NFPA 13 Beam Rule Calculator: Sprinkler Obstruction Distance

What is the beam rule?

When a sprinkler deflector sits above the bottom of a beam, duct or cable tray, the obstruction cuts part of the discharge pattern. NFPA 13-2025 controls this with two dimensions: A, the horizontal distance from the sprinkler to the side of the obstruction, and B, the height of the deflector above the bottom of the obstruction. The farther the sprinkler is from the obstruction, the higher above its bottom the deflector may be.

Which table?

Standard spray upright and pendent: Table 10.2.8.2(a) (ft–in) and 10.2.8.2(b) (mm), Figure 10.2.8.2(a).
Extended coverage upright and pendent: Table 11.2.5.1.2(a)/(b).
ESFR: Table 14.2.10.2.1(a)/(b), Figure 14.2.10.2.1.

The metric tables are not a straight conversion of the inch tables: for standard spray, A = 1 ft allows B up to 2½ in, while A = 300 mm allows up to 65 mm. The calculator reads the mm table in SI and the ft–in table in US units.

Wide obstructions

For standard spray sprinklers, sprinklers shall be installed below fixed obstructions over 1.2 m (4 ft) wide (10.2.8.2(6)). Below obstructions 1.2 m or less wide no sprinklers are needed when the table is met (10.2.8.2(7)); with sprinklers on both sides, the distance from the obstruction centerline must not exceed half the allowable spacing (10.2.8.2(2)).

The three times rule

For obstructions below the deflector, such as columns, pipes and light fixtures, the sprinkler is kept at least 3 × the maximum dimension of the obstruction away (10.2.8.3.1.4). For horizontal obstructions the required clear distance is at most 600 mm (24 in); for vertical obstructions such as columns there is no cap. In light and ordinary hazard only structural members are considered (10.2.8.3.1.5), and sprinkler piping under 80 mm (3 in) is not counted (10.2.8.3.1.9). Extended coverage sprinklers use a four times rule with a 900 mm (36 in) cap for horizontal obstructions (11.2.5.2.1.3).

Frequently Asked Questions

What do A and B measure in the beam rule?

A is the horizontal distance from the sprinkler to the side of the obstruction. B is how far the deflector sits above the bottom of the obstruction. If the deflector is below the bottom of the obstruction, the pattern is not cut and the rule does not apply.

Why do the metric and inch tables give different answers?

The metric table in NFPA 13-2025 is not a straight conversion of the inch table; both are printed separately. Use the table for the unit system you work in.

What if the beam is wider than 4 ft (1.2 m)?

For standard spray sprinklers, sprinklers shall be installed below fixed obstructions over 4 ft (1.2 m) wide (NFPA 13-2025 10.2.8.2(6)).

When does the three times rule apply?

To obstructions below the deflector: the sprinkler must be at least three times the maximum dimension of the obstruction away. For horizontal obstructions the required distance is at most 24 in (600 mm); columns have no cap (10.2.8.3.1.4).