Many years ago I was taught that a fire door assembly can have components from different manufacturers, labeled by different test labs, and with different hourly ratings. For example, a wood fire door with a 20-minute label can be installed in a hollow metal frame with a 3-hour label, but the component with the lowest rating would establish the rating of the fire door assembly; in this case the rating of the assembly would be 20 minutes.
I recently received a question from Al Rivas at Contract Hardware that had me stumped. Al pointed out a mismatch in the opening protective requirements of the International Building Code (IBC). In the 2012 and 2015 editions, Table 716.5 establishes the required rating of the opening protectives for various types of walls. The table also lists the glazing requirements, and includes a column for the minimum rating if a sidelite or transom frame is used. In many of the wall types addressed by the table, the minimum fire door and minimum sidelite/transom ratings are equal. But in 4 locations the required sidelite/transom rating is higher than the required rating of the door. For example (see partial table below), a corridor fire partition requires a door rating of 20 minutes and a minimum sidelite/transom rating of 45 minutes:
Differing ratings also occur in this table for sidelite and transom frames in 4-hour fire walls, 2-hour fire walls, and 2-hour walls in shafts, exit enclosures, and and exit passageways. I checked with the International Code Council (ICC) and also with Underwriters Laboratories (UL), and they helped to clear this up. Using the corridor fire partition as an example, a door in a 1-hour corridor fire partition is required to have a rating of at least 20 minutes. If that door is installed in a 3-sided frame, the frame could have a 20-minute label as well (although many 3-sided frames are labeled for 3 hours). If a transom or sidelite frame is used in this location, a rating of 45 minutes is required for the frame and glazing, but the door could have a 20-minute label. The same holds true for the other locations where a sidelite/transom rating greater than the door rating is required. In these locations, the door label is not required to match the frame label.