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Can anyone help with a solution?

I have a 3 wire coming from a smoke detector in the basement into a switch at top of stairs (3wire providing the hot) and from that switch it feeds two light fixtures with 2 wire. How do I install another 3way switch at bottom of stairs to control those two lights without affecting the smokes because it's clearly on the same circuit.

Thanks, much appreciated Jamesenter image description hereenter image description here

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Untangling the smokes

What happened is that a /3 cable got run from the smoke detectors to the switch box, but with the red wire capped in the switch box as the light switch should not be connected to the interconnect wire. Then later on, when the 3-way was disconnected, the homeowner was wondering what to do with the "extra" red wire, and hooked it up to the switch as "all the wires should be hooked up", in his mind at least.

However, that's not how it works. With the breaker off, take that red wire out of the switch and cap it off with a wirenut once again (you may need to tape the nut on to get it to hold). Now, you won't have to worry about any issues that could be caused with your smoke detectors by the light-switching arrangement.

What you'll need to do is run a switch spur from the existing switch to the new switch

Once you have the situation with the smoke detectors sorted, run your /3 cable between the old and new switch boxes. The ground wire in the new cable ties to a grounding screw in the old box (since it appears to use screws on a backstrap for grounding instead of the normal scheme) and to the grounding arrangement in the new box, while the black wire from the existing /3 cable nuts to the white wire (taped black) in the new /3 cable instead of being run into the switch, and the red and black wires in the new /3 cable connect to the brass screws (use the screws please!) on the existing 3-way switch. While you're here, move the black wire from the /2 cable from the backstab to the black screw terminal on the switch.

In the new switch box, tape the white wire black and connect it to the black screw on the new 3-way switch. The red and black wires then land on the brass screws of the new 3-way. Once you're done with both switch boxes, then you can button everything up, turn the breaker back on, and enjoy!

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I can't make sense of what you have going on but, the smoke detectors are simple black and white to each of them in the house and the red involved in the smokes is just a signal wire to turn them all on if one goes on. That being said the, aside from the line and neutral wire you can largely ignore the smoke detectors. And yes that picture is a 3 way switch.

  • Somehow, I think his light switch got wired to his smoke detectors... – ThreePhaseEel Feb 17 at 17:46
  • Smoke detectors are supposed to be wired with a hall light so if the breaker for the detectors trips you will know as soon as you try to use the lights. I just can't figure out why it shows a 3 way and he wants to add a 3 way. Seems like a box got covered maybe? – Joe Fala Feb 17 at 17:50
  • I believe there was a second 3 way switch before the previous homeowner removed a wall opening up the stairs and did not change this switch to single pole. Now I am installing a new wall to reinstate this second switch. Just need to wire the second box – James H Feb 17 at 17:55
  • @JoeFala -- no, what happened is the interconnect line for his smokes got wired into the switch somehow – ThreePhaseEel Feb 17 at 18:03
  • Oh I see, I'm not sure if I can help but I would approach it like this. Disregard the smoke detectors to avoid any confusion. Hook up your replacement 3 way. Make sure your have it connected properly. Then add in your line and neutral. That way your smoke alarms will always be powered – Joe Fala Feb 17 at 18:05
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I can't make sense of what you have going on but, the smoke detectors are simple black and white to each of them in the house and the red involved in the smokes is just a signal wire to turn them all on if one goes on. That being said the, aside from the line and neutral wire you can largely ignore the smoke detectors. And yes that picture is a 3 way switch. In these pictures you can see the proper way to connect SDers and 3way switches It's okay to use the hot and neutral front the SDs to power your light but not the red one. The red wire in the SD circuit is a signal wire. So you need to add your 3way according to the image and then pick up hot and neutral from the SD circuit.

You just need to pull one 3 conductor cable to the second 3 way and connect it like this. Disconnect the red from the SD cut the bare copper and wire nut it off. enter image description hereenter image description hereenter image description here

  • If I remove the 3wire that is running between last sd and switch, then run black and white from sd to switch to provide the line for the switch and then run new 3wire between two switches two make the 3way setup I am trying to achieve? – James H Feb 17 at 20:27
  • Based on what we have discussed, that is accurate. But you don't need to remove the cable, just the red wire. Do you have a multimeter? Or can you see the wire completely from one end to the other? – Joe Fala Feb 17 at 20:37
  • I have a multimeter and I can see the cable from one end to the other. – James H Feb 17 at 20:39
  • Let me find a 3way diagram that fits exactly what you need.... – Joe Fala Feb 17 at 20:46
  • Sorry I misread, don't remove the 3 wire cable. You're only disconnecting the red wire from the SD to the switch – Joe Fala Feb 17 at 20:56

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