Why is my circuit breaker being tripped after removing a light switch?

Question

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:

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

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

Breaker box:

Answer 1

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.

Answer 2

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.

Answer 3

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