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I'm trying to understand why a switch loop in my house was wired in this manner and if I need to correct it. I'm assuming it is incorrect because I cannot turn the light off and on from both switches. I included a diagram of the current wiring I can see and pictures of the switch receptacles. I'd like to avoid running new Romex if possible. The source comes into the light socket via the attic (Black and White only,) down to the first switch, and down to the second switch. Black, White, and Red wires are present with no ground between the switches.

My plan was to swap the bulb out with a recessed light fixture. I'd love to be able to add a smart switch in eventually, but it's not a dealbreaker. I need to hook up the light as well so guidance on that would be appreciated! Thanks!

Diagram

First (Left Switch)

enter image description here

First (Left Switch)

Second (Right Switch)

Second (Right Switch)

Second (Right Switch

This is within the first switch box.

first

To First Switch (Left in Diagram)

To First Switch (Left in Diagram)

Source Source

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  • Do both switches operate the light? What are you trying to correct? Also if the wires are run in conduit, the conduit would be the ground.
    – JPhi1618
    Sep 21, 2021 at 16:20
  • They do not operate the light correctly, no. If the light was turned off at the left switch it cannot be powered on via the right.
    – sixtysix
    Sep 21, 2021 at 16:22
  • If those are both 3-way switches, two screws should be one color and the other screw another color (ignoring ground which would be green). The two same-color screws should be connected to the travelers and it shouldn't matter which traveler is connected to which. Show pics of both sides of both switches.
    – FreeMan
    Sep 21, 2021 at 16:45
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    I added the pictures above. Thanks!
    – sixtysix
    Sep 21, 2021 at 17:01
  • Looks OK to me, but I'm not the pro here. I'm sure someone will chime in with some more info for you shortly.
    – FreeMan
    Sep 21, 2021 at 17:04

2 Answers 2

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Let's review how 3-ways work.

enter image description here

Note the 2 travelers: one is hot, and one is not. They must necessarily always be in the same cable, and be the same on both ends.

That uses up all the brass screws and leaving only 1 screw left per switch.

Once you get the travelers right, there are precious few wires left. Hooking up the remaining wires is fairly obvious.


Now, there's one wrinkle. Because it's a switch loop, neutral is not present in either cable. And when neutral is not present, there are strict rules about what white can be used for.

  • It must be used as the always-hot if always-hot is present in the cable.
  • Otherwise it can be used as a traveler. It can never be used for switched-hot.

So the black-white splice is actually a Code violation.

I recommend you rearrange the wiring so that white is always-hot "all the way". That means black and red are your travelers. So, #1 Rearrange the switch wiring so that black and red (from the same cable) are the travelers on the brass screws.

Then #2 break up that black-white splice that should have never been, and splice white-white.

Then pretty much a victory lap... both common terminals are empty at this point, so #3 hook up the remaining 1 wire in each box to the common terminals.

Up at the lamp, make sure you splice supply-hot to the white wire in this cable (so the black-white splice occurs there)... and use the black from the 3-ways as the switched-hot.

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    Every time you write this up, it makes my head hurt that the white must be the always hot. Maybe some day, I'll have that digested and in me...
    – FreeMan
    Sep 21, 2021 at 18:22
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    @FreeMan For a very good reason... the doctrine of NEC is "Trust nothing. Measure". So 23 years later, Goober wants to extend for a receptacle, and is following NEC doctrine, probing around hoping to find always-hot and neutral. What NEC wants to happen: Goober tests the white wire: Always-hot! "Clearly this won't work!" The bad thing: Goober tests the black wire, always-hot. Tests the white wire, cold (the switch happens to be off). "Found it!" Sep 21, 2021 at 18:27
  • Ahhh... makes sense! hopes to remember this example
    – FreeMan
    Sep 21, 2021 at 18:34
  • Did a conduit job a few weeks ago and kept hearing you say" travelers are yellow"... so I ran yellow.. lol
    – JACK
    Sep 21, 2021 at 19:12
  • Awesome explanation. Thank you!
    – sixtysix
    Sep 22, 2021 at 12:12
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Try testing each switch to make sure you have assigned the correct wires to the correct screws. Disconnect all wires (mark them so you know which was connected where!) and then use a voltmeter in "ohms" mode as follows. Connect the probes to any 2 screws. If flipping the switch causes the resistance to go from zero to Infinite, then you have one probe on the single side and the other probe on one of the "double" side screws. Move one of the probes and test again. If flipping the switch does not ever give you continuity, i.e. zero ohms, then those two are the "double" screws and the third is the "single" side.
Once you know that, assuming the switches function properly, you can reconnect the wires to the proper screws on each switch. If the light still responds to only one switch, then the wiring for that light does not go to the other switch.

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