Projected Beam Detector Spacing
Spacing for projected beam smoke detectors — the transmitter-and-receiver detectors that protect large, high-ceiling spaces where spot detectors do not work. A beam is treated as a row of spot detectors: no more than 60 ft between beams, 30 ft from a side wall, with beam lengths up to about 300 ft. Verify the length against the detector's listing.
Projected beam spacing
| Parameter | Value |
|---|---|
| Maximum spacing between beams | Not more than 60 ft (each beam acts as a row of spot detectors) |
| Maximum distance beam to a side wall | One-half the beam spacing = 30 ft |
| Maximum beam length (transmitter to receiver) | Up to ~300 ft — set by the product listing, not a fixed code number |
| Typical application | High ceilings (roughly 25–30 ft and up) where spot detectors are impractical |
| Very high ceilings (stratification) | Place one beam at the ceiling and a second at a lower level to catch stratified smoke |
A beam is a row of spot detectors
The easiest way to lay out beam detectors is to picture each beam as a straight line of spot detectors. The 60 ft maximum spacing between beams is the same as the 30 ft smooth-ceiling spot spacing doubled — a beam covers the strip on either side of it — and the 30 ft wall limit is again the half-spacing rule. Where a spot smoke detector protects a circle around a point, a beam protects a band along a line.
Beam detectors exist because spot detectors fail on tall ceilings — smoke dilutes and cools before it climbs 30 or 40 feet, and mounting and servicing spot detectors that high is a maintenance headache. A beam shot across the top of an atrium or warehouse covers the whole span from two accessible end points. The catch on very high ceilings is stratification: smoke can spread out at a warm layer below the ceiling and never reach a beam at the very top, which is why tall spaces get beams at more than one height.
Common questions
How far apart can projected beam smoke detectors be?
A projected beam acts like a long row of spot detectors, so the beams are spaced no more than 60 ft apart, with the outermost beam no more than 30 ft — one-half the spacing — from a side wall. This is the same logic as smooth-ceiling spot spacing, applied to a line instead of a point.
What is the maximum length of a beam detector?
Up to about 300 ft between the transmitter and receiver, though the exact maximum is set by the product listing rather than a fixed NFPA number — some units are rated a bit farther. Always confirm against the specific detector's data sheet.
When do you use a beam detector instead of spot detectors?
On high ceilings — roughly 25 to 30 feet and up — where spot detectors would be impractical: too many to install and maintain, and too high for smoke to reach reliably. A single beam can protect a large open volume like an atrium, warehouse, or gymnasium. For very high ceilings, beams are placed at more than one level to catch smoke that cools and stratifies before reaching the top.
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