I use extension cords all the time within the class 1 zone. Maybe I'm wrong for using such but in 25 years I've never had a problem. I've also used listed equipment(portable pumps) with SO cord on them.

See 514.7 for wiring and equipment above Class 1 locations. It refers to 511.7, all fixed wiring above shall be in Metal raceways, etc. it goes on and lists all kinds of different conduits but no NM-b, interesting [Linked Image]