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

Drill a hole through the OSB and the 2x4 big enough to feed the cable and to accommodate 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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  • I like this answer but do not understand fixture mounting and wiring. Shouldn't that nipple exit inside the box? Shouldn't the fixture backing plate form a metal enclosure around all wiring? As it appears, the wires will run outside the box along the exterior facing and then into the fixture. And how does the huge fixture mount to the wall? Just a pair of horizontally oriented box screws?? Yuck. If so, the framing suggested in this answer would be highly advised. And then I'd replace the box screws with 3" wood screws and go right through the back of the box into said 2x4.
    – jay613
    Feb 25, 2021 at 16:36
  • Driving mounting screws through the fixture into the framing isn’t necessarily a bad idea. But how heavy a fixture are we talking about, really? 5 pounds? 10? Meh. ;-) There’s no room inside the box for the clamp. It has to go outside, thus drilling a hole big enough for the clamp in the backing. The cable will run straight through the wall from the inside of the garage. It has to be clamped (and protected by the clamp) where it enters the box, and according to the rules inside the garage, which means appropriate nail plates where it runs through studs, and so on.
    – Craig Tullis
    Feb 25, 2021 at 16:44
  • 10 pounds cantilevered 10 inches on an outside wall with a racoon sitting on it or a human grabbing it while removing the cover to change the bulb ... all hanging off a box screw? I maintain: yuck. Agreed re clamp ... would a simple plastic Halex strain relief be adequate here? That would require the smallest hole and would not require access to tighten anything laterally.
    – jay613
    Feb 25, 2021 at 16:54
  • The raccoon probably isn’t too much of a problem, but the human would be. You can probably find one of the shallow boxes with doubled up threads like you find in ceiling fan boxes. The metal box does have some strength.
    – Craig Tullis
    Feb 25, 2021 at 17:11
  • I would probably just use a metal jake clamp. Use the appropriate red plastic insert, if needed, to protect the cable. The only tightening that needs to be done is a threaded nut inside the box, which can be done with the box mounted on the wall. Even at that if any maintenance ever needed to be done, just pull the box off the wall, do the maintenance, then put it back.
    – Craig Tullis
    Feb 25, 2021 at 17:14

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