I'm trying to replace an existing switch (at the top of the basement stairs) with a 3-way switch by adding an additional switch at the bottom of the basement stairs.
The existing switch box has 4 romex wires coming into it. Three of them are 12-2 and one is 12-3. The 12-3 is already connected to the switch like shown in this picture: (maybe they pre-wired it for a 3-way switch install at a later date?)
I'm trying to trace the wires and it appears to go into a strange junction box in my basement that looks like this:
Does anybody have any idea what is going on here? The incoming wire looks huge like an 8-3 or 10-3 wire (?). What is this junction box and what are the tiny telephone (?) wires coming off of it? The smaller side has 12-2 wires going to the switch, and the larger wire appears to come from somewhere (upstairs electrical box?). The tiny telephone wire appears to go back upstairs through the floor boards to somewhere I don't know.