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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?

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  • Which type of cable do you have extra of in your inventory? – Harper - Reinstate Monica Jul 21 '20 at 22:24
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    I have a reel of 250ft of 12/2 but picking up some 12/3 is no big deal. I care more about my time, and doing it right, than if I have to spend an extra $15 or $20. – Jason Jul 21 '20 at 22:56
  • Ummm... 12/2 is 2 conductors plus a ground. Are you sure yours is ungrounded? Did the previous "electrician" just snip the grounding conductors at the end of the jacket and not connect them at either end? (Yes, I know this is about the neutral, not the ground, but that jumped out at me.) – FreeMan Jul 21 '20 at 23:10
  • @FreeMan yes it’s ungrounded. I’m saying 12/2 because it has two conductors. But this is old NM cable, it doesn’t have a ground wire. By replacing it with newer 12/2 romex (which does have a ground wire) I can now ground the electrical box too. – Jason Jul 21 '20 at 23:11
  • OK, just checking - I've not worked with actual 2-wire NM, so I wasn't aware of that (I fall into the "electrician" category, not the electrician category). Also, I like your attitude of "do it right, even if it costs more"! – FreeMan Jul 21 '20 at 23:12
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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.

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  • Yes, I should have been clear, the diagram says 14/2 but I’ll be using 12/2 as it’s on a 20A breaker. I also will replace the existing switch box, mainly because smart switches are so much bigger so I’ll need more space for the extra wires. And if the switch loop is stapled, can I just leave it in the wall unterminated at both ends? I might not be able to fully pull it out. – Jason Jul 21 '20 at 22:55
  • If you are replacing the box, I would think you could get a hand in and pull out the staple. – Jim Stewart Jul 21 '20 at 23:31
  • if there’s one staple. If there’s more then one it’s a bit harder. – Jason Jul 21 '20 at 23:32

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