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I have an exterior, wall-mounted light fixture that I want to attach to my new garage. It looks like this:

Rear of exterior fixture

Here's a PVC board the contractor left to mount it. There appears to be OSB sheathing board behind the PVC, then the house-wrap, then the OSB sheathing that's nailed to the garage studs. The whole thing is about 2 inches deep. There is no hollow in it. I know this because I drilled a hole through the center of it. You can see the hole in the picture.

PVC fixture mount on exterior siding

This is what it looks like on the inside of the garage. In the center of the photo, you can see the same hole.

Interior of garage behind fixture mount

My light needs a junction box behind it to connect the wires to NM cable inside the garage. Since the mounting block is solid, I can imagine routing (not cutting) an octagon into it to insert a very shallow junction box, or I can imagine cutting an octagon all the way through the wall to insert a deeper junction box.

What I want to do instead is just to mount a junction box inside the garage, and pull the wires from the fixture through the hole in the wall. My connections would all be inside the garage and I'd avoid a big hole.

Is this allowed? If there is a problem with pulling those little unshielded wires through an exterior wall, could I get around it by inserting a 2" length of PVC or metal conduit through the wall?

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Strictly speaking, that round metal mounting bracket on the back of the fixture is supposed to be secured to an electrical box.

It can be a shallow box, but it's supposed to be a box. You must protect the wires. You want that new garage to still be there 10 years from now.

I can't really tell from the photo, but the right shallow metal box should fit within that recess on the back of the fixture without you needing to cut a hole in the siding. But if you have to cut a hole, then you have to cut a hole (if you do, rent, borrow or buy a holesaw for your drill big enough for the box).

Shallow box Shallow box Shallow box

You could attach the box through to the oxboard with screws, although personally, I'd lay a 2x4 flat side against the oxboard inside the garage, toenail it into the studs on both sides, and screw the box directly through the oxboard into that 2x4. So a stiff breeze or errant basketball doesn't knock your light fixture off the wall. :-)

Drill a hole through the oxboard and the 2x4 big enough to feed the cable and to accomodate the clamp that holds the cable in the box. The cable has to be clamped into the box.

Then attach the round mounting bracket that came with the fixture to that box, wire everything up, attach the light fixture to the bracket that you attached to the box. That's the way it's designed.

If it's a metal box, it has to be grounded (screwing the grounded mounting bracket to it will accomplish this).

You also have to secure the cable inside the garage as dictated by your local electrical code.

Finally, I'd use a dose of waterproof silicone or caulk around the box, and then again around the fixture itself after it's mounted, to make sure water doesn't seep inside.

Good luck!

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