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I'm installing a 24V DC low voltage under-cabinet lighting system in my new kitchen. I had electricians run some 18AWG runs before the drywall went up from under every top kitchen cabinet, all terminating in one central location (under the kitchen sink). They also put another j-box in that central location with some 14/2 romex (120VAC) to power the transformer itself.

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I've managed to get it working by pulling out the individual 14/2 conductors and terminating them inside the transformer. But obviously I know I can't leave unprotected conductors in the air under a kitchen sink, so I need to figure out a way to protect them between the j-box and the transformer.

I'm also bonding the 7x 18AWG cables for all the separate lighting runs together using WAGO 221s, but since there are so many of them I don't have space to do this in the tiny compartment inside the transformer, so I have to join them outside and just get pigtails into the transformer itself. I'm guessing I also can't (and don't want to) leave these exposed in the air, but I'm not sure how to protect low voltage wiring between transformer and j-box.

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(Note: I've only bonded 3 of the low voltage runs in this picture)

My questions:

  1. What is an acceptable way to protect the 14/2 Romex here? Should I wrap it in metal flex conduit? If so, how can I terminate this on both the wall j-box side and the transformer side?
  2. Same question but for the 18AWG low voltage wiring. The main difference there is that the "bonding" of all the wires is done outside the transformer, though perhaps I can push those into the low voltage in-wall box and only run / protect the pigtail into the transformer.

My transformer has 3/8" knockouts btw


Update for future readers: I ended going with a mix of Ecnerwal's solution (for the low voltage) and Harper / jay613's (for the high voltage). This let me avoid conduit or Flex / MC cable altogether which was a win!

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As Harper said I couldn't find 8-port wire nuts, and my big box store didn't have the 8-port push in connectors, so I just used 2 5-port wago's with a pigtail between them. I pushed them all in the wall and added a single 18/2 cable coming out into the power supply.

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I also couldn't find a grounding bushing for 1/2" (my home depot only has those for 1.5" and up), so I drilled a hole in the extension box and screwed in a green 10-32 screw with a bare wire, nutted to the ground from the panel.

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    Since the transformer is handling 120V, you need to install it according to the instructions and labeling, which were approved by UL/CSA/ETL as conforming with UL standards for such devices. If it is not an approved device then back to Amazon it goes. Actually reading that, I see it has standard KOs for conduit. Apr 20, 2023 at 20:08
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    Note that the 120V box must be flush to the finish surface unless a) the material in question is non-flammable, and b) it's less than 1/4" sunk. If that's not solvable, you'll need a box extension sleeve there. Apr 20, 2023 at 20:15
  • "the 120V box must be flush to the finish surface" <-- I did not realize this, thanks for the heads up. I'll measure the difference and consider whether to add a box extension based on that. I'm guessing a metal box cover with a long screw wouldn't be sufficient here if the gap between the finish surface and the in-wall box is > 1/4"?
    – peter
    Apr 20, 2023 at 20:26
  • Also, just for my own learning, where in NEC can I find requirements about how the 120V box must be installed into the finish surface?
    – peter
    Apr 20, 2023 at 20:26
  • NEC 314.20. They also make [box extension rings](homedepot.com/p/Box-Extender-2-Pack-BE1-2/202708650, see "customers also viewed" above). However now that I look at it, I see it's just hte back of a cabinet. That's probably fine e.g. if you use an extension box anyway to get a side knockout entrance. Apr 20, 2023 at 21:04

3 Answers 3

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The PSU is well chosen and is either UL listed, or would be if they applied for it. How refreshing to look up a street address on Google Earth and find an industrial park, instead of a Mailboxes Etc. or a registered agent office.

The equipment provides a standard 1/2" trade size knockout (7/8" diameter actual) and you will be able to fit standard fittings into that. I think Ecnerwal has the best plan for getting from there to the junction box - use a steel cover with a 1/2" knockout on the face of the cover.

A standard 1/2" KO is a 7/8" hole. I have never heard of 3/8" conduit and I do a lot with conduit. If the hole is 7/8" diameter that's a 1/2" KO. I do see 3/8" mentioned on Romex cable clamps for standard KOs, but I assume that refers to the Romex somehow. The thread on them is standard 1/2" pipe thread.

enter image description here

To get between knockouts, I would - honestly, I like jay613's idea of sticking a steel extension box on there, and then just use a box spacer nipple or offset nipple to go right into the 1/2" KO on the power supply. Easy peasy, no need to deal with cutting or terminating metal jacketed flex, or lids with wires on them.

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Not actual size.

The low voltage side is not critical, in fact the normal low-voltage box is a "wish box" which is made of orange plastic and has no sides or back. It just opens into the stud cavity. However since you do have a junction box, it may be an appropriate place for your big 8-wire join to happen. Note that Wago lever-nuts don't exist in 8 ports and you need to follow their instructions and labeling regarding number of wires per port. Don't use the cheap Cheese counterfeit Wagos off eBay/Amazon Marketplace which may have 8 ports. They do make 8-port push connectors that may be just right. As for wire nuts, they don't seem to list them for more than 6 wires.

