I'm running 4 x 4AWG copper wires through 1-1/4"conduit. I have 6 elbows in the 125' run so I need to insert a junction box before the last two elbows. It will be a straight pull through the junction box. By code, I could have used 1" conduit (~38% fill) but chose to go up a size to 1-1/4" (resulting in ~22% fill). 314.28(A)(1) states that the junction box must be 8 times the size of the conduit for straight pulls, or 10" in this case. However, if I am interpreting 314.28(A)(3) correctly, I can use a smaller junction box if I am less than the maximum conduit fill (which I am). Since, I could have used 1" conduit does this imply that I could use an 8" junction box even though I used 1-1/4" conduit?
Electrical – Junction box down-sizing when using oversized conduit
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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).
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!
Stripping the jacket off the cables is not an option. The wires inside are only rated as part of a cable assembly, and cannot be used as individual wires.
I didn't check your numbers, but the process you followed is correct.
If you're dead set on installing cables, you might want to bump the conduit up a size. The extra space will make the pull a bit easier. Also make sure you use plenty of lube. Though since you're installing conduit anyway, consider installing individual wires instead of cables.
It also might make more sense to bump up the size of the feeder a bit, and eliminate the two branch circuits. Increase the size of the secondary panel, and supply the 20 ampere branch circuits from there instead. This might save money on materials, make the pull easier, and reduce the size of the conduit (not sure about this, as I haven't run any numbers).
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Best Answer
Don't worry about it. Just use the standard C-type conduit body, which is designed to exclusively be an access point for pulling in a straightline section. That is literally its one purpose, so obviously, UL wouldn't list it if it couldn't comply with Code. As far as your frustration finding the right thing, the caliber of your work is beyond the local big-box store (which, ironically, doesn't do well stocking big boxes) -- you need the real electrical supply.
You can use C-type conduit bodies anywhere they will remain accessible. However, the better plan, generally, is to use LB, LL or LR conduit bodies as access points instead. That lets you kill 2 birds with one stone: it eliminates one of your four allowed 90 bends while also creating an access point. However, when you do this, you must use the access point to pull - you are not allowed to drag wires around conduit body corners even if that is feasible. They're not made for it and will damage the insulation. The only corners you can drag around are sweeps that have typically a 6-12" radius.
Also, the first rule of conduit is that the conduit route must be finished before you start pulling wire through it. That's not a Harper Rule, that's actual Code. I mention that because it sounds like you are building the conduit over top of the wires. That's not allowed. It's too easy to leave a conduit route that is unpullable.
Once somebody used one of these to go around an inside corner - the cover was facing the wall and could not be removed. Can't do that! And obeying the "pull the wire after construction is complete" rule takes care of that!