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g/fire_alarm • Posted by u/smoke_detector_steve •

Smoke detector placement in a 32-foot clear height warehouse — NFPA 72 math

NFPA 72 Table 17.6.3.1.1 gives reduced coverage for high-ceiling spaces. At 32 feet, spot-type smoke detectors are essentially not listed for the application — the standard requires beam-type or air sampling systems.

I keep running into GCs who want spot detectors because they're cheaper and don't understand the code limitation.

Quick reference for anyone who needs to cite the code in a meeting:

  • Spot smoke detectors: listed to a maximum ceiling height of 30 feet per most manufacturer listings
  • At 30 feet, required coverage area is already reduced to 0.7 of the standard 900 sq ft
  • Above 30 feet: use projected beam detectors (NFPA 72 Section 17.7) or air sampling (VEWFD, Section 17.8)
  • Beam detector spacing: maximum 60-foot beam spacing, mounted at or near ceiling level

At 32 feet on a large warehouse, projected beam detectors are the code-compliant and cost-effective choice. Installed cost is more than spot type but less than the retrofit when the AHJ rejects it.

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u/suppression_sue •

Don't forget: VESDA (air sampling) in high-bay warehouses with racking is increasingly specified because it samples at multiple levels, not just at the ceiling. For a warehouse with 30-foot-high racking, a ceiling-level beam detector won't catch a smoldering fire in the middle of a rack. Something to raise with the design team if it's a storage occupancy.

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u/fire_code_fred •

This deserves to be pinned. The GC budget conversation is something every fire alarm tech has had. Once you show them the reject/rework cost of installing non-compliant spot detectors at 32 feet vs the upfront beam cost, the math is obvious. Beam detectors aren't even that expensive on a per-coverage-area basis for large spaces.

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