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We have two dimmer switches in a finished basement room. Switch 1 controls wall lights, and Switch 2 controls overhead lights.

We are hoping to replace them with smart switches that require a neutral.

It looks as if Switch 2 is wired using a switch loop. (The white wire connected to Switch 2 is wrapped in black electrical wire, to indicate that it is being used for that purpose.)

From what I understand, the switch loop is an obstacle to using a smart switch, because the neutral wire is being repurposed.

What I'm wondering is whether it's possible to rewire (ideally without having to run new wire) to take advantage of the fact that there is a true neutral that is accessible within the box from the cables that are connected to Switch 1.

Wiring diagram

  • I’m dying to know, how do you easily get the black outline on the white wires? Only times I’ve tried, I’ve duplicated the line and made the front one a narrower stroke, not satisfactory. – Harper - Reinstate Monica Jul 8 at 15:07
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    Haha, that's exactly what I did. In Inkscape, I made a black stroke at 3.5 width then duplicated it and made a white strike at 3.0 width on top of it. – Shaun Gallagher Jul 8 at 16:17
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Can’t do it.

There’s a “Great Wall of China” between the left switch (and its cables) and the right switch. You cannot cross that with any wire, except safety ground.

It’s a Code violation, it’s a safety hazard to workmen working on the left circuit... and what’s more, if there are any GFCIs or AFCIs in either circuit, it will trip them.

What you might be able to do is reverse flow.

Depending on load factors, you might be able to flipper-oonie (that’s a technical term) the right-side lamp circuit so it isn’t powered at the lamp anymore. Cap off that supply at the lamp. Then flip this former “switch loop” so it is not a switch loop and powers the light in the normal way.

By “load factors” I mean you’re now throwing all of that light’s current onto the left circuit. If it’s a large light, that could be a provisioning issue.

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  • OK, so you propose that the left circuit powers all of the lights in the room, and we just ignore the supply at the lamp? There are six low-light wall lights and six brighter recessed ceiling lights, all of which either are (or will be) LEDs. So it's just a matter of figuring out whether the circuit can tolerate that? If I were to go down that road, how janky would it be? Would some future owner or electrician hate me? – Shaun Gallagher Jul 8 at 14:58
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    @ShaunGallagher yeah, that’s what I’m proposing. Reversing a circuit like that is not that weird. The electrician will instantly recognize what you did. Here’s the main thing: the Great Wall still exists, you’re just moving it to the lamp box. So hot AND neutral of the old supply must be capped off safely. (Neutral can be hot, that’s why it has insulation). – Harper - Reinstate Monica Jul 8 at 15:02
  • You can tunnel through the Great Wall safely with a relay though – ratchet freak Jul 9 at 9:48
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It does seem like the center top cable in your diagram supplies the light fixture, and that the cable at switch 2 is a loop. If that's the case there's no way to reconfigure connections and maintain switching at switch 2 without adding a conductor.

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If you had more space than a light switch box would provide has you could use a relay to separate the smart switch from the circuit powering the light.

If you don't mind adding a decently large electrical box you can still do it.

Then the configuration would be,

  • bottom cable powers the switch and it outputs a switched live.
  • That switched live and neutral from the bottom cable go to the coil of the relay.
  • The switch loop cable is connected to the common and normally open contacts of the relay.
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