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In my dining room, there is a 2-gang switch box that houses two switches. Switch A is on a completely separate circuit from Switch B. Switch A controls a single light in the dining room, and is the only switch that does so. A while back, I replaced this with a Kasa smart switch, which went very smoothly. I left Switch B there. Switch B controls a light in the adjacent kitchen, and is a 3-way switch - there is another switch in the kitchen that does the same thing. Switch A's circuit is shut off at the circuit breaker in the basement, Switch B is one of the circuits in the box in the kitchen. There are neutral and ground wires in the box.

Yesterday, I went to replace the smart switch (Switch A) with a smart dimmer switch, and remove Switch B, connecting its line and load wires with a wire nut in order to make the other switch in the 3-way configuration the only one that controls the kitchen light. I joined the neutral wire that was attached to this switch to the rest of the neutral wires (I don't fully understand neutral wires). It seems like there were some issues with this process. When I first restored power to both circuits, Switch B's light remained off (I don't think the breaker was tripped though), as did Switch A's. I found that some of the ground and neutral wires were loose since there were too many for the wire nuts, so I fixed that. Switch A worked as expected, but the light was still off for Switch B (not sure if this was because it was tripped).

At this point, I tried using a wire nut to join Switch B's neutral, line, and load (which I now understand is a bad idea), but this tripped the circuit breaker. I restored it to the previous configuration, but the circuit breaker remained tripped when I tried to turn it back on. I read that sometimes you need to push the circuit breaker harder when you turn it off, and got it to turn back on by doing so (at which point the rest of the appliances on the Switch B circuit turned on, but not the light it controls). A slight jostling of the wires caused it to trip again, though, and now it won't turn back on. I have made sure the neutrals and grounds are all in contact, Switch A works normally, and I've opened up the other switch on Switch B's 3-way configuration to get a picture.

Here is a picture of the box from when I was taking out the original non-smart switch, in which you can see Switch B still in there, with a red wire and white wire visibly attached:

Switch B

Here's what it looks like now with Switch A on the right and the red and black wires joined:

Wire Nut

And the other 3-way switch on Switch B's circuit in the other room:

Other box

Breaker box:

Breaker box

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    Aside from the super dangerous stuff mentioned in the answers, the quality of the wiring connections does not look good. For example, in your first photo, the white and red wire ends under the screws just seem to be straight, not in the proper looped "Shepherd's crook" for good secure contact with the screw head and terminal. The twisted-together wires look insecure to me as well. Why not practice a bit with some wire and a switch on your kitchen table to become proficient at attaching looped wire ends to screw terminals, and making good strong wire nut connections that won't come loose?
    – Armand
    Jul 8, 2022 at 4:09
  • @Armand Appreciate the suggestion, however I didn't make those straight connections and I only twisted the other wires together because I didn't have a big enough wire nut at the time.
    – Alex
    Jul 8, 2022 at 12:36
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    With the mess created I see a Darwin Award in your future. It appears you connected a hot white to the neutrals (Dead short breaker trip) we do not combine neutrals of separate circuits as it creates a shock hazard and is a code violation. How is it a shock hazard you can turn 1 circuit off to work on it but the other circuit is live and the neutral is the return so open it from being grounded and now there is line voltage just waiting to provide your award as mentioned.
    – Ed Beal
    Jul 9, 2022 at 14:34
  • Does "I went to replace the smart Switch A…" mean you did more than remove a light switch, or not? Jul 11, 2022 at 21:51

3 Answers 3

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STOP

Sorry to be blunt, but you have no idea what you are doing and it is only because you have good circuit breakers that your house has not burned down. Seriously.

A few key pieces, but nowhere near enough to figure this all out. I recommend either some serious studying of how US electrical wiring is done before doing anything else at all, or call an electrician. Really. And I don't say that often.

Never Mix Circuits

You can't mix neutrals between different circuits. Since you know that A and B are on different circuits, you must never connect any of their wires together, except for green and bare grounds (which always go together). Mixing neutrals can lead to overloaded neutrals, which is a fire hazard, because in most cases neutrals do not have over-current protection, and also to unexpected "hot" wires because you might turn off a circuit to work on it and find that the neutral wire is still carrying current. Neutrals are bonded to ground, but they still carry current, which is why they are insulated (unlike ground which can be equally bare or green or touchable metal conduit).

Colors Don't Matter in 3-Way Circuits

Or rather, they do matter but they are not so simple. A standard US cable has black and white (for 2 wire cables) or black, white and red (for 3-wire cables). There are a bunch of different ways of wiring up a 3-way circuit, and in many of them, in at least some part of the circuit white is not neutral. So even though usually all the whites are neutrals and go together (plus pigtails off to switches or other devices that need neutral), there are a number of situations where white is not neutral and does not get connected to the other white wires, and in those cases connecting the white wires together can result in short circuits (breaker trips - a good thing!) or dangerous situations.

Always Do One Thing At A Time

Even if switch A and B were on the same circuit, there would be some real advantage to working one each one separately. You managed, in the end, to get switch A working but not necessarily safe. Once you start pulling wires off of two unrelated switches at once, it becomes easy to get things confused.

Labels

If you work on switches one at a time, and know exactly what you are doing, it can still be helpful to label the wires as you go. In particular, 3-way switches get confusing. Different manufacturers (and sometimes different models within a manufacturer) will put the common vs. traveler wires on different locations on each switch. So a good work process, if there is any possibility of confusion (e.g., two black/white cables, one from the panel, one to the light) is to use colored tape to mark things. For example, red for switched hot, yellow for travelers, etc. In particular (actually required, but not always done but definitely best practices) mark any white wire that is not an actual neutral.

