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I have a question similar, but not identical to this one: Transformer location

Can I install a 24V transformer inside of interior wall cavities, with no access other than removing drywall?

My house is a split-level ranch with 5 heating zones. As a split-level, it has no basement, ie the ground floor is on a slab. The boiler is in the garage. There is no access to pipes, wiring, etc., except by opening walls.

I am changing mercury thermostats to smart thermostats, which require 24V power. Yes, you can run them on batteries, but they don't last more than a week.

Because of the layout of the house, it was easy to run a wire from an unused transformer near the boiler to the thermostats downstairs. They are working great. But to run a wire to the upstairs transformers will take a lot more time than I can afford now.

Both upstairs thermostats are near wall outlets, so I could just use plug-in transformers. But these look ugly, and I wold really like a permanent solution.

Question: can I install 2 transformers inside of the walls, attached to the outlet metal box, via the knockouts? I know that the transformers can be installed this way, and in fact, they are designed to be installed in knockouts. The question, more specifically, is if I can have a 24V transformer inside of a wall cavity with no access (other than removing the drywall, of course)? Would it be up to code if I install one of those plastic access doors? (the outlets are behind furniture)

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    Transformers do not last forever. Ripping out drywall to replace them does not sound like fun.
    – crip659
    Commented Oct 23, 2023 at 15:30
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    An access door would definitely be code-compliant as it does not require any tools to open it, and in many cases even easier than removing a receptacle face plate.
    – manassehkatz-Moving 2 Codidact
    Commented Oct 23, 2023 at 15:32
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    one thing you will want to do is check the range of the input voltage, that might open up an option for a smaller transformer than the typical half brick
    – ratchet freak
    Commented Oct 23, 2023 at 15:35

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The line voltage connections would, as required, be accessible in the outlet box, with the typical "nipple into the box" style of transformer. The box needs to be sized for adequate box fill due to adding extra wires; either it's big enough, or it gets replaced with a bigger one.

Making the transformer and low voltage connections inaccessible would be foolish, but not a code violation. Putting in an access door (plastic, metal, whatever) would remove the foolish aspect.

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Yes you can. Where the 120 volt line connects to the transformer, a junction box is required and it must remain accessible and covered but the transformer can be hidden but not a good idea to do it. An access door like you have shown is a good way to cover it. Have the space large enough to you can replace the transformer if need be in the future.

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    OP was asking about the transformers by themselves being in the wall. The connections would be in the accessible outlet/receptacle box. Does bring up if the box is big enough.
    – crip659
    Commented Oct 23, 2023 at 15:49
  • @crip659 Right, got it.
    – JACK
    Commented Oct 23, 2023 at 20:48
  • Was not sure about the transformers themselves. Low voltage can be hidden, but the transformers are half 120 volts.
    – crip659
    Commented Oct 23, 2023 at 20:54

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