Using 2" PVC electrical conduit
That would be much easier! I can't tell you how many threads I found with people arguing about whether NM is allowed in conduit. It's forbidden, allowed by the NEC but forbidden by local ordinances (most people seem to be concerned about excessive heat), possibly allowed based on the size of the conduit, allowed but not recommended, etc. Very confusing!
Last question: what's the proper way to secure the 6/3 NM at the top of the 20' vertical conduit run? Can I just staple it to a stud and then continue up the stud to the sub-panel? Will a single SEU staple be enough to hold the weight of 20' of cable? I may need to jog to the side a bit to find a suitable place to install the panel, and I assume it needs to be fully secured on the vertical before bending.
(BTW, how strict is the no-more-than-4-bends-between-jboxes rule... I have to assume on a large cable like this, a few extra bends would be better than a splice, no?)
Oops, one more: from the responses, I assume there is no problem with the NM just being exposed on either side of the conduit? Both spaces are unfinished. Other than sanding it down, do I need to somehow protect the cable from scraping against the cut ends of the conduit? Is it enough to just secure the cable (with staples) on the top and bottom, forcing it to stay perfectly parallel to the conduit, or is there some kind of protective cap or bushing for 2" PVC?
I've found plenty of other threads on how to wire up the sub-panel (use separate ground and neutral bus bars!) but getting this cable to the attic is looking to be much more difficult than I expected, given that I have the stupid conduit already. Thanks for all your help! :-)