Notification Appliances and Circuits
Audibility
NFPA 72 requires audible appliances to produce at least 15 dBA above the average ambient sound level, or 5 dBA above the maximum sound lasting at least 60 seconds, whichever is greater, measured 5 ft above the floor. In sleeping areas, the sound must reach at least 75 dBA at the pillow. If ambient exceeds 75 dBA, add 15 dBA above ambient.
Visibility
Strobe coverage is selected from NFPA 72 Table 18.5.5.1 by room size and ceiling height. A common wall-mounted progression is 15 cd for a 20 ft × 20 ft room, 30 cd for 30 ft × 30 ft, 75 cd for 40 ft × 40 ft, and 110 cd for 50 ft × 50 ft. Wall strobes mount 80–96 in. above the floor. Visible appliances in the same field of view must flash in synchronization.
NAC design
NACs are supervised power circuits, usually 24 VDC, with an end-of-line resistor or addressable module. Total appliance current at the selected candela and tap settings must not exceed the NAC rating. Voltage drop must leave each appliance at or above its listed minimum operating voltage.
Worked example
A 30 ft × 30 ft room with a 10 ft ceiling needs a wall-mounted strobe. The table requires 30 cd for that area; a 15 cd strobe is undersized even if it is closer or the room has a lower ceiling. The reasoning: candela rating is tied to coverage area, not to the number of appliances.
Traps
Do not size strobes by square footage alone while ignoring ceiling height or mounting height. Do not forget the 75 dBA pillow requirement. Do not leave strobes unsynchronized in one view. Do not place the NAC end-of-line resistor at the panel instead of the end of the circuit.