Traditional 3-Way Circuits have TWO Travelers

These are a key. The first 3-way switch doesn't have "hot/line" and "switched hot/load" like a regular switch. It has "hot/line" and two "travelers". The second 3-way switch doesn't have "hot/line" and "switched hot/load", it has two "travelers" and "switched hot/load". So you "connecting its line and load wires" doesn't actually make sense. It is either "connect its line and travelers" or "connect its travelers and load", and in either case that's three wires, not two. There are actually better ways to remove a 3-way switch, but done correctly that will work.

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    +1 for the first two paragraphs alone (in this particular case)
    – maples
    Jul 7, 2022 at 22:10
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    Fair enough, my reach may have exceeded my grasp somewhat on this one. This is definitely not my area of expertise, I shouldn't have jumped to the conclusions that three-way switch wiring and wire coloring would work the same as standard switches, and once I thought more about the fact that they were on two separate circuits while writing this question I realized that I shouldn't have crossed those wires, so to speak.
    – Alex
    Jul 8, 2022 at 12:49
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    It might be worth noting in this answer that mixing neutrals can create both a fire hazard and an electrocution hazard. Jul 8, 2022 at 19:44
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    @EdBeal Mixing neutrals could lead to overloaded neutral. Hypothetical: Two 15A circuits, each has 12A (heater) and 1A (light). The 12A from one goes on the other neutral (and the 1A from the other neutral goes the other direction) - now instead of 13A on each you have 24A on one and 2A on the other, but the hots are each 13A so no breaker trips. Extreme but technically possible with just a couple of wires moved around. Jul 10, 2022 at 2:02
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    I guess an internet electrician might think this but the branch circuit is protected by the breaker so joining them all together creates multiple paths , cross connecting a neutral could over load it where the splice going back to the panel is on a 14 awg but the hot coming from the panel is 12 awg protected by a 20amp breaker but this is not what the op did. All the neutrals combined create a parallel path and could never be overloaded.
    – Ed Beal
    Jul 10, 2022 at 14:15
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I joined the neutral wire that was attached to this switch to the rest of the neutral wires (I don't fully understand neutral wires).

That right there.

#1 that wasn't a neutral wire.

#2 even if it was a neutral wire, you don't just join all neutral wires.

remove Switch B, connecting its line and load wires with a wire nut

3-way switches don't have "Line" and "Load". And they don't have "neutral" either.

I'm just trying to illustrate how far off the rails you are, so you can avoid a Dunning-Kruger skill pit. That's when someone learns just a little bit, and confuses that with knowing everything. And then, their brain cannot accept evidence of not knowing everything, which shuts down all learning! Read the scientific paper, it's astounding.

Put it back the way you found it, then don't worry about it.

Not least, there may be a Code requirement for a 3-way to be here. Builders don't install unnecessary switches for their health.

Honestly your best exit is to simply restore the original mechanical 3-way switch, as you found it. It appears to me that a black-white-red went on the original switch, most likely all from the same cable.

I honestly cannot tell you which one is "common" and which are "travelers". You shouldn't experiment generally. But in this particular case since a) they are all hot wires, and b) it is a plain mechanical switch, this time only there is no harm in just trying all 3 possibilities (white on common... black on common... red on common). The other 2 wires are interchangeable. I would start with white=common, just a guess.

Oh, and if you don't like the color or style of the original 3-way, feel free to change it out to a simple mechanical 3-way that is white, Decora, whatever floats your boat. I really don't see why you need to remove a Decora 3-way switch - they do no harm just sitting there, and a missing switch is a really weird aesthetic.

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It's incorrect to assume you cannot share neutrals. Load on the breakers from individual circuits with shared neutrals on standard breakers can trip if the combined circuits exceed capacity of either breaker. Also you cannot share neutrals between 2 circuits if either one is an arc fault breaker. Breakers using shared neutrals also must be connected with a bar so that both trip together. The problem isn't the switch it's in the rewire however photos provided do not show enough of what was rewired in the boxes to definitively provide an accurate assessment. We can only theorize that it is how things were rewired. It's important to look at code as to permitted conditions .https://www.neca-neis.org/code-question-of-the-day/code-question/cqd-10-10-2019

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  • This doesn't answer the question and would be better as a comment once you have enough rep.
    – JACK
    Jul 9, 2022 at 15:39
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    A multi-wire branch circuit (singular) is 1 circuit not 2. It is not "shared neutral circuits". It is not multiple breakers, it is one breaker optionally field-assembled with a handle tie. It is 1 circuit with 1 neutral and multiple hot wires. It only counts as multiple for provisioning (e.g. 2 kitchen circuits). This is a Q&A site, and answers are answers to a question, never a blanket statement. Therefore you should not nitpick and "yeahbut" as one might with a blanket statement. It's also not a discussion forum and this is not an answer. Jul 10, 2022 at 8:52
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    @Harper-ReinstateMonica: It may also be good to state why connecting neutrals of unrelated circuits is dangerous: if two circuits have separate non-connected neutrals and one of the neutral wires fails, the circuit in question will pass no current. If neutral wires are connected and one fails, the other will have to pass the current for both circuits. If both circuits are are at their maximum peak load, that wire will heat up four times as much as its maximum peak load, which may in turn in turn be four times its continuous rated load, without tripping any breakers.
    – supercat
    Jul 10, 2022 at 14:18

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