Ceiling Strobe Candela Chart — by Room Size & Ceiling Height

Minimum candela for a ceiling-mounted fire alarm strobe, by room size and lens height, per NFPA 72 Table 18.5.5.7.1(b) (2022 edition; numbered 18.5.5.4.1(b)/18.5.5.5.1(b) in the 2013–2019 editions). Unlike a wall strobe, a ceiling strobe's requirement climbs with mounting height, and each height band caps at a different maximum room size — 44×44 ft at 10 ft, 46×46 at 20 ft, 70×70 at 30 ft. Wall-mounted strobes use a separate table.

Ceiling-mounted strobe candela

NFPA 72 Table 18.5.5.7.1(b)
Minimum candela for one ceiling-mounted strobe, by maximum square room size and maximum lens height, values per NFPA 72 Table 18.5.5.7.1(b) (2022 edition). A dash means that room size is not a printed row in that height band — use the next larger listed row.
Max room size≤ 10 ft ceiling≤ 20 ft ceiling≤ 30 ft ceiling
20 × 20 ft15 cd30 cd55 cd
30 × 30 ft30 cd45 cd75 cd
40 × 40 ft60 cd——
44 × 44 ft75 cd75 cd—
46 × 46 ft—80 cd—
50 × 50 ft——95 cd
53 × 53 ft——110 cd
55 × 55 ft——115 cd
59 × 59 ft——135 cd
63 × 63 ft——150 cd
68 × 68 ft——177 cd
70 × 70 ft——185 cd
Mounting a lens lower than a band's height is not precluded by the code, so a taller band's row may be used at any lesser height (a 50×50 ft room takes 95 cd at any lens height up to 30 ft). Above 30 ft: suspend to 30 ft, use the performance-based alternative (18.5.5.9), or wall-mount. The table assumes the appliance is at the center of the room — otherwise double the distance to the farthest wall to get the room size.

Why ceiling height matters here but not for wall strobes

A ceiling strobe throws light down and outward, so the taller the ceiling, the farther that light spreads before it reaches a standing or seated occupant — and the more candela it takes to deliver the required intensity at eye level. That is why the same room needs progressively more candela as the ceiling rises from 10 to 20 to 30 ft. A wall-mounted strobe sits in the 80–96 in band regardless of ceiling height, so its table keys on room size alone.

Ceiling mounting is the natural choice for open areas where a wall strobe would be too far from the center of the space, and it is required where a low ceiling cannot fit a wall strobe's lens at 80 in. For narrow corridors, neither room table applies — use the corridor spacing chart.

Common questions

How is a ceiling-mounted strobe sized?

A ceiling strobe is sized by both room size and lens height (NFPA 72 Table 18.5.5.7.1(b), 2022 edition). For a 20×20 ft room the minimum rises from 15 cd at a 10 ft lens height to 30 cd at 20 ft and 55 cd at 30 ft — the higher the strobe, the more candela it takes to reach the occupant's eye level. A 30×30 ft room runs 30 / 45 / 75 cd across the same three height bands, and the table tops out at 185 cd for a 70×70 ft room.

What if the ceiling is higher than 30 feet?

Where the ceiling exceeds 30 ft, NFPA 72 18.5.5.7.6 gives three options: suspend the appliance at or below 30 ft, use the performance-based alternative (18.5.5.9) to justify the actual mounting height, or switch to wall mounting sized by the wall table. Very high spaces are common candidates for wall-mounted strobes.

Why are some cells blank on this chart?

A blank cell means the code table doesn't print that room size in that height band — each band has its own set of maximum room sizes (the 10 ft band tops out at 44×44, the 20 ft band at 46×46, the 30 ft band at 70×70). Use the next larger listed room size, and note the code's own footnote: mounting a lens lower than a band's height is not precluded, so a 30 ft-band row (like 50×50 → 95 cd) may be used at any lesser lens height.

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