FM Global DS 3-26: Worked Water Demand Examples for Nonstorage Areas

For a nonstorage area the water demand comes out in four steps: determine the hazard category, confirm that the storage really is incidental, read the density and area from Table 2.3.1.10, then add the hose allowance and multiply by 60 minutes. This article walks through those four steps with section numbers and four numerical examples.

The source is FM Global DS 3-26, Fire Protection for Nonstorage Occupancies (April 2019, Interim Revision April 2025, 27 pages). For hazard category mapping and the overall framework see the DS 3-26 nonstorage water demand article, and for the calculation steps the hydraulic calculation guide.

Step 1: Hazard Category

Section 2.2.2 (p.3) determines the category from the predominant occupancy; Table 2.2.2 (p.5) gives the definitions. HC-1 covers areas with light overall combustible loading and limited combustibles in processes, or low-hazard operations; it does not include incidental storage of plastics or plastics used in wall and ceiling construction. Examples are residential, offices, noncombustible manufacturing and hospitals. HC-2 covers moderate continuous combustible loading, or operations of moderate hazard due to limited plastics or ignitable liquids; machine shops, woodworking, electronic assembly, retail, theatres and food production are examples. HC-3 covers generally continuous, heavier combustible loading with more plastics; plastic manufacturing, vehicle manufacturing and printing plants are examples.

The table note matters: the examples are typical but may need to move to a higher or lower category depending on the occupancy details. Section 2.2.3 classifies buildings of wood construction that otherwise contain no combustibles as HC-1; Section 2.2.5 requires protection for the greatest hazard where mixed occupancies are not separated by fire partitions. Section 2.2.4 opens a separate item: spaces concealed from ceiling sprinklers that have combustible construction or contain combustible material are identified and protected, and combustible concealed spaces are treated as HC-1 per DS 1-12.

Step 2: Is the Storage Really Incidental

This is where the calculation most often slips. Section 2.3.2.1 (p.7) treats solid-pile, palletized, rack, shelf or bin-box storage of Class 1-3 commodities up to 10 ft (3 m) high and no more than 200 ft2 (20 m2) in area as incidental to the occupancy, protected with Table 2.3.1.10. Section 2.3.2.2 narrows the limit for plastics: in HC-2 and HC-3 occupancies, plastic commodities up to 6 ft (1.8 m) high and no more than 64 ft2 (6 m2), approximately four pallets, count as incidental. Section 2.3.2.3 states that multiple storage areas within these limits may still be treated as incidental if separated by aisles at least 8 ft (2.4 m) wide.

If the area limit is exceeded but the height limit is not, Section 2.3.3.1 applies: the storage is treated as low-piled, and Table 2.3.3.1 plus the DS 2-0 installation and obstruction rules for storage sprinklers are used. If the height limit is also exceeded, Section 2.3.4.1 (p.10) sends the area straight to DS 8-9.

Section 1.1 (p.3) and Section 2.3.1.10 list the situations where the table does not apply: ignitable liquid storage or use above incidental quantities, combustible dust or lint collected on horizontal surfaces, plastic construction elements, concealed and shielded spaces with combustible construction or contents, and storage heights or areas beyond incidental storage.

Step 3: Design Density and Area

Table 2.3.1.10 (p.7) gives the design. SI values, density in mm/min and area in m2:

The notes shape the design. Note 3 and Section 2.3.1.10.1: the 30 m ceiling designs are limited to sprinklers of K14.0 (K200) and larger; for a dry system under a 30 m ceiling, design guidance is currently unavailable. Note 1: for dormitories, residential and dwelling type areas, the demand area may be based on the largest room, but not less than four sprinklers, provided compartmentation with at least a one-hour fire rating is present; corridors are treated as rooms in that determination.

Section 2.3.1.11 sets a floor: regardless of the table demands, a minimum design pressure of 7 psi (0.5 bar) is provided at the most remote sprinkler. Section 2.3.1.14 treats the manufacture of large, contiguous components (large aircraft, boats, wind turbine blades) separately: Table 2.3.1.10 is used below 18 m ceilings, and above 18 m a design of 12 sprinklers at 50 psi (3.5 bar) with K25.2 (K360) sprinklers applies.

