The NEMA reference should be good enough for an AHJ, but if it isn't the UL Whitebook Category Code (AALZ) ELECTRICAL EQUIPMENT FOR USE IN ORDINARY LOCATIONS states:
Note it is a general permissive, the mark may or may not be there but it is nevertheless permitted just for being one of the "Types."
A Type 3, 3S, 5, 12K, or 13 enclosure may be marked ‘‘Dusttight’’
At one time NEMA 250 and UL 50 were virtually identical. They have departed recently with the UL recogniation of IEC equipment.
Last edited by rbalex; 06-17-09 at 06:14 PM.
Robert B. Alexander, P.E.
Answers based on 2014 NEC unless otherwise noted.