Doors and frames are rated and given a label after extensive testing by the Underwriters Laboratory. Rated doors must be self-closing and must have a positive latch.
|UL Label||Rating||Rating||Max Glass Area|
|A||3 HR||180 MIN||100 square inches per leaf|
|B||1 1/2 HR||90 MIN||100 square inches per leaf|
|C||3/4 HR||45 MIN||1296 square inches per lite|
|D||1 1/2 HR||90 MIN||Refer to local codes|
|E||3/4 HR||45 MIN||Refer to local codes|
|1/3 HR||20 MIN||1296 square inches per lite|
UL Labels D and E are for exterior doors that are subject to fire hazards from the outside of the door.
To learn more on the safe of Fire Rated doors read the link below: