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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?)

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I'm trying to trace the wires and it appears to go into a strange junction box in my basement that looks like this:

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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.

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That looks like a doorbell transformer. Those smaller wires should lead to your doorbell chime unit (assuming you have a doorbell). The junction box is there simply to provide power to the doorbell transformer.

From the picture it looks like you have 12/3 and 14/2 in that box, but it is difficult to tell. How confident are you the larger wires are larger than #12?

Figure out what breaker is supplying power to that circuit. As long as it is sized appropriately for the smallest conductors on the circuit, it will be fine. For example, if that is indeed a mix of #12 and #10 copper, your breaker must be 20A or smaller.

Also as a side note, NM cables entering that metal junction box need to be secured with a cable clamp. Any unused knockout holes need to be plugged. It looks like you've got a couple of code violations there at the top of your picture.

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  • Thanks so much. I'm almost positive that the larger wire is larger than 12 gauge. I'm not even sure I've seen electrical wire that large before. Anyways, these wires and box are from the previous homeowner and I'm just trying to add the 3-way switch. I'll be sure to plug the holes though to bring them up to code and double-check the breaker size.
    – SofaKng
    Commented Aug 16, 2015 at 1:00
  • That transformer mounting is the standard, approved way to feed a low-voltage transformer from 120V wires; it meets the code requirements to keep line voltage and low voltage separated. Thus, a transformer inside a box would be a violation, as would running 120V wires outside a box to an external transformer. As for the heavy ga, let's assume the prior homeowner got a bunch for free, or leftover from some other project, and wanted to use it. I find 12ga hard enough to work with, frankly.
    – CCTO
    Commented Feb 4, 2021 at 18:32

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