Recently got a new video doorbell and noticed that it was flaky, especially at night when the IR was active. Tech support suggested my 16v 10va transformer could be the reason and suggested a 24v/40va model, which I got.

When I opened the panel, I noticed my old transformer has a 3rd green ground wire, but the new one does not have one (see photos). I don't know what to do with that ground wire, so I'm thinking to send this transformer back and get one with a ground wire - I'm obviously not an electrician but I can connect 3 primary wires - but thought I'd check with those more qualified on here first.

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  • This is yet another reason metal boxes are better than plastic. The metal chassis of the transformer is the ground. With a metal box, ground wire from the circuit connected to the box with a proper screw, a metal cover plate attached to the transformer and metal screws connecting the transformer to the cover plate and the cover plate to the box, ground just works. Nov 14, 2022 at 19:03
  • you don't really need a ground (in this application) anyway, so don't worry about it.
    – dandavis
    Nov 14, 2022 at 20:20
  • 4
    @dandavis The main point of grounding, by a mile, is safety. If this hunk of metal is mounted to a plastic box (and therefore not grounded) an exposed current-carrying wire inside the box could touch the device, electrify the outside (and possibly the low-voltage side as well), and injure someone. Grounding it properly (according to the manufacturer's instructions) is mandatory.
    – nobody
    Nov 14, 2022 at 23:45
  • 4
    Yes, it would "work" (operate) without ground, but it would not be safe.
    – nobody
    Nov 14, 2022 at 23:45
  • @nobody isn't the low-voltage side completely made of wraps of insulated wire? If so, how would external current get to the secondary wiring? Such transformers provide a galvanic isolation from the grid, so the only apparent voltage would be +/- 24v, which isn't typically considered dangerous, though with wet hands you might feel it. Most also have thermal one-time fuses, so even a short that melted coil insulation should go open before it goes mains. Given all that, I don't see a safety issue.
    – dandavis
    Nov 15, 2022 at 4:32

2 Answers 2


You may not mount this to a plastic junction box. There is a sticker on the device that clearly says "When mounting in a knockout, install on metal box only."

You must mount it to a grounded metal box, or exchange it for one that has a ground wire if you want to mount your doorbell transformer to this plastic box.


No need to return it.

The new transformer does not comes with ground wire attached.

But it has a connection for it.

It is that screw on it, where you can connect your ground wire.

If you do not have metal but have plastic box, the screw is where you can ground it.

  • Ah, I didn't realize that screw was for the grounding, I thought it was only for tightening it to the face plate. Thank you!
    – Vance
    Nov 14, 2022 at 19:29
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    The screw is not for grounding. The sticker on the side of the transformer in the photo very explicitly says: "When mounting in a knockout, install on metal box only." You may not install this in a plastic box. Period. Aside from violating the instructions, the mounting screw head doesn't look big enough to securely contain a ground wire.
    – nobody
    Nov 14, 2022 at 23:41
  • 1
    Yes I can. And that screw is obviously not for retaining a ground wire. It lacks the large flange seen on legitimate ground screws.
    – nobody
    Nov 15, 2022 at 4:10
  • 1
    The screw is for mounting the transformer in a knockout. Same effect as the threads and nut on the other one, but a different method.
    – nobody
    Nov 15, 2022 at 4:14
  • 3
    That screw secures the transformer to a metal cover or metal jb after being installed through a knockout. There are tabs on each side of that bracket that fit into a knockout. I've installed no less than 50 of them over the years. If that screw was meant to hold a grounding wire, it would be green, like all the outlets and switches have.
    – JACK
    Nov 15, 2022 at 14:54

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