When the fire marshal refers to "Appendix B" or "Appendix C" fire flow, they are referencing specific appendices of the International Fire Code (IFC) or California Fire Code (CFC) that establish fire flow requirements. The distinction is straightforward but critical: Appendix B applies to commercial and multifamily buildings, Appendix C applies to one- and two-family dwellings. Using the wrong appendix produces the wrong fire flow, the wrong hydrant spacing, and potentially an undersized water system.

Appendix B: Commercial and Multifamily

Appendix B (CFC/IFC Section B105) establishes required fire flow based on:

1. The building's construction type (Type IA through VB per the building code)
2. The building's total floor area (including all stories)

The higher the building area and the less fire-resistive the construction, the higher the required fire flow. The range is 1,500 GPM to 8,000 GPM.

Sprinkler reduction: For buildings with an approved NFPA 13 automatic sprinkler system, the fire flow can be reduced by up to 75 percent, with a minimum of 1,500 GPM. For NFPA 13R systems (common in multifamily residential), the reduction may be limited to 50 percent depending on the AHJ.

Duration: The required fire flow must be sustained for 2 to 4 hours, depending on the fire flow quantity. Higher flows require longer durations.

Appendix C: One- and Two-Family Dwellings

Appendix C (CFC/IFC Section C103) provides a simplified table based solely on building area:

Sprinkler reduction: For NFPA 13D sprinkler systems (the residential system), many AHJs allow a 50 percent reduction, with a minimum of 500 to 1,000 GPM depending on local amendments.

Which One Applies to Your Project?

• Single-family home or duplex — Appendix C
• Townhouse project (3+ attached units) — Appendix B. Townhouses are not "one- and two-family dwellings" under the fire code, even if each unit has its own entrance.
• Apartment building (3+ units) — Appendix B
• Mixed-use building (retail + residential) — Appendix B, using the total building area including all uses
• Commercial (office, retail, industrial) — Appendix B
• ADU (accessory dwelling unit) — Appendix C, based on the total area of the primary dwelling plus the ADU (or the ADU alone, depending on the AHJ's interpretation)

How This Affects the Water System

The required fire flow directly determines:

• Water main size: A 6-inch main can deliver approximately 1,000-1,500 GPM depending on pressure and length. Flows above 1,500 GPM typically require an 8-inch main. Flows above 3,000 GPM may require a 12-inch main.
• Hydrant spacing: Higher fire flow = closer hydrant spacing, as described in the hydrant spacing article.
• On-site water storage: In rural areas where the water system cannot deliver the required fire flow, an on-site fire water storage tank may be required. The tank volume must equal the required fire flow multiplied by the required duration (e.g., 1,500 GPM x 2 hours = 180,000 gallons).

Local Amendments

Many fire districts adopt local amendments that modify the fire code appendices. Common amendments include:

• Increasing the minimum fire flow above 1,500 GPM (some districts set a blanket minimum of 2,000 or 2,500 GPM regardless of building type)
• Limiting the sprinkler reduction to 50 percent instead of 75 percent
• Requiring a minimum residual pressure of 20 psi at the design fire flow (standard) or 40 psi (some districts)
• Adding hydrant spacing requirements for specific occupancy types (schools, hospitals, hazardous materials facilities)

Always check the local fire district's adopted code and amendments before calculating fire flow. The IFC/CFC provides the baseline, but the AHJ's amendments may increase the requirements significantly.