I have a box that has 2 3-way switches. One goes to control the basement lights. The other one controls a light outside the basement. There is also a 3-way switch outside the basement that controls that light. The switch for the basement lights works fine, but the other 2 switches no matter what configuration I have tried I can't even get the light outside the basement to light. I have a diagram below. The dots next to the switches is what I believe is the correct setup, but it does nothing. Here is the weird thing. If I take a wire from the red connection on Switch A to the red connection on Switch B then I can turn the light on and off from switch B. I know that isn't right because then the lights to the basement don't work correctly from other switches for the basement lights. Switch B & C never worked correctly, at one point if you switched it on from C you had to turn it off from C. Any help is appreciated.
Whomever did this fundamentally misunderstood how three-way switches work. A three-way switch changes which wire (in your case the white/brass screws) is connected to the black screw. One of the two brass screws will always be hot. We call the wires connected to them traveler wires.
There's no easy fix with the existing wires here because they tried to make one cable do the work of two. I put in a three-way last year and my setup is
- Line (hot and neutral)
- Load (2-line cable going to my fixture)
- 3-line cable running between the switches (two travelers and the switched hot)
That's all going into what is your B switch. In other words that's three cables where you have two. The good news is the 3-line cable can still be used. The bad news is you really need a 2-line cable run between B and your fixture. I suspect they tried (and failed) to use the unused white wire to do that. The problem is that you can't run a multi-way without two travelers.
Per the images you've added here's how I would fix this
- Go buy some equivalent gauge metal-clad (MC) cable. I can't tell if that's 12 or 14 from the jacket so use a wire stripper to confirm size. You'll need two cable clamps as well. I think both boxes are a combo 1/2" and 3/4" but measure a knockout to be sure (1/2" clamps are sometimes confusingly labeled as 3/8"). It's also a good ideal to buy some cable clamps to secure it to the wall
- You need to bond your metal boxes. Buy some ground screws (they're green) and add some wire between the box and your ground nut. Some places have ground pigtails if you don't have enough spare ground wire.
- Run your MC cable between the B box and the middle box. You do not need to remove any existing cable. Knock out a hole in each box and clamp the cable
- Inside the box, disconnect everything except the ground (hopefully you bonded in step 2)
- In the B box add your MC black wire to that black wire nut for the other switch. Add the MC white to the white nut.
- B will now hook to the 3-line black screw, and the 3-line white and red go to the brass(white) screws (order does not matter). Hook the ground wire up as well. B is now done
- Rewire C to swap the black and white (this helps the sanity of wiring). Black is now on your black screw.
- In your box hook the B and C 3-lines up as such: C-black to MC black, C-red to B-red and C-white to B-white. I would mark the 3-line whites with a black sharpie so you know it's a traveler and not a neutral.
- The 2-line is your load to the fixture. The white from this goes to your MC white (neutral), and the black connects to the B-black (switched hot)
When this is all done what you're doing is piping your hot to C, creating two proper travelers back to B, and hooking the fixture to the switched hot from B.
A three-way switch is what in the computer world is called an "A/B switch." That is, there are three power-carrying wires, A, B, and C, and two of them are connected at any time.
If the switch is "up", wires C and A are connected.
If the switch is "down", wires C and B are connected.
Wires A and B are never, ever connected.
You should be able to verify this with the resistance function of your multi-meter, or by wiring up a table lamp or something. Hook up two of the terminals, and check connectivity. Then flip the switch and check connectivity. If there is never a connection, you have hooked up A and B. If there is sometimes a connection, you have found A and C but you don't know which yet. Move one of your probes over to B, and repeat the test. You'll know whether you have B and A or B and C at that point.
Essentially, a 3-way switch acts like either a splitter or a joiner. Power goes in C, and splits out to either A or B. At the far end of the circuit, power travels in on either A or B, is joined up and comes out (or not) on C.
This creates the "switched from either end" behavior of stairway lights by requiring agreement between the two switches. If power goes into a switch and comes out on A, then the second switch must connect A to C or there is no circuit. If the first switch changes to B, then the second switch must connect B to C or there is no circuit.
The wiring for a three-way switch should be like this:
Power comes from "upstream" to terminal C on switch 1. Two traveller wires connect from terminals A and B on switch 1 and head for switch 2.
The traveller wires arrive at terminals A and B on switch 2. Terminal C is connected to the "load" circuit.
Your wiring diagram looks like you have connected a white wire to "terminal C" on switch "C". That is wrong. Connect the wire carrying power out to the lights (black or red) to the C terminal of the switch. The white wire should continue through the box to the light fixture.
As @Machavity mentioned in his answer, it also appears that your switch A is splitting wires that you run to another light circuit. If there's not another 3-way switch, then you should just disconnect one of the wires from this system -- it's just an on/off switch at this point, there's no reason to carry power down a wire you aren't using.