Why is hazard classification the first decision in NFPA 13?
The NFPA 13 density/area method sets the sprinkler demand for almost every non-storage building. Its single input is a judgement: the occupancy hazard classification of the space. The class reflects the quantity and combustibility of the contents and the expected heat release rate, and it comes in five groups: Light Hazard (LH), Ordinary Hazard Groups 1 and 2 (OH1, OH2) and Extra Hazard Groups 1 and 2 (EH1, EH2). The class sets density, design area, hose allowance, water supply duration and sprinkler spacing together, so a wrong class invalidates everything downstream in the hydraulic calculation.
Annex A example occupancies
Alongside the class definitions, the standard's annex lists typical occupancies for each class, and the list in this calculator is built from those examples. The lists are not exhaustive: if your facility is missing, choose the closest analogue and check the real fire load, storage height and presence of flammable liquids.
- LH: places of worship, clubs, educational buildings, hospitals, libraries (except large stack rooms), museums, nursing homes, offices, residential, restaurant seating, theatres (excluding stages), unused attics.
- OH1: car parks and showrooms, bakeries, beverage manufacturing, canneries, dairy products, electronic and glass plants, laundries, restaurant service areas.
- OH2: cereal and feed mills, dry cleaners, exterior loading docks, library stack rooms, machine shops, mercantile, paper and printing, repair garages, stages, textiles, tyres, tobacco and wood machining.
- EH1: combustible hydraulic fluid use areas, die casting, metal extruding, plywood and particleboard, saw mills, rubber processing, upholstering with plastic foams.
- EH2: asphalt saturating, flammable liquid spraying, flow coating, open oil quenching, plastics manufacturing, solvent cleaning, varnish and paint dipping.
Reading the density/area curves
NFPA 13 gives each class a straight-line density/area curve rather than a single point. The designer may select any point on it: a smaller design area needs a higher density, and a larger area allows a lower one. The calculator interpolates linearly along the curve and marks the chosen point on the chart.
| Class | Curve start | Curve end | Hose allowance | Duration |
|---|---|---|---|---|
| LH | 0.10 gpm/ft² @ 1500 ft² | 0.07 @ 3000 ft² | 100 gpm | 30 min |
| OH1 | 0.15 @ 1500 ft² | 0.12 @ 4000 ft² | 250 gpm | 60–90 min |
| OH2 | 0.20 @ 1500 ft² | 0.17 @ 4000 ft² | 250 gpm | 60–90 min |
| EH1 | 0.30 @ 2500 ft² | 0.28 @ 5000 ft² | 500 gpm | 90–120 min |
| EH2 | 0.40 @ 2500 ft² | 0.38 @ 5000 ft² | 500 gpm | 90–120 min |
For SI values use 1 gpm/ft² = 40.75 mm/min and 1 ft² = 0.0929 m²; the OH2 start point, for example, is about 8.15 mm/min over 139 m².
Qsprinkler = density × design area → 0.20 gpm/ft² × 1500 ft² = 300 gpm (≈ 1136 L/min)On the chart, the shaded rectangle is as wide as the design area and as tall as the density, so its area is the sprinkler flow itself. Moving right along a curve lowers the density, but the area grows faster, so the total flow still rises. That is why most projects are designed at the start of the curve.
Dry pipe and preaction systems
In dry pipe and double-interlock preaction systems water reaches the sprinklers later, so more heads open. NFPA 13 therefore increases the design area by 30% without changing the density. The calculator reads the density at the un-increased area and then multiplies the area by 1.30; the extra area appears on the chart as a hatched extension.
Hose allowance and duration
Total water demand is the sprinkler flow plus the combined inside and outside hose allowance. The lower end of the duration range is acceptable where waterflow alarms are monitored at a remote station; otherwise use the upper end. For tank volume, multiply total flow by duration or go straight to the fire water tank calculator.
Sprinkler spacing and coverage
For standard spray sprinklers in hydraulically calculated systems, the maximum protection area per sprinkler is 225 ft² (20.9 m²) for LH, 130 ft² (12.1 m²) for OH and 100 ft² (9.3 m²) for EH. Maximum spacing is 15 ft (4.6 m) for LH and OH and 12 ft (3.7 m) for EH. The calculator divides the design area by that coverage to show the minimum number of sprinklers expected to operate.
Common mistake: protecting shelving or stacks above 3.7 m (12 ft) in a shop or workshop with these curves. At that height the commodity class and storage arrangement govern; the occupancy curves no longer apply.
Boundary cases
- One building can contain several classes; each area is calculated with its own class, and where there is no physical separation the higher-hazard design is carried into the adjoining area.
- Machinery using combustible hydraulic fluid moves an OH2 plastics fabrication area up to EH1.
- Aircraft hangars governed by NFPA 409, special occupancies and storage areas follow their own standards and chapters, not these curves.
- Area adjustments for high ceilings, sloped roofs or dry systems must follow the edition accepted by the authority having jurisdiction.
Frequently Asked Questions
What NFPA 13 hazard class is an office building?
Offices, including data processing areas, are listed as Light Hazard (LH): 0.10 gpm/ft² over 1500 ft² (≈ 4.1 mm/min over 139 m²), a 100 gpm hose allowance and 30 minutes. Archives, storage rooms or kitchens within the office should be assessed separately.
What is the difference between OH1 and OH2?
Both are ordinary hazard, but OH2 has more combustible content and higher stockpiles. At the start of the curve OH1 needs 0.15 gpm/ft² and OH2 0.20 gpm/ft²; over 1500 ft² that is 225 gpm against 300 gpm of sprinkler demand. The hose allowance (250 gpm) and duration (60–90 min) are the same.
Which point on the curve should I choose?
Any point on the curve complies. A larger area allows a lower density, but because the area grows faster the total flow increases. In most projects the start of the curve (1500 ft² for LH/OH, 2500 ft² for EH) gives the most economical water demand; high-ceiling or dry-system adjustments then enlarge the area.
What if my occupancy is not on the list?
Pick the closest analogue and compare fire load, flammable liquids and stockpile height. If in doubt, use the next class up and record the reasoning for the authority having jurisdiction and the insurer. For storage above 3.7 m (12 ft), move to the NFPA 13 storage chapters.
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