The minimum rating for visible indicating appliances in corridors is 15 cd (candela), as specified by NFPA
72, National Fire Alarm and Signaling Code. This requirement ensures that the visual signals are bright enough to be seen by occupants during an emergency, providing adequate warning to facilitate evacuation.
The 15 cd rating is the standard for most corridors to ensure visibility over the required distance, especially in areas with no natural light or in emergency lighting conditions.