Does code allow for in-wall mounting of low voltage transformers?

Question

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

Answer 1

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.

Answer 2

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.

Answer 3

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.