The NFPA 13 density/area curve for Light Hazard (LH) runs from 4.1 mm/min over 139 m² (0.10 gpm/ft² over 1,500 ft²) to 2.9 mm/min over 279 m² (0.07 gpm/ft² over 3,000 ft²). Hose allowance is 379 L/min (100 gpm) and water supply duration is 30 minutes.
Definition
Light hazard covers spaces where both the amount and the combustibility of the contents are low, so the fire you expect to fight has a relatively modest heat release rate. Furniture, paper and fabric are present, but not in dense stockpiles, and a fire usually stays at the scale of a single item or room.
Design parameters
| Parameter | SI | US units |
|---|---|---|
| Density – start of curve | 4.1 mm/min | 0.10 gpm/ft² |
| Design area – start of curve | 139 m² | 1,500 ft² |
| Density – end of curve | 2.9 mm/min | 0.07 gpm/ft² |
| Design area – end of curve | 279 m² | 3,000 ft² |
| Theoretical sprinkler flow (start / end) | 568 / 795 L/min | 150 / 210 gpm |
| Hose allowance (inside + outside) | 379 L/min | 100 gpm |
| Water supply duration | 30 min | 30 min |
| Maximum area per sprinkler (standard spray) | 20.9 m² | 225 ft² |
| Maximum distance between sprinklers | 4.6 m | 15 ft |
Theoretical flow is simply density × area. Actual system demand is higher because of hydraulic imbalance and the minimum sprinkler pressure (0.5 bar, 7 psi). Coverage and spacing values are for standard spray sprinklers; smaller limits apply under combustible obstructed construction and in pipe schedule systems.
Typical occupancies
- Offices, including data processing areas
- Schools and other educational buildings
- Hospitals, nursing homes and other institutional buildings
- Places of worship and clubs
- Residential areas
- Restaurant seating areas (not kitchens)
- Museums; libraries without large stack rooms
- Audience areas of theatres and auditoriums (not stages)
- Unused attics
Using the curve
Any point on the curve may be chosen: a smaller area at a higher density, or a larger area at a lower density. The chosen density must be delivered over the whole hydraulically most remote area. In practice most designers work at the start of the curve because it usually gives the lowest water demand. For dry and double-interlock pre-action systems the area is increased by 30%.
With quick-response sprinklers on a wet system and a ceiling below the height limit, the design area may be reduced by up to 40% depending on ceiling height. This reduction is specific to LH and OH; confirm the conditions in the edition in force.
The inside hose allowance (0, 50 or 100 gpm) is part of the total above. The lower end of the duration range is accepted where waterflow and supervisory devices are electrically monitored at a constantly attended location.
Comparison with EN 12845
The closest match is the EN 12845 LH light hazard class (2.25 mm/min, 84 m², 30 minutes). The two standards do not measure the same thing: the start of the NFPA 13 LH curve, roughly 4.1 mm/min over 139 m², sits well above the EN values. EN 12845 also restricts LH to a narrower set of uses, so some offices and schools that are LH under NFPA may be treated as OH1 under EN.
The class comes from how the space is actually used. A project title or the occupancy on a permit is not enough; contents, stack height and flammable liquids are checked on site and the class is recorded with a written justification. For a quick first pass, use the NFPA 13 hazard classification tool.
Checklist
- Is the activity and type of contents recorded in writing?
- Are stack heights within the class definition; if not, have storage criteria been checked?
- Is the system type (wet/dry/pre-action) reflected in the area adjustment?
- Are hose allowance and duration included in the tank volume?
- Are areas of different classes in the same building calculated separately?
Frequently Asked Questions
What is the minimum water demand for NFPA 13 LH?
In theory, at the start of the curve 568 L/min (150 gpm) of sprinkler flow plus 379 L/min (100 gpm) of hose allowance, multiplied by 30 minutes. The real figure comes from the hydraulic calculation and is usually higher; the fire water tank sizing calculator helps with the volume.
What NFPA 13 hazard class is an office building?
Typical office floors are light hazard (LH). Archive rooms, dense paper storage or plant rooms can fall into a higher class and must be assessed separately.
How much area can one sprinkler cover in light hazard?
With standard spray sprinklers on a hydraulically calculated system under noncombustible construction, 225 ft² (20.9 m²), with a maximum spacing of 15 ft (4.6 m).
SprinkCalc — Fire Sprinkler Design Across Three Standards
SprinkCalc covers hazard classification, design density and area, K-factor selection, water demand and hydraulic calculations for NFPA 13, FM Global and BS EN 12845 in a single iOS app, and exports a professional PDF report.
Download SprinkCalc on the App StoreNFPA 13, Standard for the Installation of Sprinkler Systems (current edition) · BS EN 12845:2015+A1:2019. Values are for information; the binding text is the edition in force and the decision of the authority having jurisdiction. Conversions: 1 gpm/ft² = 40.75 mm/min, 1 ft² = 0.0929 m².