Step 4: Hose Allowance, Duration and Four Worked Examples

Section 2.3.1.12 (p.7) gives the hose stream allowance: 250 gpm (950 L/min) for HC-1 and HC-2, 500 gpm (1,900 L/min) for HC-3. Section 2.3.1.13 gives the duration: the water supply must provide the design sprinkler flow plus the hose allowance for 60 minutes for all categories. The rest is arithmetic: sprinkler flow = density x design area; total = sprinkler flow + hose allowance; volume = total x 60 minutes.

Example 1. HC-2 electronic assembly hall, wet system, 8.5 m ceiling. From the table, 8 mm/min over 230 m2. Sprinkler flow 8 x 230 = 1,840 L/min; total 1,840 + 950 = 2,790 L/min; volume 2,790 x 60 = 167,400 L, about 167 m3.

Example 2. The same hall on a dry system because it cannot be heated. From the table, 8 mm/min over 330 m2. Sprinkler flow 2,640 L/min; total 3,590 L/min; volume 215,400 L, about 215 m3. Going dry enlarges the tank by about 29% without changing the density. For the trade-offs between system types see wet, dry, alternate and preaction systems compared.

Example 3. HC-1 office floor, wet system, 4 m ceiling. From the table, 4 mm/min over 140 m2. Sprinkler flow 560 L/min; total 1,510 L/min; volume 90,600 L, about 91 m3. Note that in this case the hose allowance is about two thirds of the total flow. In HC-1 areas it is the hose allowance, not the sprinklers, that sizes the tank.

Example 4. HC-3 plastics manufacturing hall, wet system, 17 m ceiling. From the table, 20 mm/min over 280 m2. Sprinkler flow 5,600 L/min; total 7,500 L/min; volume 450,000 L, that is 450 m3. Had the same hall been built with a 9 m ceiling, it would need 12 x 230 = 2,760 L/min for sprinklers plus 1,900 L/min hose, 4,660 L/min in total and a volume of about 280 m3. The ceiling height decision alone raises the tank volume by about 61%.

Note 2 of Table 2.3.1.10 offers a reduction: for HC-2 and HC-3 occupancies with ceilings not over 9 m on wet systems, the design can be reduced to 12 mm/min over 140 m2 (at least 6 sprinklers) with 160 °F (70 °C) K11.2EC (K160EC) upright sprinklers, or to 12 mm/min over 90 m2 (at least 4 sprinklers) with K14.0EC (K200EC) upright sprinklers. For HC-2: 12 x 90 = 1,080 L/min for sprinklers, 2,030 L/min in total, a volume of about 122 m3. That is a tank about 27% smaller than in Example 1.

As a final check, remember the 0.5 bar requirement at the most remote sprinkler. Using the relationship between K-factor and flow (see what is the K-factor), a K11.2 (K160) sprinkler at 0.5 bar delivers 160 x square root(0.5) = about 113 L/min; at 8 mm/min this flow feeds an area of about 14 m2. Actual spacing and coverage limits come from DS 2-0 (Section 2.3.1.9), but this simple check shows why the design pressure is set as a floor.

Field Note

Where this calculation most often breaks down on site is storage that creeps into the production area over time. The project is handed over as HC-2 with a 167 m3 tank; two years later, raw material pallets in the same hall start being stacked above 3 metres. Once the 3 m and 20 m2 limits of Section 2.3.2.1 are exceeded, the area is no longer within DS 3-26; under Section 2.3.4.1 it moves to DS 8-9 and the design changes completely. The only mechanism that catches this is turning the exclusion list of Section 2.3.1.10 into an operational check item: once a year, measure the storage height and area in the hall and compare them with the design basis.

The second point is concealed spaces. Section 2.2.4 requires spaces concealed from ceiling sprinklers that contain combustibles to be identified and protected; it gives as examples equipment with combustible material, areas obstructed by ductwork, light fixtures or hoods, and hidden combustible construction. On site this usually means cable trays and ducts added later; none of them goes back to the fire protection design, and the calculation stays right on paper while turning wrong in the room.

