I understand that NM wire is not permitted to be used in wet or damp locations. That said, it seems generally permissible to run NM wire into the back of an outdoor box (such as a breaker panel). Unfortunately, I have a fixture that doesn't provide any back knockouts (and cutting my own is inadvisable here) so the only solution is to use the bottom entry provided.
To what extent can NM wire be run from inside an exterior wall through conduit outdoors to the intended fixture?
I have drawn some representations of three different cases below:
Case 1
In this case, I would have a simple elbow running into the bottom entry and into the exterior wall. The NM cable (in orange) would run from the hole in the wall through the elbow and into the fixture. This is the simplest option, but not exactly my preferred option.
Case 2
This is what I would prefer, though I somewhat doubt it is legal. In this case, the NM cable would enter an outdoor junction box (so I can also have another circuit providing a GFCI receptacle here), travel through a minimal conduit segment into the device, and terminate as before.
My guess is that the running on NM cable through the conduit is not legal for any length outdoors so I probably can't do it.
Case 3
I presume this to be the most likely legal solution. In this case, the NM terminates immediately, gets pigtailed into some THHN/THWN (indicated by the colored dots), and the clearly legal individual wires go through the conduit into the fixture.
While I am fairly certain of the legality of this solution, I still don't know what is considered acceptable when NM wire goes outside for short segments (such as to a breaker) and if it's required to be through a back knockout or any knockout.
Best Answer
Running the NM cable at all outdoors would have me concerned about capillary action wicking moisture upcable and back inside, leading to premature failure of the NM. I'd use a short length of /2 UF from an accessible box on the inside through the wall and then directly into a J-box on the outside for the GFCI. You can then run UF through the nipple that connects the weatherproof J-box to the fixture in question.
Don't forget the "while in use" cover on the GFCI-box, and to make sure that there's a 1/8" drain hole in the bottom of that box. (Elbowing up a conduit directly to the fixture from the inside isn't the greatest idea because then you have raceway arranged to drain...to the inside of your house. Oops!)