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Your PSU is nicely designed to interface with a metal junction box and to provide HV/LV separation to code. I'll focus first on the 120V side. Because you have plastic wall boxes you cannot install this power supply directly if you want to attach the PSU directly you'll need a couple of appropriate parts, see other answer. So.

The HV Side

  1. Either replace the 120V plastic wall box with a metal one (requiring some drywall work), or install a metal surface mount box on top of it, one with a rear knockout. If you mount one on top, put a strain relief nut in the rear knockout, mainly to protect the cable as it enters (the plastic box should provide the actual strain relief). Attach the ground wire from the NM cable to the back of the metal box as required.
  2. If you replaced the plastic box, get a metal cover with a knockout hole and a short FMC connnector. Mount the PSU nearby to the wall and use the knockouts in the box and the PSU to connect it. Run appropriate loose wire (THHN) through it. OR, if you mounted the metal box on the surface, put a blank cover on it or an outlet just for convenience (having nothing to do with this lighting). Use a side knockout. See if you can connect the box and PSU with a 1-inch nipple, or else use some FMC as described. Make sure there's room for the LV wires to exit IE that knockout should not be blocked by your metal box.

Now the LV side

You can do anything you want with this, but as a guide to doing it nicely: Buy a plastic project box. Knock out or drill a hole in the back and mount it over the LV box in the wall. Run the LV wire from the PSU into the project box through a side knockout. You can protect it in any way you like or not at all.

Inside the box don't use Wagos ... they are not ideal for connecting a large number of things in parallel. I hate to say it but twist nuts are probably best, you just need two of them to tie everything together. And be careful twisting #18 not to break it.

Maybe, if you don't want anything surface mount you could follow the other answer for the HV side and for LV replace the wall bracket with a 4x4 plastic box big enough to house all the wires and connections. I wouldn't want the connections just dangling inside the wall. If you use this approach you just need a way for the LV wires to enter from the PSU ... for that you can use a plastic cover with a small round hole, such as for cable TV.

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  • Thank you! This is very helpful, though I'd prefer to avoid surface mount boxes here. I think my LV box is deep enough to house all the wires so I can probably do what the other answer suggested and get 2 covers with a knockout. Sounds like for the LV I don't actually need flex if I don't want to?
    – peter
    Apr 20, 2023 at 18:06
  • Also the tip about wire nuts is useful. I initially thought they'd be stranded and wanted to avoid wire nutting 7 stranded conductors (it's hard enough to get a tight bond with 2!) but now that I know my 18AWG is solid I'm more confident in nutting them together.
    – peter
    Apr 20, 2023 at 18:08
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    It's not clear in the picture but I'm not sure what you have there for the lighting wires is a box at all, maybe just a frame? It's up to you if you want the wire connections to be in the wall but not in a box. I would want some strain relief on the connections. You could rig that up with bits of string of course!
    – jay613
    Apr 20, 2023 at 18:08
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    You don't need flex for the two wires from the PSU to the LV box. You can use nothing (will not look professional) or drinking straws (kidding) or I think for a better look heat shrink tubing. Or a nother length of FMC ... not necessary but would make it all look impressive. And if you don't like twisting wires (I don't) you could use terminal blocks or heat shrink terminals.
    – jay613
    Apr 20, 2023 at 18:10
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    Heat shrink to provide covering. Heat shrink terminals to make connections.
    – jay613
    Apr 20, 2023 at 18:19
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You can't do that, but you can get close. Either you told them what to install, or your electricians did you no favors on box choice here...

Buy two "Single gang covers with knockout"

image removed for draconian attribution reasons

Technically, you can't "protect" the 14/2 conductors because you have stripped the jacket off them - so make a junction in that junction box to either MC/MC-lite cable, or THHN wires, and use the appropriate flex conduit or MC fittings in the knockouts and your transformer, which hopefully has standard threading or size input points. I think MC fittings in 3/8" are available.

Move the low voltage junction into the LV junction box (you HAVE a normal size non-tiny junction box right there in the wall. Why did you make the junction out in space at the transformer????) and use flex conduit and flex conduit fittings to make that run protected, though the extent to which that's required is somewhat less - but it will be a lot nicer looking (and look less like a hack job.)

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    Elegant, I didn't know there's a metal cover for a plastic box. How does this cover and the cable jacket get ground from the plastic box? I think there's a special connector that can provide that ... don't remember what it's called.
    – jay613
    Apr 20, 2023 at 17:44
  • Bonding bushing is the term, I think.
    – Ecnerwal
    Apr 20, 2023 at 17:46
  • FWIW the electricians selected all the boxes and stripped the Romex jacket before I got to this, and I did the initial junction outside just to have more space, knowing I'll probably need to redo some part of this when I figure out how to properly protect it.
    – peter
    Apr 20, 2023 at 18:02
  • For the bonding bushing, would I install this on the back of the knockout on the metal wall plate? Then just wire nut it with ground? Also are those available for 3/8"? I need a bonding bushing a few months ago and they were really hard to find at anything other than 1/2"
    – peter
    Apr 20, 2023 at 18:03
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    At that end I think you'd be using a fitting for a 1/2" knockout with connection to 3/8" 3/8" as anything other than metal flex or MC cable is pretty rare.
    – Ecnerwal
    Apr 20, 2023 at 18:20

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