Initiating Devices and Detection Circuits
Spacing and mounting
- Spot-type smoke detectors on a smooth ceiling are commonly spaced at 30 ft (9.1 m), with no point on the ceiling more than 15 ft (4.6 m) from a detector. Ceiling-height correction factors apply above 10 ft.
- Keep smoke detectors at least 4 in. (100 mm) from a wall; wall-mounted units go between 4 in. and 12 in. below the ceiling. Keep them at least 3 ft from supply diffusers and out of dead-air pockets.
- Heat detectors use their listed spacing, often 50 ft for ordinary-temperature units, with half-spacing to walls.
- Manual pull stations mount 42–48 in. above finished floor to the center of the handle, within 5 ft of the exit door, and unobstructed.
- Duct detectors go in the main trunk or branch, with sampling tubes perpendicular to airflow and access for testing.
IDC rules
Conventional IDCs are supervised two-wire circuits with an end-of-line resistor. Class B loses devices beyond an open; Class A returns the circuit and survives a single open. Waterflow switches must alarm within the code-allowed time after flow, and supervisory switches report valve position.
Worked example
A 60 ft × 40 ft office has a smooth 10 ft ceiling. Along 60 ft, 30 ft spacing allows detectors at 15 ft and 45 ft: two devices, 15 ft to each wall. Along 40 ft, one row would leave 20 ft to the sidewalls, so use two rows at 10 ft and 30 ft. Total: 2 × 2 = 4 detectors. The reasoning: the 15 ft wall distance is a maximum, not a target.
Traps
Do not use 30 ft as the distance to the wall. Do not mount a pull station with its top at 48 in.; measure to the handle. Do not omit the duct detector remote test station or access panel. Do not treat a heat detector like a smoke detector for spacing.