I'm putting some light fixtures in my basement stairwell. There's a box with two switches in it pictured here. I'm adding a third switch, and I don't know which wires to tie the black and white from the fixtures into. I plan to take the two switch box out, and recreate this exactly as I found it so my other basement lights continue to work. If it helps, I believe that these two switches pictured are on different fuses.

 

The switch on the top is a switch leg only. It can't be touched.
The bottom switch is the start or end of a three way circuit.

You would need a voltmeter or voltage tester to see if that two wire cable where the black connects to the switch is the feed in or out to the lights.

They are most likely on two separate circuits too.

 

Thanks! Are there possibly two different configurations that I could test in order to determine which way is right? My local hardware store closed at 7 and I don't have a voltmeter.

 

If you have some kind of test light/socket.

Unfortunately that two wire cable is either the hot in or the switched line to the three way switch.
There is no way to guess.

.... and if that is indeed the switched line out to the lights..... you can't add anything there.

 

I already have the fixtures mounted and wired up, light bulbs and everything, ready to hook into the box as soon as I know which wires to use. I was going to use that as a test.

 

So you are running another cable to this location and adding another switch, keeping everything else as is??

 

Yes, the new cable goes to the fixtures, and has black, white, and bare ground wires.

 

Oh.... ok. Then remove the black wire that connects to the dark colored screw. It would be the black wire from the two conductor cable. Connect that to your new lights black wire. Connect the white wire to that two wire splice.

If you lights come on your in luck.

DO NOT remove the wires from the single pole switch.

 

Do you have an extra outlet laying around and some romex?

If so, (shut off power) remove wires from 3 way switch. Pay close attention to the wiring.... The wire going to the marked terminal has to go back exactly as it was. Marked terminal is dark.
Take some romex and wire to outlet, white to silver screw, hot to dark screw.
At the 3-way wires:
connect white from outlet to 2 neutrals at 3-way
connect black from outlet to wire from marked terminal on 3-way.

If the outlet works, you can run wires here. If it doesn't work you can't.

Doing this test saves you from removing existing box if you can't use it.

 

I tried as PJmax suggested, and took the black wire from the black screw of the 3 way switch (from the middle cable coming into the box), and wired it to my new switch. I sent the black from my new cable to the other end of the new switch. I connected the ground and the white to the existing splices.

No light

So now what?

 

Connect your black wire from your new lights to the black screw on the switch.
See if your new lights work with the 3 way switches. If they do.... game over.

 

Yep, they sure do... It's beautiful.

So if I am understanding correctly, there is no way for me to use the available wiring in this box to run my lights.

If I splice them into the wiring previously on the 3 way, will they turn on when I turn on the other lights that were also previously on the 3 way? Or will it all just not work entirely if I do that...

 

Correct.

You can connect them exactly like the two wire cable is set up and then the three way switches will work the old lights and your new ones.

 

Can you be a bit more specific? There's two wires in my new cable: black and white. There's three wires in the 3 way switch: the black on the black screw, a red on a gold screw, and a black on a gold screw on the opposite side of the other two.

Assuming I wire the white from my new line into the white bundle, what do I do with the black from the new cable?

 

You would take the black wire off the switch.
Connect that black wire to your new lights black wire.
Add a short piece of black wire for a tail. Wire nut that three wire splice.
Put the black wire (tail) on the black screw.

Connect your new white wire to the existing two wire splice.

 

Works like a charm, and thank you very much for all the help, and for being so timely!

Last question: if I splice both my new white and black into the white and black in the OTHER switch (the one you called a switch leg), would it operate in tandem with the lights on that circuit, or would it all fall apart?

 

If you connected you new lights to that single switch..... when the switch was off..... all the lights would be on dim. If you turned the switch on..... your new lights would be off and the old ones would be full bright.

That switch is only switching hot. There is no neutral there for it.

 

Thanks for saving me the effort of trying it and being all confused lol.

And thanks again SO SO SO much for the help!

 

Your are very welcome.