NFPA 13 Design Criteria, Occupancy Classification, and System Types
This domain tests whether you can turn a building's use and contents into a defensible NFPA 13 design: the hazard class, the density/area point, the hose allowance, and the system type that fits the space.
Hazard classification drives everything
NFPA 13 groups occupancies into light hazard (LH), ordinary hazard Group 1 and Group 2 (OH1, OH2), and extra hazard Group 1 and Group 2 (EH1, EH2). The baseline density/area points are:
- LH: 0.10 gpm/ft² over 1,500 ft²
- OH1: 0.15 gpm/ft² over 1,500 ft²
- OH2: 0.20 gpm/ft² over 1,500 ft²
- EH1: 0.30 gpm/ft² over 2,500 ft²
- EH2: 0.40 gpm/ft² over 2,500 ft²
Hose stream allowance is added at the base of the riser, not at the remote sprinkler: typically 100 gpm for LH and 250 gpm for OH and EH. Do not add it inside the hydraulic calculation of the remote area.
System types
A wet-pipe system is the default where piping is above freezing. A dry-pipe system uses air or nitrogen and must deliver water to the test outlet within 60 seconds; systems that cannot meet that limit need a quick-opening device. Preaction systems protect water-sensitive areas: single-interlock releases on detection alone, double-interlock requires both detection and a sprinkler-operated pressure switch, and non-interlock releases on either. Deluge systems use open sprinklers and release all at once on detection, typically for special hazards.
Worked example
A mercantile occupancy is classified OH2. The density is 0.20 gpm/ft² over 1,500 ft², so the sprinkler demand is 0.20 × 1,500 = 300 gpm. Add the 250 gpm OH hose allowance at the riser: the water supply must prove 550 gpm at the required residual pressure. The remote-area calculation proves 300 gpm; the supply analysis proves 550 gpm.
Traps that cost marks
Classifying a storage area as OH2 when commodity and storage height push it into a storage chapter. Adding hose allowance at the last sprinkler. Using EH density with a 1,500 ft² area. Choosing a dry-pipe system for a heated building, or forgetting that dry-pipe demand gets a 30 percent area increase in the calculation.