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I am asking if NEC code permits for in-wall (gang box) mounting of low-voltage transformers and power supples, and if so do these product have a name or term?

My use case is low-voltage electronics mounted to drywall: a small car amplifier, an Arduino, Raspberry pi, some 12v LED strips, powering car-type DC accessories. I am looking to hide as much wiring as possible.

I realize you can't dissipate unlimited heat in the wall. 55 watts - the Class 2 limit - comes out to 12v 5.5A if I did the math right and that's easily enough power.

As examples of failed terms I tried: "doorbell transformers 1 gang" and "in-wall Power over Ethernet supply". We've all seen the 4-port USB 1-gang devices - I want that for 12v.

Please do not answer with specific products/links. I am asking if these things EXIST and whether I have been calling them the wrong name. I have already posed this question before but it was deleted over the holidays (reason: "no shopping for links") so to avoid that experience again I need this strongly worded disclaimer. :-) Thanks all!

https://diy.stackexchange.com/questions/180414/does-anyone-make-a-12v-in-wall-gangable-power-supply-like-the-usb-ones

3 Answers 3

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You can do this, but you're best off ditching gang-boxes to do so

What you want is a 12VDC NEC Class 2 power supply, listed to UL 508. These are made as more-or-less commodity items, which is the good news. However, since they are mostly used for industrial controls work, they come in a DIN rail module form factor, which doesn't fit well into typical junction boxes. Instead, you're expected to mount them into a NEMA enclosure, as one would use to house an industrial control panel. Fortunately, flush-mountable NEMA 1 enclosures are readily available through your friendly local electrical supply house, or through MRO/industrial controls suppliers online for that matter.

Note that these DIN rail supplies are designed to be used without a barrier; mains comes in one end, and low-voltage DC comes out the other, easily allowing the installation to meet the ¼" rule found in NEC 725.136(D) point 1:

(D) Associated Systems Within Enclosures. Class 2 and Class 3 circuit conductors in compartments, enclosures, device boxes, outlet boxes, or similar fittings shall be permitted to be installed with electric light, power, Class 1, non–power-limited fire alarm, and medium-power network-powered broadband communications circuits where they are introduced solely to connect the equipment connected to Class 2 and Class 3 circuits, and where (1) or (2) applies:

(1) The electric light, power, Class 1, non–power-limited fire alarm, and medium-power network-powered broadband communications circuit conductors are routed to maintain a minimum of 6 mm (0.25 in.) separation from the conductors and cables of Class 2 and Class 3 circuits.

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By the way, 55W heat dissipation is not allowed inside a 1-gang box. The 55W restriction is not about that, it's about something else. Other than that...

Nothing in Code prohibits this

But I'm not aware of any products that offer this in 12 volts DC. The lingua franca of low-voltage wiring in the US is 24 volts AC, and always has been. They certainly do make that in a "junction box cover" form-factor; however it's pretty industrial looking since it's made for installation in unimproved spaces, and the low voltage side (and often the whole transformer) sits on the outside of the box.

In any case, any such product should include the necessary dividers, to keep low voltage and mains rigidly separate. LV and mains are allowed in the same multi-gang junction box if a divider is physically installed.

I'm concerned that such a thing might not exist. Fitting inside a gang-box, it definitely has to be UL-listed. Going through the UL listing process may be too onerous for a niche product with a limited market.

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  • I can tell you offhand that stuff for this exists for 12VDC, it's just that they're UL 508 listed industrial controls supplies and thus usually found in DIN rail form factors for mounting into control panels and suchnot; thus, getting them into a gang box will be hard. Better to use a NEMA 1 flushmount enclosure... Jan 18, 2020 at 20:55
  • @ThreePhaseEel Never thought of using a NEMA enclosure. Sounds like you might have a useful answer, unlike mine... Jan 18, 2020 at 20:57
  • Than you both. I wish SO permitted dividing acceptance, since I learned something from each response. I shouldn't have "in-wall gangable" I should just have specified "in wall". Jan 19, 2020 at 20:36
  • @ScottPrive oh, you'll never go wrong selecting a ThreePhaseEel answer. The rep point system does not reflect skill or experience, only activity. Jan 19, 2020 at 21:12
  • I have 24VDC relays in my system that have the terminals all on one side so I could not use a divider. I talked to the electrical inspector and he stated his only requirement was that the low voltage wires have the same or higher voltage rating then the mains. I did it that way and it was approved as were some updates years later.
    – Gil
    Sep 1, 2021 at 1:30
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I am asking if NEC code permits for in-wall (gang box) mounting of low-voltage transformers and power supplies?

I just went over this with my electrical contractor for a low-voltage doorbell circuit with transformer remote from bell, in a 1-gang box. (This 2021 construction in Snohomish County, WA.). The layout has both primary power wires and low-voltage wires led into the same box. The contractor explained that the high/low wire separation requirement may be met with the transformer inside the box by including an insulating barrier (plastic sheeting, for example) between the two wires. It doesn't have to be air-tight (gaps on edges would be fine) but just has to separate the two sets of wires.

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  • Ours which passed inspection has the transformer mounted on the cover of an octagon box in the basement with the screw terminals for the low voltage outside easy to reach. The mains are in the box and only accessible if the cover is removed. You state "in-wall mounting", inside the wall. that should never be allowed, how is it going to be replaced?
    – Gil
    Jul 27, 2021 at 17:20
  • Mounted like a wall switch or outlet, with a box inside wall, flush with outer surface of drywall, with a blank screw-on cover. What I ended up actually doing is based on Edwards Adaptabel, including their transformer. The transformer barely fits in a single-gang box so I added a two-gang box nearby and ran two plastic conduits for separate low and high voltage runs (and dividers in each box). Upside is that it clearly has separation of voltages, connections and components accessible behind plates. I passed electrical final with no glitches, but inspector did not probe this item in detail. Aug 31, 2021 at 14:53
  • You can always get a wall wart with an AC output then simply plug it in.
    – Gil
    Aug 31, 2021 at 21:18

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