Junction boxes
 

Enclosed in wall means covered and inaccessible. A junction box with a screw cover can not be covered or enclosed as to be inaccessible by normal means. Covered with drywall. Anything that has a splice can not be inside of a wall without normal access. The cover or access point must be visible and indicate that it is an access point.

Ah, I think I get it. So if I had the transformer in the wall and only the wiring connections in a junction box with a screw plate, that would still not be enough since the transformer itself needs to be accessible and hence inside of a junction box with a screw plate accessible from the outside?

That means that any transformer that is too big to fit inside of a junction box is a no-go to begin with, am I understanding you correctly?

Junction Boxes
 

Yes, transformers in particular produce heat and can short circuit or start on fire. New electronic transformers are not as prone and the newest codes may have exemptions as such. Connection such a solder or wire nuts are what they don't want t be enclosed as well as they are prone to being loose and creating hot connections.

Things like door bell transformers are typically located in the attic mounted to an octagon box exposed in the open air.

I would check with your local code authorities as they will be the governing authority.

Yeah, I've got all the connections in junction boxes, but not the transformer itself. I guess I'll have to redo that part :)

Before I installed it, I was a little concerned with the current through 12V wiring, but taking into account that in the office I'm only running about 2.5A through the wires (14ga stranded) if all lights are on, I'm not too worried.
The kitchen is going to be closer to 4A total - I expect to use the existing 14ga solid core wires and just transform directly before or after the light switch, so that shouldn't pose a problem either.

I like how well the lights turned out, with your new transformer size it should fit in a deep switch box behind your light switch very nice. I agree with leave your normal house wiring in and transform at the switch box then 12 volts to switch and so on. I would think you could then also use ground from 110 side to switch. question about mounting leds, could you use the holes in the center and add stainless screws to hold them in place? What if you run a thin strip of aluminum under them between the leds and wood and if you are going to use the home wiring use the ground and mount to strip? this way the whole system is grounded all the way back to breaker box in case transformer fails or ect. maybe to much but I dont think it could hurt? Again very nice

Interestingly enough, the transformer itself doesn't have a ground wire, only hot and neutral. It's in a plastic housing, so I guess they deem it unnecessary - on post-transformer side, if you ever end up with current going through something touchable, it's only 12V, so grounding isn't as crucial.

In the kitchen, I'm going to use the existing Romex from the switch to the lights. The light switches themselves are going to be grounded if I transform after the switch - I guess I could hook the ground conductor from the lighting wires up to the incoming ground and ground the modules through that, but I don't think a module failure that would have 12V running through the module backplate is very likely.

You can conceivably use screws to hold the modules in place. In fact, you could directly screw them into drywall - they only get a little warm to the touch. I just decided on gluing because it was easier with the mounting method I used ;)

Unfortunately the transformer I posted a picture of isn't going to work - turns out it outputs 12V AC, which these modules don't work with. I've ordered a different one which should fit into a 6" switch box (it's 5.1"x2"x1.5"). I'll post about installation as soon as I get it (should be about a week).

Just a thought but instead of putting the transformer in the switch mount if you can on the back of the lighting mounts you make? like the one you put into the ceiling, I know in my area anyways as long as you can access the transformer or splices your good for code. Like any normal light fixture the wire splice is in the ceiling and covered with the light fixture and to access all you need to do is remove fixture. Mount transformer on backside of your plywood were wires from ceiling come out, you could even put in a junction box for the transformer and connections, that way you dont need a 6" box at a switch that will be 3 times bigger than needed and not look as good, also if you or someone else ever wants 110 back to that light source they just have to remove the transformer and install different light fixture.

I've thought about that, but then I'd need 3 or 4 transformers just for the kitchen. I figured it would be easier to just use one for all of the fixtures ;)

Im caught between building them or buying the bulbs. After you build and purchase the transformers the cost is high compaired to the bulbs that you could connect into any 110 fixture. I bought one of the really cheap led light bulb type on ebay just to see and it works fine, but still like the idea of building my own. Is there a reason you choose to build your own instead of purchasing them say safety or just for the the fact of building your own and design?

For several reasons in my case. Primarily, the location is a new partition wall with no existing fixtures, so I'd have to shell out for the bulbs and fixtures (and man, are decent looking light fixtures expensive!).
Also, I wanted something that would fit well into the spot visually and not protrude into the hallway much. It was just difficult to find a fixture with the shape I was looking for ;)
I also had most of the materials already available - Plywood, acrylic and the frosted glass spray were left over from previous projects, so that made it quite a bit cheaper, too. For the kitchen, I plan on building because again, I'll need new fixtures to replace the pretty ugly ones in there right now. I've still got enough acrylic, which should make the total cost for three similar fixtures for the kitchen about $80 or so, everything included.