What is the occupancy factor used by the Insurance Services Office (ISO) for computing required fire flow for C-3 (Combustible)?

The Insurance Services Office (ISO) method for calculating Needed Fire Flow (NFF) uses an Occupancy Hazard Factor (Oi) to adjust the calculation based on the combustibility of the building's contents. The FSRS categorizes occupancies from C-1 (Noncombustible) to C-5 (Rapid-Burning). A C-3 (Combustible) classification represents occupancies with contents that burn with moderate rapidity, such as department stores, paper manufacturing, and woodworking shops. For this specific classification, the ISO Fire Suppression Rating Schedule (FSRS) assigns an occupancy factor of 0.85. This factor is multiplied by other variables in the NFF formula to determine the water supply required for firefighting operations.

A. 0.75: This is the occupancy factor for a C-2 (Limited-Combustible) classification, which represents a lower hazard level.

C. 1.00: This is the occupancy factor for a C-4 (Free-Burning) classification, which represents a higher hazard level.

D. 1.15: This is the occupancy factor for a C-5 (Rapid-Burning) classification, representing a significantly higher hazard level.

1. National Fire Protection Association (NFPA). (2008). Fire Protection Handbook (20th ed., Vol. 2). Section 18, Chapter 4, "Water Supply Requirements for Fire Protection," Table 18.4.1 "Occupancy Hazard Classification Factors," p. 18-66.

2. Insurance Services Office (ISO). (2012). Fire Suppression Rating Schedule (FSRS). Item 530, "Needed Fire Flow," Part B, "Occupancy Hazard Factor (Oi)."

3. Oklahoma State University, College of Engineering, Architecture and Technology. (n.d.). FST 3433: Fire Protection Hydraulics and Water Supply Analysis. Course materials on Needed Fire Flow Calculations, detailing the ISO formula and Occupancy Hazard Factors. (This courseware directly references and teaches the FSRS methodology).