The NFPA 13 density/area curve for Ordinary Hazard Group 1 (OH1) runs from 6.1 mm/min over 139 m² (0.15 gpm/ft² over 1,500 ft²) to 4.9 mm/min over 372 m² (0.12 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 1 describes contents of low combustibility present in moderate quantity. Stockpiles do not exceed 8 ft (about 2.4 m) and the expected heat release is moderate. Production and service areas handling low-combustibility goods typically fall here.
Design parameters
| Parameter | SI | US units |
|---|---|---|
| Density – start of curve | 6.1 mm/min | 0.15 gpm/ft² |
| Design area – start of curve | 139 m² | 1,500 ft² |
| Density – end of curve | 4.9 mm/min | 0.12 gpm/ft² |
| Design area – end of curve | 372 m² | 4,000 ft² |
| Theoretical sprinkler flow (start / end) | 852 / 1,817 L/min | 225 / 480 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
- Car parks and car showrooms
- Bakeries
- Beverage manufacturing and canneries
- Dairy product manufacturing
- Electronics assembly plants
- Glass and glass product manufacturing
- Laundries
- Restaurant kitchens and service areas
- Plant rooms
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 LH, quick-response sprinklers on a wet system under a suitable ceiling height allow a design area reduction of up to 40%. The reduction is for wet systems only; if a car park uses a dry system there is no reduction and the area instead increases by 30%. 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 EN 12845 matches are OH1 and OH2 (5.0 mm/min; 72 m² and 144 m²; 60 minutes). The start of the NFPA OH1 curve, about 6.1 mm/min over 139 m², is slightly higher in density and considerably larger in area. There is no one-to-one mapping, and the example occupancy lists differ between the two standards.
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 OH1?
In theory, at the start of the curve 852 L/min (225 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 an enclosed car park under NFPA 13?
Car parks and showrooms are typically OH1. If a dry system is chosen because of freezing risk, the design area is increased by 30%.
What is the difference between OH1 and OH2?
In OH1 the contents have low combustibility and stockpiles stay below 8 ft. In OH2 quantity and combustibility are moderate to high, and the stockpile limit is 8 or 12 ft depending on heat release.
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.