This thing is on my junction box but i don't know what it is or what it's for.Junction box



  • Can you post a photo of the label on the transformer? Commented Sep 17, 2017 at 21:41
  • @ThreePhaseEel i edited my post. This is the only label i can find on it.
    – hack3rfx
    Commented Sep 17, 2017 at 21:46
  • So, you want to have an outlet wired downstream of this box, on the same circuit as the transformer, right? Can you get us a photo of the inside of the box (with the breaker off, of course)? Commented Sep 17, 2017 at 21:54
  • @ThreePhaseEel see my edit please for photo
    – hack3rfx
    Commented Sep 17, 2017 at 22:31
  • 1
    @Toby Yes, or at least in these specific photos
    – hack3rfx
    Commented Sep 18, 2017 at 12:31

2 Answers 2


This is easy

What you have here is a low voltage (NEC Class 2) transformer that steps house voltage down to a safe voltage -- they're typically used to power doorbells or HVAC controls, but may be found in other places as well.

Simply get your new cable into the box with the power off (use one of the built-in NM clamps you can see on the bottom right), then connect black to blacks, white to whites (+a black from the transformer), and bare to bares/greens. While you're at it, use a grounding screw and pigtail to attach the box to the bare/green ground wire bundle.

Then you can install the new receptacle and its box on the other end of the new cable.

  • 3
    Also used to power alarm systems, garage door sensors, etc. The exposed side is low-voltage and safe to touch. If you put a multimeter on it, set to A/C VOLTS, you'll probably read 12V, although it could probably be anywhere from 5V to 24V. If you want to, you can unscrew and disconnect either wire and then go test the doorbell, alarm, etc. to see if it's stopped working. Commented Sep 18, 2017 at 17:52
  • I have one that powers my thermostats and a cable running from the same junction box powers the heating and hot water systems. In my opinion (not an electrician) you should be cautious about extending the circuit if it's powering something critical such as HVAC, security system, etc.
    – Jamie Ide
    Commented Sep 19, 2017 at 0:12

It is a transformer. It purpose is to reduce your house voltage to something lower, typically used for doorbells.

  • I'm trying to wire a new outlet to this box. Is there anything special i need to do with this transformer?
    – hack3rfx
    Commented Sep 17, 2017 at 21:24
  • 2
    Leave the box and transformer as they are, unless you want to disconnect it and not have a functioning doorbell. If you want a receptacle in that location then mount another box nearby for the receptacle and pigtail off the wires in the existing box to power it. Commented Sep 17, 2017 at 22:39
  • Looks like he'll have to move the dedicated transformer plate to put a front plate with an outlet on it. Might be easier to gang another box next to this one. There are specific rules/codes for this stuff, so make sure you run your plan by an experienced person here or at your local hardware store. Commented Sep 18, 2017 at 17:56
  • 1
    @ScottSmith It might have been miscommunication. The outlet that I installed was downstairs. I just had to power it using this junction. I was able to simply run the new line into this one, keeping the transformer intact.
    – hack3rfx
    Commented Sep 18, 2017 at 18:16

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