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I am installing a smart switch, so I need a neutral. I am a bit confused which wire is which in my wall switch box. Below is a picture of the wires:

The switch box

I am thinking that the red wire (circled in red) is a random live wire that I am not too sure about. The white wire (circled in green) is a neutral wire. And the 3 black wires (circled in blue) are all live wires, with one going to the power box, and two going to two appliances. The thing is, this switch only controls one light, so I am not sure why there are two black live wires going out to appliances.

The switch box with circles

Any help?

  • What are you trying to accomplish here? – ThreePhaseEel Feb 23 at 4:07
  • I am trying to figure out which wire is which so I can replace the switch with a smart switch. – potay Feb 23 at 5:34
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    @potay that being the crux of your question, I edited the question to make that the lead. If that doesn't match your intent, hit edit and tune it as you like. – Harper Feb 23 at 13:09
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You are in the conduit wiring method.

The rules here are different.

  • most likely, the ground is the metal conduit itself. That's done!

  • it is normal to have wires pass through your box without stopping. Those wires are not usable by you, and should be disregarded. (White and red)

  • Neutral will not be provided in switch loops because it is easy to pull neutral if it is needed in the future.

Your assessment of the purpose of the three black wires is correct, though more likely the 2 blacks are the always-hot, and the 1 black is the switched hot to the lamp. There are 2 always-hot blacks because power goes onward to other outlets or points of service. The neutral you see is surely for that onward use of power.

Note that the switched lead doubles back in the same conduit. That is fine. I'm a little disappointed that the installer didn't any of the 6 other allowable colors for the switched hot, to distinguish it, or at least mark it with tape.

I gather you are not familiar with multi-wire branch circuits (MWBC). I don't know what the red is, but it's probably an MWBC. This means there will be 2 breakers you must shut off to deenergize both black and red, and if you don't get them both, you will get electric shocks when separating the neutral. They may already be handle-tied in the panel. They should be by modern Code. (if it's a double-stuff, let us know!)

Getting neutral

You do not have neutral in that box. That loop is not available to you and might as well not exist. Don't even think of trying to bite onto that somehow, if you're gonna do something that nasty, you might as well bootleg neutral off the metal chassis of the box.

To get neutral to that box, you will need to pull it. As it happens, there is already a "rope" in the conduit fit for pulling a new neutral, so this will be easy but will involve gasp material cost. The material is #12 THHN, THWN-2 or XHHW wire that is white. Your pick, solid or stranded. You will be pigtailing both ends, so stranded will work great.

You see the white looping through. Follow it in both directions to whichever other box it goes to. You may need to open up a lot of junction boxes, although it's a guess that one direction goes toward/to the light. You shove the white wire up the conduit about an inch, and see if it moves on the other end. Also, get a handle on how stiff it moves, you are interested in the direction with the easier pull. You are not interested in a location where the white wire "loops through" like it does here. It needs to terminate there. We are going to use that white wire to pull in a new white wire.

Now, you've identified the easiest pull. All breakers must be off, don't let the MWBC bite you. Now, unhook that white wire. Straighten it out and lay your new white wire alongside it at least 9" of distance, so they overlap, and tape them to each other with electrical tape the whole length of overlap. They mustn't separate! Use tape at the lead to make sure the front of the new wire doesn't present a hard corner, or it will snag going through.

Now push this into the pipe. I prefer to push than pull, honestly, but eventually you'll hit a snag and pulling will make it easier. It's less brute-force, and more teasing it through. Twisting the wire often also helps.

You finish when you have all 3 ends of the white wire sticking at least 7" past the surface of the wall (10" past the end of the conduit). It can now be snipped to that length. See why we couldn't use the white as-is?

Now, we talk about a fundamental rule of MWBC. You must be able to remove a device without interrupting the neutral. Now since I gather your smart switch will come with wire leads, forcing you to pigtail, you will need to double-pigtail the neutral here. That way you can remove the smart switch later without having to separate the two wall neutrals. So wire-nut the two wall neutrals to a short 6" piece of white wire. This shorty then gets pigtailed to the smart switch's neutral wire.

Presumably, the other box you just ran white from will also have been pigtailed already, if not, pigtail it.

The two blacks that were on the old switch together, get pigtailed to the (presumably black supply) wire of the smart switch.

The remaining black goes to the switched-hot lead of the smart switch.

  • I'm confused, are you pulling one white or two white wires? – Platinum Goose Feb 23 at 13:35
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    You're using half of the loopthru white to pull a new white. The other half of the old white remains. That accounts for the 2. – Harper Feb 23 at 13:37
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The two black wires on the bottom are the "hot" wires and the black one on top is the "switch hot". There are two wires on the bottom because the "hot" goes on to another load. Possibly a receptacle or something like that. The red wire should be taped or marretted (wirenutted) and the green screw should be grounded. It looks like there is only 1 neutral or it's possibly connected to the red? If that's the case don't touch them. You should also tape around the switch. Like this. enter image description here

  • Thanks! So I think there's no ground. The red and white just go from one hole in the box to another. I believe they are not connected to each other but go directly from the power box to the light? What is the red wire for? Thanks for the advice regarding the tape. I'll do that. – potay Feb 23 at 5:37
  • It's hard to say what the red is for but any unused wires should be taped or marretted (wirenutted) – Joe Fala Feb 23 at 5:44

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