Taking NM cable, stripping off a length of the outer sheath, inserting it into a junction box, and then going out of that junction box though a length of FNMC as a whip doesn't strike me as a way to do less work or save money; it takes time and effort to strip that cable and then fish it.
I see as benefits: not having to go out and find the small length of THHN/THWN, and not having to make a splice in the junction box.
The downside is that the conductors inside of NM-B are THHN conductors, and are not individually printed as being THHN conductors; they are part of a cable assembly. Given the materials that they are made of, the conductors would _probably_ be able to pass the tests to be THWN conductors, but they have not been tested.
Question: in this same situation would it be _better_ to simply use UF-B cable, both for the interior wiring (where it is permitted to be used as NM-B cable) as well as inside the FNMC whip? UF-B is rated for wet locations and for direct burial. I presume that the whip would need to be large enough to properly hold the UF-B cable assembly.
Further question: What about using the UF-B with the sheath stripped off in the FNMC whip, as in the original post? In this latter case, if the individual conductors are not labeled THWN, but a data sheet for the cable lists that the conductors are THWN, would having this datasheet make a difference in terms of allowing this installation?
-Jon
