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So I have this electrical box that is housing 2 switches, one of which isn't currently controlling anything, the switches aren't even grounded either, so I'm getting rid of the useless switch and cleaning up that box.

The power enters the box from the 12/3 cable at the bottom right of the box (both red and black are already hot entering) and then from that box there are two 12/2 cables, one going to outdoor lights, and the other one to the doorbell chime box.

For reference, here is the current box, so that right switch is not currently serving any purpose since red and black are already hot coming in. (I'm sure I could re-wire the previous 2 receptacles in the circuit correctly as to have that switch control them, but I don't use them so I'm not interested in that, I'd rather remove that switch altogether):

enter image description here

Here is how I'm planning to re-wire it (and yes, I will also use a wire nut to properly tie all ground wires), and I mapped the full circuit for reference, a few receptacles before this box, and it is not a MWBC, 12/2 from the breaker box, the 12/3 only starts from the third receptacle :

enter image description here

This looks right to me, but would appreciate a sanity check. Thanks!

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    @Dord If you measure 240 volts between red and black, it's a MWBC. If you measure 0 volts between red and black, but 120 volts between either and white, both wires are on the same leg. If they are still on separate breakers, it's dangerous because the white neutral can be overloaded.
    – DoxyLover
    Jun 18, 2022 at 2:04
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    @knowitall your contempt for code and your fellow participants is obvious. It really seems like this platform may not be a good home for you. Maybe you're on the wrong platform. Try 4channel.org/diy
    – Harper - Reinstate Monica
    Jun 18, 2022 at 2:07
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    @knowitall Suicidal? now you're just fear mongering, or you're confusing a multimeter with a voltage tester. A voltage tester to make sure the circuit is off is all that's needed to safely perform basic electrical tasks such as replacing a switch or a light fixture. A multimeter comes in handy for troubleshooting and is necessary for some more advanced electrical tasks, but that's it.
    – Dord
    Jun 18, 2022 at 16:06
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    @knowitall You've missed the point, you seem confused about the difference between a multimeter and a voltage tester, please inform yourself with a quick google search to clear that up. You'll realize that a simple voltage tester pen will prevent all the deadly scenarios you're describing, while being much cheaper, so my point stands: as long as you have a voltage tester, you can safely do basic electrical work around the house without the need for a multimeter.
    – Dord
    Jun 18, 2022 at 19:07
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    @knowitall Stop making assumptions please, I have both contact and non contact voltage testers, both of which are instruments, so I do have instruments to safely change that switch :) What I don't currently have handy is a multimeter, but I maintain that I don't need one to safely change a switch (because again, I have a voltage tester, yay!). It's ok to be wrong and move on my guy, not sure what you're trying to prove to strangers on the internet.
    – Dord
    Jun 18, 2022 at 19:34

2 Answers 2

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First, as far as the mystery switch, that usually controls receptacles. One of two things is likely the case:

  • It controls a receptacle, but you never checked for that. (you may have a "matching" dead receptacle that you never thought to try with the "dead" switch turned on... why would you?)
  • It used to control a receptacle, but a novice changed receptacles and didn't know what they were doing, and shorted out the switch by not "breaking off the tab" where that is required.

Now as far as your drawing, that looks alright, but a multi-wire branch circuit is providing 4800 watts of power for about 25 watts of total load, which makes no sense to me. Are you sure you are not mistaken about that circuit? Are you sure it only powers the one light? Have you turned the breaker(s) off and searched the house for what else died? I would expect it to power at least receptacles in 1 room, or a lot more lights.

It does not need to be wired as a MWBC (and probably shouldn't be, as the 15 watt doorbell transformer can easily coattail on an ordinary circuit. To remove its MWBC character, the black and red could be pigtailed to a single wire, and that wire can be landed on the breaker.

Keep in mind that a "single breaker" with two independent handles is not a single breaker at all, and is the absolute worst possible place to put a MWBC, i.e. that's how you set your neutral on fire.

