The UL 10C steel door core is the tested interior of a steel fire door assembly. When a spec calls for a UL 10C door, it means the complete door, frame, and hardware were tested together to limit the temperature rise on the unexposed side. Understanding what the label covers helps you order the right door the first time.
What UL 10C tests
UL 10C is the standard for positive-pressure fire tests of door assemblies. It measures how long the door holds back fire and how hot the unexposed surface gets. The temperature rise limit matters in corridors and stairwells where people may pass on the other side of the door. A door that passes UL 10B may not pass UL 10C because the temperature rise requirement is stricter.
The core is part of the assembly
The “core” is the insulation inside the steel faces. Mineral-based cores are common in fire doors because they hold the rating and resist sagging. But the core alone is not the door. The label covers the complete assembly: leaf, frame, hinges, closer, and locks. Buyers should ask for the test report that names the full assembly, not a core material spec.
What to check on the label
The label states the rating in minutes, the standard (UL 10C), the maximum opening size, and the hardware used in the test. If your opening is larger than the tested size, the door does not qualify. If the hardware is changed, the assembly must be retested or the door loses its rating. Keep the label documentation with the building records.
When you need UL 10C
Specify UL 10C when the building code requires positive-pressure testing or when the corridor on the other side is an occupied space. Common examples are stairwells, elevator lobbies, and corridor doors in commercial buildings. Confirm the required rating with the architect or the code official before ordering, because 10B and 10C are not interchangeable.
Send your opening size and required rating to Yesoon Door. Browse fire rated doors and the UL certified 3-hour steel fire door, or read our UL rated fire door guide. Yesoon Door
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