled out of the box has two black wires and one white wire wire-nutted together.

So the first question is, which box has the direct connection to the power?
I think that the right picture has the connection to the power line and it will therefore need the Zooz switch installed there., but I’m certainly not sure.

Question #2, which of the following diagrams is appropriate for my wiring?

Thank you in advance.

Am I seeing correctly that the switch on the left has red and white going to the same screw, and that there are scorch marks around that screw? :grimacing:

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Well, partially correct… There IS a scorch mark :grin:

The red wire on the left picture/left switch is actually connected to the screw on the other side of that same switch - just above the ground. The scorch is from me getting sloppy as I prepared to take some readings with my multimeter and then take the pictures. The switch on the left, bumped the switch in the middle and that resulted in some sparks and the scorch mark. The two circuit breakers did their jobs. All’s well now and the breakers reset fine.

BTW, I’ve not disconnected anything from any of the switches yet.

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Crowded switch boxes are SO much fun. At least that isn’t the box where you have to fit the Zooz…

(Disclaimer: not an electrician!)
From your explanation and pictures, I would agree that the box on the right has both your line and load connections, and the box on the left just has the 3-way traveler. Your setup seems to match the first Zooz diagram, but the black and white wires going to the remote (left) switch are reversed as below (blue substitutes for white in my sketch). When you rewire everything, I would match the colors back up to the diagram to avoid confusion, particularly since the white wire in the remote switch will actually be neutral.


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It’s good to add a little excitement to mey day! :flushed:

Thanks for your help. I agree it looks like diagram #1 too. I’ve run out of time for today but will put it to god use when I go back to tackle it again.

Thanks again!

As an aside to the above discussion: Before I started working on this project, I was looking around for the proper way to determine the LINE and LOAD wires in a 3-way . I found this YouTube Video that seems to do a good job of explaining it. Since I need to rewire both switches differently than they are now in order to set up the Zooz switch with the existing 3-way switch, the installation part of the video and instructions below are not exactly correct, but it appeared to me to be the easiest way that I’ve seen to determine LINE and LOAD when working with any 3-way setup.

Anyway, I then took the information from the video and put it in the text that I am sharing below:

How to find the LINE and LOAD wires in a 3-way

  1. Take pictures of both switches before starting.
  2. Turn the circuit OFF
  3. Disconnect all three wires from BOTH of the switches
  4. Turn the power back ON
  5. Go to one of the boxes. After grounding the meter, measure each of the (3) wires to see if any have 120 volts. If not, go to the other boxes and do the same thing. ONE of the wires in one of the boxes will read 120 volt. Mark this wire as the LINE or HOT wire. This is switch ‘A’. The other switch is switch ‘B’.
  6. Turn the power back OFF
  7. Go back to switch ‘A’ and tie all three wires together electrically.
  8. Turn the circuit back ON
  9. Go to switch ‘B’ and measure each of the (3) wires to see which wire does NOT have 120 volts. That’s the LOAD wire. Mark it.
  10. Turn the power back OFF
  11. Go to switch ‘A’ and attach the HOT wire to the common screw
  12. Attach the remaining (2) wires (travelers) to any of the remaining two screws
  13. Go to switch ‘B’ and attach the LOAD wire to the common screw
  14. Attach the remaining (2) wires (travelers) to any of the remaining two screws


Yes, I did something similar. Many of the instructions here are very complicated with instructions to actually figure out the wires into the lamp etc.

Also, there is one simplification you can do, get a non contact voltage tester. Then all you need to do is see if the tester beeps when you touch the tip to one lead.

So go to switch 1, flip one direction, test wires in both boxes, flip switch 1 again, check all wires all boxes for voltage.

Then go to switch 2, flip and test, flip and test. then you are done, all the info needed is gotten. No need to turn power on and off.

You will know which box the line comes into, because that line will always be hot regardless of how the switches are switched.

In the other box, with some configurations of switches, no wires will be hot if your wiring is line to one box and then the other box. Or line to one box, then light, then box.

I think that usually one box gets line first, is the most common way to wire.

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Also the newer zooz zen26 is LESS picky on wire requirements. In my case I only had neutral in one box, so I needed the ZEN 26. Or at least it made it simpler to wire one zooz 26 and one dumb paddle switch.

Works great!


A quick (and sad) update to this. :frowning_face:

All three of the switches in the first picture in my post above are on seperate circuits. So, turning off the breaker for the switch I was working with was not sufficient to avert a larger issue than I originally thought. As it turns out, my 55" Samsung TV (UN55B8000) happens on the same circuit as the middle switch that bumped - and sparked - with the one on the left. As a result, the TV took a hit and will no longer turn on/boot up properly. It makes a clicking sound instead and the power light flashes on and off. My intuition and a quick follow up online search, tells me that it’s probably the Power Supply board (BN44-00272A). I’m hopeful that I can buy a replacement and that will solve the issue. If successful, it will cost much less than having it repaired at a shop and certainly less than buying a new TV.

The morale of the story is: Be very careful when working with electricity. A few sparks can cost you!

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@lflorack, Just my two cents and completely off topic. The power board on my Panasonic failed and I had the flashing power light that you describe. I removed the SC board and sent it in to be repaired (https://www.ptselectronicsinc.com) for $125. The SD board was also bad so I had to send it in later for another $125 repair. Both worked correctly upon return but the connections on the SD board were very difficult to line up correctly. ALL recommendations stated to adjust those connectors without power and re-test. Too time consuming to keep turning off the power…So I proceeded with caution…and POOF, a small contact arc and the TV was toast.