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5
  • I've been scratching my head about this switch since we moved in, I've pretty much checked the entire inside and outside of the house, it doesn't control anything. The mystery switch was between the black and the red of the 12/3, but the black of that 12/3 was also tied to the other switch and the black of the second 12/2, before the mystery switch, so it was blocking current from going from one live to another live? Not sure what would be the purpose. That breaker also powers all the receptacles in the room, not just this one light, and all are before the box. (1/2)
    – Dord
    Jun 18, 2022 at 2:58
  • I think I better understand what MWBC is now, and you're right, I don't think I have that, something I read researching made me think I might, but I definitely only have 1 breaker for that circuit, I only have 1 breaker, with one handle. I'll get a multimeter tomorrow and confirm to be sure tho, thanks! (2/2)
    – Dord
    Jun 18, 2022 at 3:06
  • It does not makes sense. First I think that was not a Switch, it was a circuit barker. If you have 12/3 with two hot lines, it should have a dual circuit barker in the main panel (bridged together). But you say you only have single CB in the main panel. That would say that only one wire in the 12/3 has power.
    – asinine
    Jun 18, 2022 at 3:46
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    Not necessarily, the 12/3 comes from receptacles that are on the circuit before that junction box, so I'm starting to think it's probably a 12/2 from the panel that goes to a receptacle and then from that receptacle it's a 12/3 for the rest of the circuit, all the way to that junction box, and both the black and red are getting hot from the receptacle, like with a 3 way switch. I'll open the receptacles on that circuit tomorrow to find out for sure.
    – Dord
    Jun 18, 2022 at 4:37
  • Edited OP with more pics/details for clarifications.
    – Dord
    Jun 18, 2022 at 15:56
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So the black and the red wire are hooked up together at the 3rd receptacle is what you are saying. It has to be. So just disconnect the red wire at the 3rd receptacle, strip it off and label it copper ground and connect it with the copper ground there.

And do the same up here at the junction you are working with. Strip it off and connect it to the copper ground.

No need to feed a new wire at all.

Feed the door bell ( as you say) with the black wire which was actually done of course and therefore been working and there has not been any fire or explosions. That is certain.

So whoever wired it like this shows two reason why it was wired like this.

It is the only wire he had, no copper ground. He did not have a cover with one opening for the one switch only, the cover had two opening for two switches.
He had another switch. So he simply used that extra switch to fill in the opening on the cover. So it ended up a dummy fill in switch.

That is what my insight tells me.

Over and out. :-)

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  • @manassehkatz-Moving2Codidact, My main interest (assumption) was why would someone wire it the way you noticed it and having caused no problems. I have a terrible habit with assumptions. I seem to assume all the time as a matter of habit. I had little interest in you stripping any wires. It was merely another way to skin a cat. Why stripping anything when you can run fresh new wires, indeed. A person shouldn't go around stripping wires, not good.
    – Meta_Alchemy
    Nov 17, 2022 at 4:22
  • You see I have been doing wiring for over 3 decades. It has little to no meaning to strip that entire wire at all. The color of wires are just for general purpose identification. By code one should not rely on color of wires to make critical decisions and that is the real code. Labeling wires supercede all wire colors. It is an absolute fact that not all electrical devices have the same value for the color of wires. Hence wiring diagrams hold paramount status. I have little interest in convincing someone, worse yet accepting what I say. Find out..."Wire tagging", maybe not.
    – Meta_Alchemy
    Nov 17, 2022 at 5:01
  • @manassehkatz-Moving2Codidact. Google this: "Can you use both insulated and uninsulated wires as ground". And you don't have to believe the answer you find. That is not my objective. I just brainstorm, that is all. It is not intended to convince you. Just make a note of it. Electricians, when they see a wire labeled, they take that seriously more than all the rest. It is the holy grail. And you can use insulated wire as ground but never ever the other way around, a bare wire as power wire.
    – Meta_Alchemy
    Nov 17, 2022 at 5:21
  • @manassehkatz-Moving2Codidact. Main reason why ground wire is bare and not insulated? To save on cost and you don't have to strip. It has no other real meaning. In all matter of truth, insulated wires are much more meaningful to be used as ground conductor. However, I do not find it imperative to explain why it is truly more meaningful to do so. But for now I will leave this to ponder over. there is a potential difference between netural wire and the bare copper gound wire.... and that is a fact. Find out for yourself. Not good going around and stripping wires, nothing more to be said.
    – Meta_Alchemy
    Nov 17, 2022 at 5:40
  • @manassehkatz-Moving2Codidact, There is potential difference between the neutral and the power wire. Therefore there is a potential difference between the ground and the power wire. Ideally there must be no potential difference between the netural and the ground bare wire. The netural wire is insulated.... that is interesting...why? It should not be. Find out why neutral wire is insulated while ground wire is bare. It is simply assumed and expected that there is no potential difference between bare copper and neutral. At times this assumption can be deadly. That is another story.
    – Meta_Alchemy
    Nov 17, 2022 at 5:51

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