Special Occupancies
What to master
Articles 500 through 555 address locations where the ordinary rules bend. Classified locations (Articles 500–506) use Class I gases and vapors, Class II combustible dusts, and Class III fibers; within each class, Division 1 is the normal or frequent hazard and Division 2 is abnormal or intermittent, with the Zone system as the international alternative. Health care facilities (517) introduce the essential electrical system with life safety, critical, and equipment branches. Also review 511 commercial garages, 514 motor fuel dispensing, 518 assembly occupancies, 520 theaters, 547 agricultural buildings, 550 mobile homes, and 555 marinas and boatyards.
Traps
Assuming all hospital receptacles must be hospital grade (only patient care areas need them); forgetting boundary seals within 18 in of a classified-location conduit entry; confusing Division 1 with Division 2; and applying dwelling-unit rules inside a commercial garage.
Worked example
A conduit leaves a Class I, Division 1 area and enters an unclassified room. A sealing fitting is required at the boundary - the seal must be within 18 in of the boundary on the Division 1 side, and the fitting must be a listed sealing fitting, not a standard junction box with putty.