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 2 12/2 wires, 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!

EDIT: Added pic of current box, and mapped full circuit + clarifications.

  • Can you measure the voltage between the black and red wires? Jun 18 at 1:22
  • Unfortunately I don't have a multimeter, what would be the concern with the voltage?
    – Dord
    Jun 18 at 1:50
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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 at 2:04
  • 2
    @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 Jun 18 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 at 16:06

1 Answer 1


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.

  • 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 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 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.
    – Ruskes
    Jun 18 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 at 4:37
  • Edited OP with more pics/details for clarifications.
    – Dord
    Jun 18 at 15:56

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