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I am working on upgrading some of the light switches to smart light switches for lighting automation.

I bought this GE Enbrighten Z-Wave Plus. This requires a neutral wire.

When I opened the light switch box, there was only one switch with the neutral wire connected and the other one had only two black wire (load and line) without any neutral.

I replaced the light switch that already had a neutral connected with the new smart light switch and it is working fine. How I can replace the other light switch? Can I just extend the existing neutral wire and run it to the other light switch? Is one neutral wire enough for both light switches?

I do see a white wire in the back with yellow cap, can I use that one?

enter image description here enter image description here enter image description here This is from a different box. The one on the left is part of a 3-way lightswitch and the one on the left is an outdoor lightswitch

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    Am I correct that the yellow wirenut has 4 blacks: black from the lower left cable, black from the upper right cable, and two pigtails? – Harper Mar 15 at 0:33
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    Can you inspect both lamp locations for any white wires attached to green or barewires? As it is wired now, I do not see how it could work. – Harper Mar 15 at 0:34
  • Harper, yes, the yellow wirenut has 4 blacks with those connections. I just added a new photo from a slightly different angle. I will check the lamp for white wires. – Chfin Mar 15 at 0:44
  • Are the two switches on the same circuit, i.e., on the same breaker? If they are on the same circuit, you can use the same neutral, but not if they are on different circuits. – Jim Stewart Mar 15 at 1:16
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The wiring as presented makes no sense.

To start with, we can determine the upper left Romex goes to the light now controlled by the left dimmer. The upper right /3 Romex red wire provides switched-hot for that lamp, and its black wire is always-hot for onward loads. The lower left Romex is presumably supply. (it's remotely possible the /3 includes supply and the lower left is onward.)

The smart switch on the left is a customer of neutral, not a provider of it. So the fact that its neutral wire goes only to the switch, simply makes no sense at all. How does the switched-hot obtain neutral, then?

Here's what happened

We get a lot of newbies who are trying to install a smart switch, but it needs neutral and the old switch did not have neutral. They say "Well, I saw in the back of the box where back in the 1960s, the builders left a bunch of spare neutrals (on the off-chance that humans would ever invent smart switches), so I just grabbed one of those spares." They're not spares. Nobody installs unnecessary wires.

Normally the newbie posts here, saying "why doesn't this work?" and we fix it. Often instead, he tries random things until he trips across something that "works" (i.e. not safe). I believe this newbie was up in the lamp box (where this /3 cable goes) and discovered if he shorts all the neutrals to ground, voila everything works. Done!

How to fix it

First you'll need to learn what a pigtail is. If you notice how 2 of the blacks in the 4-black bundle are short wires going to the switches, those are pigtails. Your neutral bundle will look like that too.

Buy 12" of white THHN wire. Probably simplest to use solid #12. Cut it in half for two 6" pigtails. You'll also need a red or tan wire nut, as 5 wires is too many for a yellow nut.

Now remove the existing white wire from that existing smartswitch, and put it back in the existing neutral bundle where it belongs, and also add your two white pigtails. You'll need the red wire nut for this.

One of the pigtails goes to the existing switch. The other goes to your new smart switch (if you're not ready yet, cover the end and bared bits with some wraps of tape).

At this point, voila, things should work. Except we're not done. We also need to find where (presumably the other end of the /3 cable) the last guy tied together neutral and ground, and we need to separate that. It might not be in that location but you gotta find it! It won't bite you until a second problem appears, but then, it'll kill you.

  • Thanks @Harper. I will try to follow these. I just added another picture to the OP (i.stack.imgur.com/HDB9X.jpg). This is from a different location. The one on the left is part of a 3 way light switch (does not have neutral connected) and the one on the right has the usual load/line connected. Are these neutrals in the back with white wires? – Chfin Mar 17 at 1:25
  • This is my first DIY, and it is getting more complicated than what I was expecting (a simple replacement). Maybe I should consider getting professional. Even if I bring in a pro and need to pull in a neutral to one of these boxes. Would pulling in a neutral to a box get expensive? – Chfin Mar 17 at 1:26
  • The home was completely renovated back in 2010, if this helps. Just imagining a building code requiring a neutral to every box. Can't help it! I am hoping something like this exists. – Chfin Mar 17 at 1:33
  • On the 3way take careful note of the brass vs black screws. The 2 brass are the travelers, and I like to mark them with the same color tape (they are interchangeable). Screw color is everything, position of the screws means nothing because every model is different. If the bundle of wires has all white wires, and it's the only such bundle, that's the neutral bunch. – Harper Mar 17 at 2:07
  • Pulling a neutral to a box is heavy fishing-and-drywall work, and can get pricey depending on how the construction has been done. Note the cables that your switch's hot wires are in, note the neutrals also in those cables, see what clump they go into, use only that clump, never any other. For instance if your 3-way has all 3 wires going into the same cable that immediately leaves the box, that means the neutral clump in that box is not yours to use. Solve that by changing the 3-way wiring to a smart switch type. – Harper Mar 17 at 2:12
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Easy peasy lemon squeezy...pigtail your neutral from the bundle to the other switch.

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