I'm installing 12V lighting in my basement in the exposed joists. I bought this LET-75 transformer to run them, but I'm not sure how to mount it. There are no mounting holes of any kind, for one thing. Also, unlike a doorbell transformer, there is no obvious way to mount it so the 120V wires are inside a panel/box and the 12V wire are outside the box.

What's the solution here? Putting the whole thing in a junction box seems like a heat issue. I guess I could zip tie it to something and run the 120V wires into a fitting on a box, but they will be non-shielded going between.

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  • You're asking all the right questions, but what makes you think there will be a heat issue? Do you have literature indicating required clearances? I'd be putting it in a double-gang fiber box with a cover. It would need to remain accessible, of course. I'm also curious what low-voltage lighting doesn't come with its own transformer setup.
    – isherwood
    Sep 1 '20 at 15:24
  • I am a bit skeptical about this device. It starts with 120V x 0.6A and ends with 12V x 6.25A. It is apparently creating 3 Watt out of thin air. At least that means you don't have to worry about overheating, since normally the lost power is converted to heat. Sep 1 '20 at 16:11
  • 1
    @manassehkatz-Moving2Codidact I see this kind of confusion on ebay a lot; those are Chinese amps, you have to divide by 2 or 3 to get metric amps.
    – dandavis
    Sep 1 '20 at 16:41

It's not approved for use in AC wiring

See the backslanted RU? That means the power supply is only RU-Recognized as a component of some future product that will then be UL-Listed.

You are not allowed to install RU-Recognized things in house wiring. You can only install UL-Listed things.

An RU-Recognized thing can be converted to a UL-Listed thing by packaging it in an enclosure that conforms to the UL White Book and sending it off to UL for approval along with labeling and instructions. One challenge you would need to resolve is how to put the mains wires inside the box and the low-voltage wires outside the box. Note most UL-listed 24V thermostat transformers do exactly that, mounting in a knockout or as a junction box lid, and providing terminals on the correct side.

As you grind through the UL Listing process, UL will notice that your active component is RU-Recognized, and they will not tear it down and do their usual testing suite, since they already have. That is the value/purpose of RU.

Typically this module would be part of a luminaire (light fixture) and the luminaire would be UL Listed in the usual way.

Other than that, you can go to the final arbiter of approval: your local electrical inspector, and ask them if they will approve some sort of mounting method.

  • Good info. More on that
    – isherwood
    Sep 1 '20 at 16:21
  • Huh, good to know. I didn't realize that's what RU meant. I'll look for something else.
    – Eric
    Sep 1 '20 at 16:48

First, see Harper's answer on the UL listing status of your device and why it's not approved for this use. I'll leave my answer under the assumption that you'll find a UL listed device instead.

Here's one idea. I'm not familiar with code regarding transformers like this, but it's not unlike a doorbell transformer and they're handled similarly. I believe that the only requirement is that the line-voltage connections are inside an approved box. Both the transformer and the box must remain accessible. In the case of the doorbell transformer the case must be grounded. I'm not sure about this situation.

Use a plastic or metal hanger strap and a couple screws to cinch the transformer to the joist tight against a standard junction box. You could fit the transformer against a joist or a retainer block to keep it sandwiched securely against the junction box.

__________        :(+): <-- mounting strap & screws
          | ______:...:_______
    (+)   ||      :   :       |=== <-- low-voltage output wires
          ||      :   :       | ____________________________________
          ||   transformer    ||                                    |
          ||      :   :       ||=== <-- high-voltage input wires    |
    (+)   ||______:...:_______||                                    |
__________|       :   :        |                                    |
  ^               :(+):        |                                    |
retaining         :---:        |            junction box            |
  block                        |                                    |
                               |                                    |

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