How much fire water does a given building need? FM Global DS 3-26 maps all non-storage occupancies — offices, hotels, hospitals, shopping centres and light manufacturing — to a hazard class and gives the corresponding water demand. This article summarises the categories used most often in practice.
Hazard Class Mapping
| Occupancy | FM hazard class | Density × area |
|---|---|---|
| Offices, hospital wards, schools | HC-1 | 4.1 mm/min × 139 m² |
| Hotel bedrooms, restaurants | HC-1 | 4.1 mm/min × 139 m² |
| Retail units, commercial kitchens | HC-2 | 6.1 mm/min × 232 m² |
| Light manufacturing (food, textile) | HC-2 | 6.1 mm/min × 232 m² |
| Multi-storey car park | HC-2 | 6.1 mm/min × 232 m² |
| Paint shops, plastic injection moulding | HC-3 | 10.2 mm/min × 372 m² |
| Timber workshops, rubber production | HC-3 | 10.2 mm/min × 372 m² |
Total Water Demand
Total flow is the sprinkler demand (density multiplied by area) plus the hose stream allowance:
- HC-1: roughly 570 L/min sprinkler demand plus around 950 L/min hose
- HC-2: roughly 1,415 L/min sprinkler demand plus around 1,900 L/min hose
- HC-3: roughly 3,795 L/min sprinkler demand plus 1,900 L/min or more hose
Duration and Tank Capacity
- HC-1: around 30 minutes, giving roughly 46 m³
- HC-2: around 60 minutes, giving roughly 199 m³
- HC-3: around 90 minutes, giving roughly 513 m³
Hose Stream Allowance
- HC-1 light hazard: around 950 L/min
- HC-2 ordinary hazard: around 1,900 L/min
- HC-3 higher hazard: 1,900 L/min and above
- Wall hydrants and hose stations add further demand
Shopping Centres and Multi-Storey Buildings
In a shopping centre the atrium smoke control demand is assessed in parallel with the sprinkler demand:
- Retail areas at HC-2, with the tank sized on that demand and duration
- Restaurants with commercial kitchens require a separate suppression system in addition
- Atrium smoke control is coordinated with sprinkler operation
- Multi-storey car parks at HC-2, with foam-water considered where fuel spill risk exists
Quick Checklist
- Hazard class correctly assigned (HC-1, HC-2 or HC-3)
- Density multiplied by area of operation gives the sprinkler demand
- Hose stream allowance added
- Duration taken from the hazard class
- Tank capacity equals total flow multiplied by duration
Frequently Asked Questions
How is the FM hazard class assigned to a building?
FM Global DS 3-26 maps occupancies to hazard classes HC-1 to HC-3 by the fuel present and the expected rate of fire growth. Offices, hospital wards, schools and hotel bedrooms fall into HC-1; retail units, light manufacturing and car parks into HC-2; paint shops, plastics moulding and timber workshops into HC-3.
How is fire water tank capacity calculated for a non-storage building?
Multiply the density by the area of operation to get the sprinkler demand, add the hose stream allowance for the hazard class, and multiply the total by the required duration. Duration rises with hazard class, which is why an HC-3 occupancy can need ten times the stored water of an HC-1 building of the same floor area.
Why does the hose stream allowance matter so much?
Because it is often comparable to, or larger than, the sprinkler demand itself. Sizing a tank on the sprinkler calculation alone leaves it substantially undersized, and this is one of the most common errors found when an existing installation is reviewed against FM criteria.
How does FM DS 3-26 relate to NFPA 13 hazard classification?
Both map occupancies to a density and area, but the class names and the specific values differ, and FM applies its own hose allowances and durations. A project should follow one framework consistently: taking the density from one and the duration or hose allowance from the other produces an inconsistent and usually undersized water supply.
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Download MEP Calc on the App StoreFM Global Property Loss Prevention Data Sheet DS 3-26 (Fire Protection for Nonstorage Occupancies); related data sheets DS 2-0 (sprinkler installation), DS 3-2 (water tanks) and DS 3-7 (fire pumps).