NFPA 72-2013 provides two candela intensity choices for wall-mounted visible notification appliances in sleeping rooms. Why are the intensities of 110 cd and 177 cd effective so different and how does one determine which to use?
The determination of whether to use a 110 cd and 177 cd effective depends on the proposed mounting height of the visible notification appliance relative to the ceiling. Following the prescriptive requirements of 184.108.40.206, wall-mounted appliances need to be mounted such that the entire lens is not less than 80 in. and not greater than 96 in. above the finished floor.
In sleeping rooms, smoke can accumulate at the ceiling without a person who is asleep being aware of it. If a visible notification appliance is being used to awaken the person in the room, smoke might partially obscure the appliance and reduce the effective intensity. Thus, if the visible notification appliance is less than 24 in. from the ceiling, it must have a higher rating (177 cd eff.).
The requirement does not change the mounting height requirement for wall-mounted visible notification appliances in 18.5.5. The hierarchy is as follows:
1. If the visible notification appliance is part of a smoke detector or smoke alarm, the unit must be mounted per the requirements for smoke detectors and smoke alarms. This requirement places the appliance either on the ceiling or on the wall within 12 in. of the ceiling. Therefore the visible notification appliance must be a 177 cd eff. appliance and listed for wall or ceiling mounting as required.
2. If the visible notification appliance is not part of a smoke detector or smoke alarm, and the unit is to be wall-mounted, it must be located at least 80 in. above the floor but not more than 96 in. above the floor. If that places it within 24 in. of the ceiling, it must have a 177 cd eff. rating. If it is 24 in. or more from the ceiling, it can be a 110 cd eff. appliance.
3. If the visible notification appliance is not part of a smoke detector or smoke alarm, and the unit is to be ceiling-mounted, it must be a 177 cd eff. appliance.