Initiating Devices and Detection Circuits
Spacing and location rules
- Spot smoke detectors on smooth ceilings up to 10 ft use 30 ft listed spacing, with no point on the ceiling more than 0.7 times the spacing (21 ft) from a detector.
- Ceiling-mounted detectors sit within 12 in. of the ceiling; sidewall-mounted detectors sit 4 to 12 in. below the ceiling.
- Keep detectors away from dead-air corners, supply diffusers (about 3 ft), and nuisance sources such as bathroom doors with showers and cooking equipment.
- Manual pull stations: 42 to 48 in. above finished floor, within 5 ft of the exit door, unobstructed.
- Duct detectors are required where other codes mandate them, commonly for air handlers at or above 2,000 cfm.
- Heat detectors use the manufacturer's listed spacing for the ceiling height; it is generally less than smoke detector spacing.
Circuits
Initiating device circuits are Class B (Style B), which tolerates a single open but loses devices beyond the break while showing trouble, or Class A (Style D), which survives a single open. Conventional zones use an end-of-line device to supervise wiring; addressable systems supervise at the device level. Alarm, supervisory, and trouble conditions must be distinct and visibly annunciated.
Worked example
A 60 ft by 40 ft room with a 9 ft smooth ceiling. Divide 60 by 30 for two rows and 40 by 30 for two columns, giving four detectors. Check walls: 30 ft row spacing puts detectors 15 ft from the side walls, and 20 ft column spacing puts them 10 ft from the end walls, both inside the 21 ft limit. Four detectors is correct.
Traps that cost marks
- Exceeding 21 ft from the last detector to a wall.
- Placing a detector in the dead-air zone at a wall-ceiling corner.
- Applying 30 ft spacing unchanged to high or obstructed ceilings.
- Forgetting that a Class B break shows trouble but leaves devices offline.