fire-door rating

A fire-endurance rating for doors, shutters, etc., established by the Underwriters’ Laboratories, Inc., or other recognized and approved laboratory: class A: 3 hr; for doorways or other openings through a wall separating buildings or dividing a single building into fire areas; class B: 1 or 11⁄2 hr; for doorways or other openings in enclosures of vertical transportation through buildings (stairs, elevators, etc.); class C: 3⁄4 hr; doors in corridor and room partitions; class D: 11⁄2 hr; doors and shutters in exterior walls which are subject to severe fire exposure from outside the building; classes E and F: 3⁄4 hr; doors, shutters, or windows in exterior walls which are subject to moderate or light fire exposure respectively from outside the building.