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I have a switch box with two switches. I'd like to replace one of them - a single-way switch controlling a light in the hallway (which is controlled by no other switches) - with a smart switch. The smart switch requires a neutral wire, and there is not one present in the box. The box has cables for the other switch (the secondary end of a 3-way with switched hot and 2 travelers), and the cables for the switch I want to replace (an always-hot and a switched-hot).

It's not very far to the light: my plan was to get the ~10 feet of insulated wire that would be required, and run a single wire up through the box into the attic (following the wires that are already there), then to the light fixture where I could tap neutral.

Reading about it, it seems like single-wire runs in free air are prohibited (I'm in the US, the construction is early 2000s). I've come up with this plan instead, and I wanted to know: (1) Is this an acceptable alternative? (2) Is this necessary or have I misread, and I could just run a single neutral?

My plan: I can get 10' of 14/3 (it's the cheapest thing by the foot that I can find available at my hardware store... I could get 12/3 for a little more, but there's not 14/2 or 12/2), and that'll be enough for the run. Patch white into light fixture's neutral, unplug the fixture's switched hot and instead connect the red conductor in the 14/3. Wire the bare copper to ground, obviously. Take the old switched-hot line and cap it. On the switch end: run the new cable into the box, tie bare copper to the GND there, tie the switched-hot (red) to the output of the smart switch, and then I'll have my neutral. Take the extra conductor (black) on the 14/3 and cap it on both ends.

I'm worried that this isn't quite right, either, since apparently the current in a cable needs to be net 0: does that mean I need to find where the always-hot is coming from into my switch, and wire that to the black cable in the 14/3? If I did so I would expect I should cap the old always-hot on both ends, but I don't know if it goes and connects anywhere else.

I appreciate your help!

Side note: the GND in the switch box gets pulled in and just kinda... seems to sit there, maybe touch the wall of the box - but doesn't seem to be directly connected to the 2 switches chassis/ground: is that kosher?

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  • if "the construction is early 2000s", you should have neutrals in there somewhere, maybe just behind the box.
    – dandavis
    May 25 at 19:24
  • @dandavis If it's behind the box, is there any way to get at it (shy of pulling out drywall)?
    – user112697
    May 25 at 19:33
  • sometimes you can fish them out or through a knock-out. sometimes you can replace the box with an old work without messing up the drywall too bad.
    – dandavis
    May 25 at 19:47

2 Answers 2

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My plan: I can get 10' of 14/3 (it's the cheapest thing by the foot that I can find available at my hardware store... I could get 12/3 for a little more

Hold on there, you need to go find your circuit breaker first. The breaker could be 15A or 20A (rarely: 30A) but #14 can only be used on 15A breaker. Breakers protect wires.

"but my lamp only draws 12 watts" We don't design electrical for normal conditions, if we did, we wouldn't need grounds or breakers at all!

Patch white into light fixture's neutral, unplug the fixture's switched hot and instead connect the red conductor in the 14/3. Wire the bare copper to ground, obviously. Take the old switched-hot line and cap it. On the switch end: run the new cable into the box, tie bare copper to the GND there, tie the switched-hot (red) to the output of the smart switch, and then I'll have my neutral. Take the extra conductor (black) on the 14/3 and cap it on both ends.

In other words, presently you have what's called a "switch loop" with a single cable from lamp to box, using /2 cable (always-hot and switched-hot). You are proposing adding a 14/3 cable (because it's cheaper) and using 2 wires out of it.

In 2011 Code changed to ban the old 2-conductor "switch loop". They can still be done, but must use 3-conductor cable, with white as Real Neutral (unused and capped off) plus the usual always-hot and switched-hot. I'll give you one guess why that code requirement was added.

So - completely unbeknownst to you - your plan is exactly what you're required to do post-2011. Use /3 cable for switch loops.

I'm worried that this isn't quite right, either, since apparently the current in a cable needs to be net 0: does that mean I need to find where the always-hot is coming from into my switch, and wire that to the black cable in the 14/3?

Obviously you have some experience with low voltage DC from vehicles or electronics (e.g. "GND" for safety ground). In DC you can run random individual wires wherever you need, you don't need to twist pair or otherwise couple wires unless there's a signal on them. However, a DC magnetic field is like a refrigerator magnet. An AC magnetic field is like a refrigerator magnet that is spinning at 3600 RPM. Not good!

So NEC 300.3 requires current to be balanced (or equal/opposite) in each cable. (a cable is several wires in a sheath). In practice that means your wiring diagram can be a tree topology but it can never be a loop. Just like on a real tree where branches never merge into each other, neither can cables.

From that there's a simple rule: A device can only take neutral from a cable that it's already taking a hot wire from.

So you must use the /3 cable as the one and only cable, and remove or destroy the /2 cable. That way, you have all 3 conductors in the same cable.

The /2 cable is unlikely to ever be useful. To destroy a cable, cut it off at the edge of the box to where it cannot be used, and push it out of the box.

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    Thank you for the clear explanations: I can go with the 12/3, that's not a big deal - and (if I understood correctly) my initial initial plan, to just run the neutral, is no-go. My second plan (to just use neutral/switched hot from the new cable I'd run) is also a no-go, because the net cable power should be 0. My final plan is OK - I'll just need to be sure that the hot I'm removing from the switch doesn't go anywhere else (or I'll need to wire that in to the new cable). As far as "destroying" a cable: I'll agree it's "unlikely to ever be useful," but is it kosher to just cap it instead?
    – user112697
    May 25 at 19:36
  • It is kosher to just cap instead. Except that often switch boxes are just barely big enough, and if the wires are left capped inside the box then they count (both code-wise and physically) towards box fill. May 25 at 19:54
  • @manassehkatz-Moving2Codidact Where can I learn/read about box fill limits? Obviously I understand the physical implications, but I'm concerned about the legal ones.
    – user112697
    May 25 at 20:13
  • Look around this site - there have been a lot of questions where box fill comes up (which is how I learned about it). Or try a box fill calculator like: constructionmonkey.com/calculations/misc/boxfill Sometimes the hardest part is figuring out the box size on an old box because it is marked somewhere... May 25 at 20:23
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    Update: I did it. Running a new cable through the fiberglass in the attic was miserable, but it's all working now. Thank you for the help!
    – user112697
    May 26 at 1:23
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To use the 14/3 the circuit must be on a 15 amp breaker, for a light circuit it is possible, most circuits with outlets are usually on 20 amp breakers now and require 12/2 or 12/3 cables.

14 gauge wire cannot be on 20 amp breakers, but 12 gauge can be on 15 amp breakers.

It sounds like you have a switch loop and to use /3 cable, one hot of /3 is connected to the hot in the ceiling to the switch.

The second hot is connected to switch and to the hot of the light.

The neutral from the ceiling is pigtail to the light neutral and the neutral of the /3 cable. the /3 neutral is capped off at the switch, till needed for the smart switch change over.

Ground should not be "hanging" in the box, but must be screwed to the box(if metal box), if plastic box must be screwed to the switch.

If a metal box usually an option to run a ground wire from box to switch, but not usually needed since the metal of the switch, grounds to the metal of the box.

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