NFPA 72 Strobe Candela — Method
Visual notification appliances (strobes) must light a room to a minimum illuminance so occupants who cannot hear the sounder are alerted. NFPA 72 publishes minimum candela by room size for a single appliance: separate tables for wall-mounted (by the largest room dimension) and ceiling-mounted (by room size and ceiling height) devices. Rooms larger than the table either step up to a higher cd or use more than one strobe; corridors and synchronisation have their own rules.
Frequently Asked Questions
Take the room's largest dimension and read the NFPA 72 minimum candela for a single appliance: e.g. wall-mounted 20 ft→15 cd, 30 ft→30 cd, 40 ft→60 cd, 50 ft→95 cd. Round up to the next standard cd rating.
Wall-mounted strobes are rated by the room's largest dimension; ceiling-mounted strobes are rated by room size for a given ceiling height. Ceiling height above 10 ft increases the candela required.
Listed strobes come in fixed ratings: 15, 15/75, 30, 75, 95, 110, 135 and 185 cd are common. Always select the next standard rating at or above the calculated minimum.
If the room exceeds the single-appliance table, use a higher-candela strobe or multiple strobes. With more than one strobe, either size each for its sub-area or synchronise them to avoid a seizure-risk combined flash rate.
Corridors up to about 6 m (20 ft) wide use a spacing rule instead of room-size: 15 cd strobes spaced not more than 30 m (100 ft) apart and within 4.5 m (15 ft) of the ends, per NFPA 72.
Strobe Candela Sizing (NFPA 72)
Visual alarms ensure that people who cannot hear the sounder — in noisy areas or with hearing loss — are still warned. The light output needed grows with room size, so NFPA 72 ties a minimum candela rating to the space.
Wall vs ceiling
Wall-mounted appliances are chosen from the largest room dimension; ceiling-mounted ones from room size at the ceiling height. The single-appliance tables assume one strobe centred/located per the standard.
Big rooms and corridors
Beyond the table, step up the candela or add strobes (synchronised to control flash rate). Corridors use a simple spacing rule with 15 cd units. Pair this with the NAC voltage drop and battery calculators to power the strobes correctly.
Related: NAC Voltage Drop, Detector Spacing, Fire Alarm Battery.