I just bought a house last year and there's this thing with what looks like string in the middle of the electrical system. The two pictures show said thing from a distance and close up.

What is this thing?


close up

For reference, the house was built in the 50s and is in the Bay Area in California.

  • Excellent photos! Well worth thousands of words....
    – wallyk
    Jul 18 '16 at 2:34

Transformer. Probably for doorbell, possibly for hard-wired controls for garage door opener. The "string" is fabric-insulated low-voltage wire, which was commonly used for that purpose.

  • Weird. Where would the low-voltage wire go from there? Would it be in the same metal conduit with the high voltage wire or is it likely behind the paneling? These days, is it normal to put the transformer closer to the doorbell? (Sorry for asking extra questions, but I'll definitely accept your answer!) Jul 15 '16 at 22:36
  • 1
    I've never found much if a rationale for where the bell transformer -- or the one for the thermostat circuit, which is also typically 24V -- are placed. From your photo, it looks like the bell wire is going back into the box and conduits, but I'm not certain.
    – keshlam
    Jul 15 '16 at 23:07
  • @keshlam It very well might serve multiple low-voltage devices and thus not have a logical location. Jul 16 '16 at 5:16

This is a common way to install a 24V low-voltage transformer. These are used for HVAC thermostats, doorbells, garage door opener controls, and other "control" applications. (For instance, if you're hooking up a Nest thermostat and looking for the "C" wire, it's one of those. The other is the "R".)

Here, the transformer is part of a junction box cover plate. The 120/240V mains-wiring parts of the transformer connection are made behind the cover plate, i.e. Inside the junction box where code requires they be made. The 24VAC is on the two exposed terminals. This conforms with NEC requirements that low-voltage wiring not be run in the same raceways as mains wiring.

This installation is improper because the 24V power is run back into the conduit with mains wiring. It should travel the house in common thermostat cable, or at least, in its own conduit.

Also, this transformer/cover plate was "hacked" onto a box that is not the correct size for the cover plate. This may not be done unless the manufacturer has asked UL to list the product to be used this way. Which may be so... since the screw-holes in use look like original equipment.

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