That story may even be sadder than mine. Based on my track record with electrical work as demonstrated above, I’ll have to especially careful with the TV replacement board.

As we were discussing my Zooz ZEN21 installation here, yesterday I also sent a request for guidance to the Zooz team. Their response was very detailed - both with their step by step instructions and their annotations on the photos I sent them. I was so impressed with their response, I’m sharing it here:

Hi Lee,

Thank you for sending the images to us and I do apologize about the late reply.

You’re right about the power line being in the box on the right where you actually also have the connection to the load so you’re dealing with Option 1 from our diagrams.

Below are the step by step instructions on how to wire the 3-way switches, I’ve also attached visual instructions for illustration.

Please make sure power is off during the installation. If you’re not confident about following the instructions or identifying the wires, please consult a licensed electrician for your own safety.

BOX 1 (the box on the right with power where your Zooz switch goes):

  1. Disconnect the white wire (we marked it light blue) which is now connected under a wire nut with 2 black wires and connect that wire to the bundle of white neutrals in the back. Then run a jumper wire (included in the packaging) from the white wire bundle to the Neutral terminal on the Zooz switch.
  2. Take the 2 black wires from that wire nut (we marked them dark blue) and connect them to the 2 holes in the Line terminal on the Zooz switch.
  3. Disconnect the black wire coming from your 14-3 romex (which is now connected to the switch, we marked it purple) and cap it .
  4. Connect the black wire from the 14-2 romex (we marked it black) to the Load terminal on the Zooz switch.
  5. Connect the red wire from the 14-3 romex to the Traveler terminal on the Zooz switch.

BOX 2 (the box on the left where the regular on/off switch stays):

  1. Disconnect the black wire from the switch and cap it .
  2. Leave the red and white wires connected as they are.

Please let me know if you have any questions about the above and I’ll be happy to help.


Support Team


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Non-contact voltage tester


I use one of these made by Klein. A great tool to check for voltage before and after the breaker is off. It’s saved my butt many times when I thought the box was “dead”.

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As suggested by @professordave, get yourself a non-contact voltage detector. Not will it make locating life and load quick and easy, it lets you poke around your multi-gang boxes with a non-conducting probe. Reduces the chance of shorting.

They cost about $20 +/-$5 on Amazon.

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As a follow-update regarding the Zooz ZEN21 installation, the instructions provided here and from @TheSmartestHouse were perfect. I’d also like to say that the switch paired easily and works very well with ST and Alexa. As a 3-way installation, it is responsive and controls my flush-mounted ceiling lights just as expected. The ZEN21 was on sale for $24.95, so it was great buy AND since it doesn’t require an add-on switch, it was even cheaper that way too. The previously existing switch that was part of the 3-way setup, continues in place and works well with the ZEN21.

I’ll be buying more! Thanks @TheSmartestHouse !

Off topic as far as SmartThings goes but I’ll let you know when I tackle the TV repair. I need help to get the 55" Samsung TV off the wall before determining and then ensuring I get the right replacement part.


Thanks for your feedback Lee! We’re happy you’re happy :upside_down_face:

Feel free to reach out to us with questions about installation anytime in the future. And good luck with the TV!

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Final Update on wiring up a new Zooz ZEN21 from @TheSmartestHouse for a 3-way setup.

  • The Zen21 is still working very well.
  • The wiring instructions from @TheSmartestHouse were excellent (see earlier post)
  • Thinking about what happened to the TV, I’ve come to the conclusion that the adjacent switches that brushed each other were not only on different circuits, they were also on different phases. Therefore, the result was a 220 volt spark - killing the TV’s power supply/motherboard.
  • My wife and I tried to take the TV off the wall to work on it but one of us :wink: was unable to lift their half of it off of the wall mount. So, considering my recent lack of electrical prowess, I reconsidered my initial plan to dismount and work on the TV electronics myself.
  • I called a A/V installer I’ve worked with before and he recommended a local repair shop. They were able to uninstall the TV, transport it to the shop, troubleshoot it, fix it with a used power supply/motherboard (they no longer make that board) and will deliver and reinstall the TV on Tuesday. Total cost, $225. Pickup and delivery was $150 of that. Only $75 was for the parts and labor.
  • All things considered, my sloppy electrical work could have been much more costly. But we’re all set now at a pretty reasonable cost.
  • Even so, I’ve learned my lesson. I’ve purchased a no-contact electrical tester as suggested by @oldcomputerwiz, @HalD and @professordave. I will also shut off all power to every circuit in the box I’m working on before I start.

Thanks for listening.


I swore I wouldn’t post again on the damage that my electrical ‘work’ did but…

Since I last updated everybody about the $225 TV repair, (when I thought that was all of the damage), I’ve found that the TV wasn’t the only casualty.

  • The ~35’ to 50’ HDMI cable from the TV to the receiver in the basement A/V closet also was fried. The cable runs through a tube/conduit through some walls and floors and it must be pulled out and a new cable pulled back in. Cost TBD
  • The AV receiver took the spark through the TV, to the HDMI cable and it lost it’s HDMI-out port . Replacement cost $900
  • As of this morning, I discovered that my BluRay player - which is on a separate electrical circuit that was not part of the wiring I was working on AND on a seperate receiver/system in the basement A/V closet, also lost its HDMI-out sound capability as well. Replacement cost $190.

I hope that this sad tale of electrical sloppiness is helpful to someone so they can avoid this kind of a mess. Just be careful! For me, I can only hope exposing my stupidity is therapeutic.