The following are general guidelines for Smoke Detectors installation and locating as per NFPA 72 standard :

• If a mechanical guard is used to protect a smoke detector it shall be listed for use with the detector being used
• Install detectors in all areas where required by the appropriate NFPA 72 standard or the Authority Having Jurisdiction.
• Support detectors, in all cases, independently of their attachment to the circuit conductors (wiring).
• Locate smoke detectors on the ceiling not less than 4” from the sidewall to the near edge, or if on a sidewall, between 4” and 12” down from the ceiling to the top of the detector.
• On smooth ceilings, place smoke detectors on 30’ centers as a guide.

Smoke detectors installation do not have a listed spacing. However. the Code does limit the spacing to 30 feet or whatever is required by the detector manufacturer. The Code also allows the use of the coverage radius to ensure that all points on the ceiling are covered.

For level ceilings with solid beams or joists. the spacing smoke detectors depends on the distance between the obstructions. the ceiling height. and the depth of the obstruction.

High air movement. caused by air conditioning. can dilute and disperse smoke particles  making smoke detection more difficult. In high airflow spaces. the coverage for spot- type detectors is reduced according to factors in a table or graph. The correction factors in the table and the graph are identical. For these circumstances. this reduction factor must be applied before any spacing reduction for ceiling height or for ceiling surface.