I bought a house recently. Some of the wiring, particularly in the basement, while not obviously unsafe, is... inspired.

On the side of a junction box in the basement, there's a device with two thin wires coming off of it, going up into a hole. I believe, but don't know for sure, that above the hole is an exterior wall, but there's nothing on that wall that would use such thin wires - just switches and outlets, which I'd hope are fed from the thicker cables also going through that hole.

So what is this thing?


4 Answers 4


Looks like a doorbell transformer. Prolly output is 24 V ac from input 120 V ac in the metal box.

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    +1, voltage and VA (volt-ampere or watts) should be marked on transformer, could also be 10v or 16v. Commented Jul 27, 2020 at 3:42
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    It should probably be secured to that joist to prevent strain on the wires.
    – Ben
    Commented Jul 27, 2020 at 12:57
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    @Ben It looks similar to one I've got in my basement; the transformer is probably mounted in one of the knockouts in the handy-box (which also serves to connect the 120V to the transformer within an enclosure). Commented Jul 27, 2020 at 13:44
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    @mustaccio with the wires going up through an exterior wall, I think it's very safe to assume it's not going to his furnace.
    – Jeremy
    Commented Jul 27, 2020 at 15:31
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    @Ben Here's an image of how mine is mounted: imgur.com/a/AV5AvCr . The transformer has a tab and a screw to secure it in a knockout in the handy box. That knockout is also how the transformer's 120V leads get into the handy box so they can be connected within an enclosure. Searching around online, other transformers appear to use similar approaches, e.g., m.media-amazon.com/images/I/616WxQiy-+L._AC_SS350_.jpg appears to have a threaded tube to fit into a knockout. Commented Jul 28, 2020 at 19:57

About 40 years ago, I saw a similar transformer in our basement. I asked my father what it was, and he thought doorbell. We disconnected a wire and the bell still worked. That night, my mom went to make a phone call and the phone dial wasn't lit.

In the old days, mid-late 70's, 4 wires to the phone. Red/Green were the phone service, the other 2 lit up the dial. Sometime later, the current in the phone line itself was enough to light the dial, and the 4 conductor wire was useful to run two phone lines.


There is a slim chance that this is electric osmotic damp treatment. You might see a couple of electrodes on the wall. We had this in a house we moved into in about 1977, we disconnected it and cured the damp by unblocking the gutter...

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    It looks like a timber framed building. Electro-osmotic damp protection is for masonry construction.
    – J...
    Commented Jul 27, 2020 at 16:10

I saw one of those things in my basement as well, and I can confirm that it is a doorbell transformer. If you want, you can get a multimeter and put the two leads on the two screws on the top. It should show a low amount of voltage ≈20-40VAC.

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