Section 17.5.3/Smoke Detector Spacing
NFPA 72 Section 17.5.3 covers spot-type smoke detector spacing including the 30-foot nominal spacing and adjustments for ceiling height.
Spot-type smoke detectors on smooth ceilings must be spaced not more than a nominal 30 feet apart, and not more than 15 feet (half the spacing) from any wall or partition. The 30-foot spacing assumes a smooth, flat ceiling up to 10 feet high. Spacing must be reduced for peaked or sloped ceilings per 17.5.3.5. Detectors in high-airflow environments or in spaces with deep beams or joists require spacing adjustments per the applicable subsections.
Why this section exists
Smoke detectors must respond quickly enough to provide early warning before conditions become untenable. The 30-foot spacing is based on testing that established the maximum distance smoke will reliably travel along a ceiling from a fire to a detector. If detectors are too far apart, smoke from a fire between detectors may not reach either one quickly enough to provide early warning.
What plan reviewers look for
Plan reviewers measure detector spacing on the reflected ceiling plan and verify that no area exceeds 30-foot spacing. They check the distance from walls and partitions (15-foot maximum). They also check for ceiling features that affect smoke travel: beams deeper than the specified threshold, sloped ceilings, HVAC supply diffusers that can dilute smoke, and high ceilings that delay smoke stratification.
Common violations
Related NFPA 72 requirements
Section 17.7.3 covers heat detector spacing (which has different requirements). Section 18.4.3 covers notification appliance (horn and strobe) placement. Section 17.5.3.5 covers detector spacing on sloped ceilings.