Doors are rated for three-fourths of the rating of the surrounding wall: A 3-hour door is used in a 4-hour rated wall; a 1-1/2-hour fire door is used in a 2-hour rated wall; and a 3/4-hour door is used in a one-hour rated wall. The notable exception is that 1/3-hour rated doors are also used with one-hour rated walls. However, a door with a higher fire rating than the opening requires may also be used. For
example, a door rated for 3 hours may be used in a 1-1/2-hour opening. All requirements for the 3 hour rating, such as maximum glass size, door size, and other restrictions for the higher rated door must be met.