Three-hour (180 minute) doors
A door with three-hour fire protection rating is usually found in walls that seperate buildings or that divide a large building into smaller fire areas. The wall rating is four hours.
1-1/2-hour(90 minute) doors
Doors rated for 1-1/2 hours are found in 2-hour rated walls. These doors are commonly located in stairwells, or the enclosures of vertical communication through a building. They also occur in boiler rooms and in exterior walls that have potential for severe fire exposure from the ourside the building.