The NFPA 13 density/area curve for Extra Hazard Group 2 (EH2) runs from 16.3 mm/min over 232 m² (0.40 gpm/ft² over 2,500 ft²) to 15.5 mm/min over 464 m² (0.38 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 2 covers spaces with moderate to substantial amounts of flammable or combustible liquids, or where combustibles are extensively shielded from sprinkler discharge. The speed of a liquid fire and the difficulty of getting water onto the burning surface call for the highest density among NFPA 13's general hazard classes.
Design parameters
| Parameter | SI | US units |
|---|---|---|
| Density – start of curve | 16.3 mm/min | 0.40 gpm/ft² |
| Design area – start of curve | 232 m² | 2,500 ft² |
| Density – end of curve | 15.5 mm/min | 0.38 gpm/ft² |
| Design area – end of curve | 464 m² | 5,000 ft² |
| Theoretical sprinkler flow (start / end) | 3,785 / 7,192 L/min | 1,000 / 1,900 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
- Flammable liquid spraying (spray booths and similar)
- Flow coating
- Varnish and paint dipping
- Solvent cleaning
- Open oil quenching
- Asphalt saturating
- Plastics manufacturing
- Assembly of modular or manufactured buildings with combustible interiors
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%.
As with EH1, high-temperature sprinklers allow a 25% area reduction, not below 2000 ft². Flammable liquid storage is not an EH2 case; it is designed to separate liquid storage criteria (e.g. NFPA 30).
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 HHP2 (10.0 mm/min) or HHP3 (12.5 mm/min), both with 260 m² and 90 minutes. The NFPA EH2 curve starts at about 16.3 mm/min. Where the liquid quantity is large, both standards accept that sprinklers alone may not be enough, and foam, deluge or dedicated liquid standards come into play.
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 EH2?
In theory, at the start of the curve 3,785 L/min (1,000 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 a spray booth EH2?
Areas where flammable liquids are sprayed are given as EH2 examples. The booth itself and its ducts also carry the requirements of the standard specific to spray application.
Can EH2 be used for a flammable liquid store?
No. EH2 is for process areas; flammable liquid storage is designed to separate criteria based on quantity, container type and storage arrangement.
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.