11.3 Density/Area Design, Adjustments & Hose Streams

Density × area is the sprinkler demand, not the water-supply demand

For new occupancy-hazard spray-sprinkler systems, NFPA 13 (2022) abandoned “any point on the curve.” Table 19.2.3.1.1 (the 2022 successor to the older Table 19.3.3.1.1 / curve figure) gives two points per hazard. The everyday point is the smaller area and higher density. The 0.07/3000, 0.12/3000, 0.17/3000, 0.28/3000, and 0.38/3000 alternates apply when 19.2.3.1.5 requires the 3,000 ft² operation for unsprinklered combustible concealed-space adjacency—not as a free designer preference. Figure 19.2.3.1.1 curves remain for evaluation or modification of existing systems.

Independent OpenExamPrep material uses the 2022 table as exam memory, not as a claim of AHJ adoption in every city. If a jurisdiction still enforces 2019, the curves can return; the NICET reference listed for this guide is 2022.

That product is sprinklers only. Hose is a separate add.

Hose streams and duration (Table 19.2.3.1.2)

Table 19.2.3.1.2 still uses inside-hose choices of 0, 50, or 100 gpm and a total combined inside-plus-outside hose allowance of:

• Light: 100 gpm, duration 30 minutes
• Ordinary (Groups 1 and 2): 250 gpm, duration 60 or 90 minutes
• Extra (Groups 1 and 2): 500 gpm, duration 90 or 120 minutes

The lower duration in each ordinary/extra pair is permitted when waterflow-alarm and supervisory devices are electrically supervised and monitored at an approved, constantly attended location (19.2.3.1.3). Inside hose is not an automatic extra 100 gpm on top of the total; the total is 100, 250, or 500 gpm. If inside hose stations take 100 gpm, the outside remainder is 0 (light), 150 (ordinary), or 400 (extra) unless the stem states a different permitted split. Hose is added at the supply connection (where the fire department actually takes water), not pushed through the remote sprinklers.

Worked water-supply totals (sprinkler product + combined hose):

• Light: 150 + 100 = 250 gpm for 30 minutes
• OH1: 225 + 250 = 475 gpm for 60 or 90 minutes
• OH2: 300 + 250 = 550 gpm for 60 or 90 minutes
• EH1: 750 + 500 = 1,250 gpm for 90 or 120 minutes
• EH2: 1,000 + 500 = 1,500 gpm for 90 or 120 minutes

Storage ESFR and CMDA hose/duration rows live in Chapter 20 and are not these occupancy-hazard numbers. Do not import 250 gpm ESFR hose onto an extra-hazard office-shop calculation.

Quick-response area reduction — use the verified percents

19.2.3.2.3 / Figure 19.2.3.2.3.1 permit a design-area reduction without changing density when all of the following are true:

1. Wet pipe system
2. Light or ordinary hazard (not extra)
3. Maximum ceiling height 20 ft
4. No unprotected ceiling pockets exceeding 32 ft²
5. No unprotected areas above cloud ceilings as allowed by 9.2.7
6. The design area never uses fewer than five sprinklers (19.2.3.2.3.2)

On the figure, percent reduction y is 40 percent at a 10 ft (and lower) ceiling, 25 percent at 20 ft, and zero above 20 ft. Between 10 ft and 20 ft the straight line is y = −1.5h + 55 (h in feet). At 15 ft: y = −1.5(15) + 55 = 32.5 percent.

Worked QR, 10 ft wet ordinary, starting area 1,500 ft²: 40 percent of 1,500 = 600. Remaining design area = 1,500 − 600 = 900 ft². Density is unchanged. If the occupancy was OH2, sprinkler demand becomes 0.20 × 900 = 180 gpm before hose, not 300 gpm. Then add 250 gpm hose: 430 gpm at the supply. Quick-response sprinklers are not permitted for extra hazard or for occupancies with substantial flammable liquids or combustible dusts (19.2.3.2.2.2), so do not take a 40 percent extra-hazard cut.

Area increases that Level I must not forget

These increase area without changing density:

• Sloped ceilings exceeding 2 in 12: +30 percent (19.2.3.2.4) for the listed spray/CMSA cases
• Dry pipe and double-interlock preaction: +30 percent (19.2.3.2.5)

They compound on the originally selected area (19.2.3.2.8.1). Wet 1,500 ft² that is both dry-pipe and steeply sloped is not in one system at once, but a dry system on a steep slope is 1,500 × 1.3 × 1.3 = 2,535 ft². Dry alone is 1,500 × 1.3 = 1,950 ft².

High-temperature extra-hazard sprinklers may reduce area 25 percent, not below 2,000 ft² (19.2.3.2.6). K-11.2 or larger on extra-hazard Table 19.2.3.1.1 points may take the same 25 percent cut, not below 2,000 ft² (19.2.3.2.7). 2,500 × 0.75 = 1,875, which is below 2,000, so the floor is 2,000 ft², not 1,875.

Unsprinklered combustible concealed-space rules in 19.2.3.1.5, if they apply, are imposed after the other modifications (19.2.3.2.8.2) and can drive the 3,000 ft² alternate densities in Table 19.2.3.1.1.

Minimum remote-area width along the branch lines is 1.2 √A (28.2.4.2). For A = 1,500, √1,500 ≈ 38.73, and 1.2 × 38.73 ≈ 46.5 ft. For the 900 ft² QR area, √900 = 30, and 1.2 × 30 = 36 ft. Level I should recognize the formula; laying out every head is later work.

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