The NFPA 13 density/area curve for Extra Hazard Group 1 (EH1) runs from 12.2 mm/min over 232 m² (0.30 gpm/ft² over 2,500 ft²) to 11.4 mm/min over 464 m² (0.28 gpm/ft² over 5,000 ft²). Hose allowance is 1,893 L/min (500 gpm) and water supply duration is 90–120 minutes.
Definition
Extra Hazard Group 1 applies where the quantity and combustibility of contents are very high and dust, lint or fibres can produce a rapidly developing fire, but flammable or combustible liquids are absent or present only in small amounts. Fires develop quickly and reach a high heat release rate.
Design parameters
| Parameter | SI | US units |
|---|---|---|
| Density – start of curve | 12.2 mm/min | 0.30 gpm/ft² |
| Design area – start of curve | 232 m² | 2,500 ft² |
| Density – end of curve | 11.4 mm/min | 0.28 gpm/ft² |
| Design area – end of curve | 464 m² | 5,000 ft² |
| Theoretical sprinkler flow (start / end) | 2,839 / 5,300 L/min | 750 / 1,400 gpm |
| Hose allowance (inside + outside) | 1,893 L/min | 500 gpm |
| Water supply duration | 90–120 min | 90–120 min |
| Maximum area per sprinkler (standard spray) | 9.3 m² | 100 ft² |
| Maximum distance between sprinklers | 3.7 m | 12 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; this applies to hydraulically calculated systems with densities of 0.25 gpm/ft² or more; pipe schedule systems are limited to 90 ft².
Typical occupancies
- Aircraft hangars (those not covered by a dedicated hangar standard)
- Areas using combustible hydraulic fluids
- Die casting and metal extrusion
- Plywood and particleboard manufacturing
- Rubber reclaiming, compounding, drying and vulcanising
- Sawmills
- Textile opening, blending and carding
- Upholstering with plastic foams
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%.
In extra hazard, using high-temperature sprinklers allows a 25% reduction in design area, but not below 2000 ft² (about 186 m²). The quick-response area reduction does not apply to EH.
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 nearest EN 12845 classes are HHP1 (7.5 mm/min) or HHP2 (10.0 mm/min), both with a 260 m² area of operation and 90 minutes. The NFPA EH1 curve starts at about 12.2 mm/min over 232 m², a higher density than the EN values. The gap comes from the different fire test basis and safety philosophy behind each standard.
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 EH1?
In theory, at the start of the curve 2,839 L/min (750 gpm) of sprinkler flow plus 1,893 L/min (500 gpm) of hose allowance, multiplied by 90–120 minutes. The real figure comes from the hydraulic calculation and is usually higher; the fire water tank sizing calculator helps with the volume.
Is an aircraft hangar always EH1?
No. Most hangars fall under a dedicated hangar standard (NFPA 409); EH1 is listed only as an example for hangars outside that standard's scope.
Why is sprinkler spacing limited to 12 ft in EH1?
At high heat release, closer sprinklers let the first heads to open control the fire while it is still small. Coverage is therefore limited to 100 ft² (9.3 m²) with 12 ft (3.7 m) spacing.
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.