Checklist

• ☐ Hazard category set from the predominant occupancy with Table 2.2.2 (Section 2.2.2)
• ☐ For unseparated mixed occupancies, designed for the greatest hazard (Section 2.2.5)
• ☐ Concealed and shielded spaces identified and protected (Section 2.2.4)
• ☐ Storage height and area within the incidental storage limits (Sections 2.3.2.1 and 2.3.2.2)
• ☐ If the area limit is exceeded, the low-piled storage table is used (Section 2.3.3.1)
• ☐ If the height limit is exceeded, the design moves to DS 8-9 (Section 2.3.4.1)
• ☐ Density and area read from Table 2.3.1.10 by ceiling height and system type
• ☐ At least 0.5 bar design pressure at the most remote sprinkler (Section 2.3.1.11)
• ☐ Hose allowance added: 950 L/min for HC-1/HC-2, 1,900 L/min for HC-3 (Section 2.3.1.12)
• ☐ Duration taken as 60 minutes for all categories (Section 2.3.1.13)
• ☐ The EC sprinkler reduction option reviewed (Table 2.3.1.10 Note 2)
• ☐ Hydrant and hose cabinet allowances of local regulations calculated separately
• ☐ An annual check of storage height and area added to the operating procedure

Frequently Asked Questions

What hose stream allowance does DS 3-26 require?

Section 2.3.1.12 (p.7) sets a hose stream allowance of 250 gpm (950 L/min) for HC-1 and HC-2 occupancies and 500 gpm (1,900 L/min) for HC-3 occupancies. The allowance is added to the sprinkler design flow. Section 2.3.1.13 then requires the water supply to provide the sprinkler design flow plus the hose stream for 60 minutes for all hazard categories.

How is the tank volume found for an HC-2 production hall?

For HC-2 with a wet system and a ceiling of up to 9 m, Table 2.3.1.10 (p.7) requires 8 mm/min over 230 m2. The sprinkler flow is 8 x 230 = 1,840 L/min. Adding the 950 L/min hose allowance of Section 2.3.1.12 gives 2,790 L/min, and the 60-minute duration of Section 2.3.1.13 gives 167,400 L, about 167 m3. This covers only the DS 3-26 demand; hydrant and hose cabinet allowances required by local regulations are assessed separately.

How much does a dry system increase the water demand in the same area?

For HC-2 with a ceiling of up to 9 m, Table 2.3.1.10 gives 8 mm/min over 230 m2 for a wet system and 8 mm/min over 330 m2 for a dry system. The density stays the same and the area grows. The sprinkler flow becomes 2,640 L/min instead of 1,840 L/min; with the 950 L/min hose allowance and 60 minutes, the total volume comes out at about 215 m3 instead of 167 m3, an increase of about 29%.

When does storage stay within the scope of DS 3-26?

Section 2.3.2.1 (p.7) treats Class 1-3 commodities stored up to 10 ft (3 m) high and no more than 200 ft2 (20 m2) in area as incidental to the occupancy. Section 2.3.2.2 treats plastic commodities in HC-2 and HC-3 occupancies up to 6 ft (1.8 m) high and no more than 64 ft2 (6 m2), approximately four pallets, as incidental. Section 2.3.2.3 states that multiple storage areas within these limits may still be considered incidental if separated by aisles at least 8 ft (2.4 m) wide.

References

1. FM Global Property Loss Prevention Data Sheet 3-26, Fire Protection for Nonstorage Occupancies, April 2019, Interim Revision April 2025. Sections used: 1.0-1.1 (p.3), 2.2.1-2.2.6 and Table 2.2.2 (p.3-5), 2.3.1.1-2.3.1.9 (p.5-6), 2.3.1.10 and Table 2.3.1.10 (p.6-7), 2.3.1.11-2.3.1.15 (p.7), 2.3.2.1-2.3.2.4 (p.7-8), 2.3.3.1 and Table 2.3.3.1 (p.8-9), 2.3.4.1 (p.10).
2. Volume and flow values are calculated in this article by multiplying the density, area, hose allowance and duration given in the sections above; intermediate results are written out in the text.
3. FM Global data sheet list: https://www.fmglobal.com/research-and-resources/fm-global-data-sheets
4. Hazard category mapping and overall framework: https://selvi.org/en/blog/fmglobal/ds-3-26-nonstorage-water-demand
5. Hydraulic calculation steps: https://selvi.org/en/blog/guides/sprinkler-hydraulic-calculation-guide
6. System type trade-offs: https://selvi.org/en/blog/comparison/wet-dry-alternate-preaction-compared
7. K-factor definition: https://selvi.org/en/blog/glossary/what-is-the-k-factor

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