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Can a low voltage transformer for a doorbell sit loose (not screwed to anything) inside the main panel or does it have to be affixed somehow? It came with holes but I'd need to drill holes in the panel to attach it, unless I can just wire it and sit it on the bottom of the panel.

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  • Just a story to show how bad some work can be -- When I moved into my current house I found the doorbell transformer had no knockout adapter. It was bolted to the outside of the service panel with the primary wires fed into the box through a clamp fitting. That's right, clamped right down onto the unsheathed stranded wires. And the hot shared a breaker screw with an existing wire The electrical inspector didn't say boo. I tore the whole thing out and installed a door knocker.
    – A. I. Breveleri
    Commented Mar 7, 2016 at 4:26
  • @A.I.Breveleri Some breakers allow multiple conductors, see this answer.
    – Tester101
    Commented Mar 7, 2016 at 4:39
  • @Tester101: Thanks. My breakers aren't marked either way. And this was almost 25 years ago, long before the miracle that is HI SE.
    – A. I. Breveleri
    Commented Mar 7, 2016 at 5:00

5 Answers 5

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You won't want to install the transformer inside the panel. Instead you'll install it on the panel.

  1. Remove the retaining nut from the threaded fitting on the other side of the transformer (the bit in your hand).
  2. Feed the wires and threaded fitting through an appropriately sized knockout in the panel.
  3. Thread the wires through the retaining nut, and tighten the nut down on the threaded fitting.
  4. Connect the primary wires from the transformer to the proper terminals in the panel.
  5. Connect the low voltage wires to the terminals on the secondary side of the transformer.

enter image description here

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  • That transformer looks to be installed upside down. The bracket should be facing the wall and screwed in place I think.
    – ratchet freak
    Commented Mar 7, 2016 at 10:23
  • Also wouldn't a fuse be advisable for the low voltage side? If the red and green short then there is a very high amperage required before the breaker on the high side will trip (5 times as much assuming it's a 120v to 24V transformer)
    – ratchet freak
    Commented Mar 7, 2016 at 11:42
  • @ratchetfreak Modern doorbell transformers have a self-resetting thermal switch internally - if the transformer starts heating up, then it disconnects the primary. Decades ago they had small fuses on both legs of the primary, but thermal detection performs the same overcurrent protection as well as thermal protection.
    – Adam Davis
    Commented Mar 7, 2016 at 12:25
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    @ratchetfreak, no, the finger nut on the inside provides the support. the screw tabs are only used if you don't use the finger nut.
    – longneck
    Commented Mar 7, 2016 at 21:25
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For safety's sake you should mount the bell transformer in a manner that the low voltage wiring does not come into the same enclosure as the mains wiring. There are available bell transformers that come already mounted on a electrical box cover. The transformer is on the outside with the low voltage leads exposed. The transformer primary leads project down through the plate so that they end up inside a box where then can be safely attached to the mains wiring.

So unless that transformer already has a threaded mount for an electrical box knockout then you should set that transformer aside and get one of these:

enter image description here

enter image description here

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No. The NEC requires a distinct separation of low voltage wiring and high voltage wiring, so you cannot have any part of the low voltage wires or the transformer itself inside the electrical panel.

National Electrical Code 2014

725.136(A) General. Cables and conductors of Class 2 and Class 3 circuits shall not be placed in any cable, cable tray, compartment, enclosure, manhole, outlet box, device box, raceway, or similar fitting with conductors of electric light, power, Class 1, non-power-limited fire alarm circuits, and medium power network-powered broadband communications circuits unless permitted by 725.136(B) through (I).

If it's a proper doorbell transformer, then it will include safety features internally to protect it from overcurrent and overheat conditions, and it will include a fitting on the primary (household voltage) side that allows it to be properly installed into a knockout on an AC electrical box. It is permitted to use a knockout on the electrical panel itself for this purpose, as long as the transformer and all low voltage wiring are outside the electrical box or panel.

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I think it shouldn't be inside the main panel at all. And it shouldn't be rattling around loose anywhere. At least bolt it to the outside of the panel.

See also this related question for advice on connecting it.

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NEC says no. Going beyond this requires some common sense. Question: Your circuit breaker panel is flush mounted with the sheet rock then what? Answer: Do not bury the transformer behind sheetrock. Use a Junction box and ensure to mount it in a location which provides access , yes it will fail. Annotate on circuit breaker panel its location.

Electrician for 30+ years says some idiot will try to connect the low voltage 12/24 to the high 120/240

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  • Heck, you could even punch a KO in the deadfront and mount the transformer to that if you really, really wanted to. Also, if you are mounting a transformer to a box faceplate, terminate the primary leads in a luminaire disconnect and then connect the incoming hot/neutral to the other side of said disconnect -- that'll make it easy to get apart in the future :)
    – ThreePhaseEel
    Commented Nov 8, 2019 at 1:08
  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know how best to contribute here.
    – Daniel Griscom
    Commented Nov 8, 2019 at 2:36

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