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Rewiring light fixture and switch to bring neutral line to the switch: one 12/3 switch-leg vs two 12/2 wire to bring power to switch?

I’m looking at rewiring a ceiling-mounted light fixture (a basic light bulb mount) and the accompanying switch in my utility room. It’s in a very accessible part of my attic, and I’d like to do it in part because it’s old wiring and also because I’m installing smart switches which need neutrals at the switch box.

For the record, I know that I can install smart switches that don’t need neutrals, like Lutron Caseta, but I also just want the wiring to be updated and it’s fairly accessible, in a wall with no insulation. So I figure this is a good place to experiment with a fairly simply DIY project. There are other reasons with smart switches why you want a neutral switch, particularly with Zigbee switches where switches that are on main power can act as repeaters, strengthening the wireless mesh network.

In any case, currently the light fixture has the circuit’s power run to the fixture directly and then a switch leg runs down the wall to the switch, with old NM 12/2 wire (ungrounded), bringing two conductive lines to a single-pole switch.

Here’s a diagram as it currently stands:

basic switch-leg

I see two ways to potentially rewire the switch:

  1. The first and simplest way is to continue to use the same single switch leg schematic, but instead of fishing down 12/2 wire (as there currently is), I’d fish down 12/3 wire, to bring down a line and load powering the light fixture, and also connect the neutral at the light fixture into the white wire in the 12/3 which would then terminate a neutral line at the switch.

  2. A second, more complex way would be to rewire the light fixture entirely so that the circuit power runs first into the switch (using 12/2 wire), and then I’d run another 12/2 up to the light fixture. So I’d have to fish down two power lines from the attic (the main power and then back up to the light). Here’s what it would then look like:

enter image description here

What’s the ideal way to wire a switch and fixture given this kind of situation? Does it really matter if power runs to the switch first and then to the light fixture, or vice-versa?

Home Improvement Asked on November 14, 2021

1 Answers

One Answer

I think 1 is better by far.

You mention 12/2 in the text, but the diagram shows 14/2 for the proposed option 2. What is the breaker on this circuit? Will you replace the existing switch box with an old work? The switch loop might be stapled.

Answered by Jim Stewart on November 14, 2021

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