Heat Detector Spacing Chart — Ceiling Height Reduction
How a heat detector's listed spacing shrinks as the ceiling rises, per NFPA 72 Table 17.6.3.5.1 (2022 edition). Multiply the detector's listed spacing by the factor for the ceiling height; the worked column shows the result for a typical 50 ft-listed detector. The reduction is applied before any further beam, joist, or slope reduction, and only heat detectors use it.
Ceiling-height spacing reduction
| Ceiling height | Factor | 50 ft listed → |
|---|---|---|
| 0 to 10 ft | 1.00 | 50 ft |
| 10 to 12 ft | 0.91 | 46 ft |
| 12 to 14 ft | 0.84 | 42 ft |
| 14 to 16 ft | 0.77 | 39 ft |
| 16 to 18 ft | 0.71 | 36 ft |
| 18 to 20 ft | 0.64 | 32 ft |
| 20 to 22 ft | 0.58 | 29 ft |
| 22 to 24 ft | 0.52 | 26 ft |
| 24 to 26 ft | 0.46 | 23 ft |
| 26 to 28 ft | 0.40 | 20 ft |
| 28 to 30 ft | 0.34 | 17 ft |
Why the reduction exists
A heat detector responds to the hot gas that rises off a fire and spreads across the ceiling. The taller the ceiling, the more that plume cools and dilutes before it reaches the detector, so the same detector effectively "sees" a smaller area — and you have to place detectors closer together to keep response time acceptable. The factor is applied to the detector's UL-listed spacing, which is established by test on a flat 10 ft ceiling (commonly 50 ft for a spot detector), so a 50 ft detector at a 20 ft ceiling comes down to 32 ft on center.
The maximum distance to a wall is still one-half the reduced spacing, and the 0.7 point rule still applies. This ceiling-height factor comes first; beams, joists, and sloped ceilings then reduce spacing further. Smoke detectors get none of this — see the smoke detector spacing chart and the detector placement rules.
Edition and table number
The factor values are stable across editions, but the table number moved: it is Table 17.6.3.5.1 in the current NFPA 72 (2013 through 2022), and was Table 5.6.5.5.1 in the 2007 and earlier editions when initiating devices lived in Chapter 5. Some field references shorthand it as Table 17.6.3.1. Confirm the edition your jurisdiction enforces.
Common questions
How does ceiling height affect heat detector spacing?
As a ceiling rises above 10 ft, the hot gas layer from a fire cools and spreads before it reaches the detector, so NFPA 72 (Table 17.6.3.5.1, 2022 edition) requires you to multiply the detector's listed spacing by a reduction factor: 0.91 at 10–12 ft, 0.77 at 14–16 ft, 0.64 at 18–20 ft, down to 0.34 at 28–30 ft. A 50 ft-listed detector at a 20 ft ceiling drops to 50 × 0.64 = 32 ft on center.
Do smoke detectors get the same reduction?
No — this reduction table applies only to heat detectors. NFPA 72 imposes no equivalent ceiling-height derating on spot-type smoke detectors; high-ceiling smoke coverage is handled through performance-based design instead. That difference is a common source of layout errors.
What is the smallest spacing I have to use?
The reduction never has to take you below 0.4 times the ceiling height. Line-type electrical-conductivity and pneumatic rate-of-rise tubing detectors are exempt from the table entirely and follow the manufacturer's instructions.
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