A few vaguely related comments.
ISTR the referenced fuses are primarily intended as an auxiliary overcurrent device for 277V light fixtures—intended to be mounted in a 7/8-inch knockout on the fixture housing.
http://www.bussmann.com/products/Electronic/Group8.asp Re Johnnyb’s comment of “…Two 20amp single pole breakers supplying 277 two each row,” note that aside from any harmonic currents, in the case of the common neutral conductor serving only two phases of a multiwire branch circuit, the neutral will see full phase current for two equal phase currents. If two circuits were routed with individual neutrals in a common raceway, the “over 3 conductor’ derating would have to be applied.
About wire nuts in fixtures—one installation error while connecting a smaller stranded {id est, 18AWG} conductor to a larger solid {id est, 12AWG} conductor, if the small strands do not “lead” the larger solid conductor, the connection will likely not be secure and have a tendency to come apart.
