The NFPA 13 density/area curve for Ordinary Hazard Group 2 (OH2) runs from 8.2 mm/min over 139 m² (0.20 gpm/ft² over 1,500 ft²) to 6.9 mm/min over 372 m² (0.17 gpm/ft² over 4,000 ft²). Hose allowance is 946 L/min (250 gpm) and water supply duration is 60–90 minutes.
Definition
Ordinary Hazard Group 2 covers spaces where the quantity and combustibility of contents are moderate to high. Stockpiles of moderate heat release material stay within 12 ft (about 3.7 m) and those of high heat release material within 8 ft (about 2.4 m). Shops, workshops and a good share of light industry sit in this group.
Design parameters
| Parameter | SI | US units |
|---|---|---|
| Density – start of curve | 8.2 mm/min | 0.20 gpm/ft² |
| Design area – start of curve | 139 m² | 1,500 ft² |
| Density – end of curve | 6.9 mm/min | 0.17 gpm/ft² |
| Design area – end of curve | 372 m² | 4,000 ft² |
| Theoretical sprinkler flow (start / end) | 1,136 / 2,574 L/min | 300 / 680 gpm |
| Hose allowance (inside + outside) | 946 L/min | 250 gpm |
| Water supply duration | 60–90 min | 60–90 min |
| Maximum area per sprinkler (standard spray) | 12.1 m² | 130 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; for ordinary hazard this limit applies regardless of construction type; smaller values apply in storage areas.
Typical occupancies
- Retail (mercantile) floors
- Machine shops and metalworking
- Repair garages
- Printing and publishing
- Textile, leather goods and tobacco manufacturing
- Paper and pulp mills
- Chemical plants (ordinary)
- Cereal and feed mills
- Wood machining and assembly
- Library stack rooms; stages
- Post offices; exterior loading docks
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 shops, shelf and stack height is the critical threshold: once stacks exceed the heights in the OH2 definition, or fall within the scope of the storage chapters, the hazard class method no longer applies and storage criteria take over.
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
On the EN 12845 side the nearest match is usually OH3 (5.0 mm/min, 216 m², 60 minutes), and for some process areas OH4 (360 m²). The NFPA OH2 curve starts at about 8.2 mm/min, well above EN's fixed 5.0 mm/min. The two classifications cannot be converted into each other; use the tables of whichever standard the project is designed to.
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 OH2?
In theory, at the start of the curve 1,136 L/min (300 gpm) of sprinkler flow plus 946 L/min (250 gpm) of hose allowance, multiplied by 60–90 minutes. The real figure comes from the hydraulic calculation and is usually higher; the fire water tank sizing calculator helps with the volume.
What hazard class is a retail shop under NFPA 13?
A typical sales floor is OH2. Where high shelving is used on the sales floor, particularly with plastic goods, storage criteria may apply instead.
What is the hose allowance for OH2?
250 gpm (about 946 L/min) combined inside and outside hose. Water supply duration is 60–90 minutes.
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.