Fire flow is the rate of water supply, measured in gallons per minute (GPM), that must be available at fire hydrants to suppress a fire in a building on your site. It is one of the first things the fire department reviews, and if the existing water system cannot deliver the required fire flow, your project cannot proceed until the water infrastructure is upgraded — at your expense.

Where the Requirement Comes From

Fire flow requirements are established by the International Fire Code (IFC), which most jurisdictions adopt with local amendments. In California, the California Fire Code (CFC) is the IFC as amended by the California Building Standards Commission. The fire code provides two methods for determining required fire flow:

  • Appendix B — fire flow requirements based on building construction type and area
  • Appendix C — fire flow requirements for one- and two-family dwellings based on building area and fire department access distance

The local fire authority having jurisdiction (AHJ) determines which appendix applies and may impose additional requirements.

Appendix B: Commercial and Multi-Family

Appendix B of the CFC/IFC establishes fire flow based on two factors:

  1. Building area (square feet) — the total floor area of the building, including all stories
  2. Construction type — Type IA (most fire-resistant) through Type VB (least fire-resistant), as defined by the building code

The required fire flow is determined from Table B105.1(2), which provides values ranging from 1,500 GPM for small fire-resistive buildings to 8,000 GPM for large unprotected buildings. Typical ranges for common project types:

Building TypeTypical Fire FlowDuration
Type IA/IB office, up to 22,700 SF1,500 GPM2 hours
Type IIA retail, 12,001-22,700 SF1,750 GPM2 hours
Type VA apartments, 12,001-22,700 SF2,250 GPM3 hours
Type VB commercial, 22,701-30,200 SF3,000 GPM3 hours
Large warehouse, Type IIB, 50,000+ SF4,000-6,000 GPM4 hours

Reductions

The fire flow can be reduced by up to 75 percent (but not below 1,500 GPM) if the building is equipped with an approved automatic fire sprinkler system (NFPA 13 or 13R). For a Type VA apartment building with a base fire flow of 2,250 GPM, the 75 percent reduction yields 563 GPM, but the 1,500 GPM minimum applies, so the required fire flow is 1,500 GPM.

The sprinkler exception is huge. Without sprinklers, a large commercial building might require 4,000 GPM. With sprinklers, the requirement drops to 1,500 GPM. This difference determines whether the existing water main can serve your project or whether you need a main upsizing that costs $200,000+. Always verify the sprinkler reduction applicability with the fire AHJ before finalizing the water system design.

Appendix C: One- and Two-Family Dwellings

Appendix C provides simplified fire flow requirements for single-family and duplex residences based on building area:

Building Area (SF)Required Fire FlowDuration
Up to 3,6001,000 GPM1 hour
3,601 to 4,8001,500 GPM1.5 hours
4,801 to 6,2001,750 GPM2 hours
6,201 to 8,4002,000 GPM2 hours

Many jurisdictions limit the residential fire flow reduction for sprinklered homes to 50 percent rather than the 75 percent allowed under Appendix B.

How Fire Flow Is Delivered

Fire flow must be available at fire hydrants located within a specified distance of the building. The water system must deliver the required fire flow at a minimum residual pressure of 20 psi at the hydrant. This means the water main, service connections, and distribution network must have adequate capacity to deliver the flow rate while maintaining pressure.

The water district (EBMUD, CCWD, DSRSD, or the equivalent in your jurisdiction) conducts a fire flow test to determine the available flow at the nearest hydrant. The test measures the static pressure (no flow) and the residual pressure at a specific flow rate, and then extrapolates to determine the maximum available flow at 20 psi residual.

If the available fire flow is less than the required fire flow, the water system must be improved. This typically means upsizing the water main from 6 inches to 8 inches, or from 8 inches to 12 inches, along the project frontage and potentially for several blocks upstream. The cost of this improvement is borne by the developer.

Hydrant Spacing

The fire code specifies maximum distances between hydrants, based on the required fire flow:

  • 1,750 GPM or less: Hydrants within 600 feet of all portions of the building (measured along a driving route)
  • 2,000-2,250 GPM: 450 feet
  • 2,500+ GPM: 300 to 450 feet, depending on the AHJ

These distances are measured from the building, not from the hydrant. The hydrant must also be located on a fire apparatus access road and be accessible to the fire engine (no fences, bollards, or parked cars blocking access). The minimum distance from the hydrant to the building is typically 40 to 50 feet (to keep the hydrant outside the building collapse zone).

Common Issues

  • Insufficient existing fire flow. Older neighborhoods with 6-inch water mains often cannot deliver 1,500 GPM. The developer must fund main upsizing. Get the fire flow test result from the water district early in design.
  • Dead-end mains. A water main that dead-ends at your site provides less flow and pressure than a looped main. The fire department may require the main to be extended and looped as a condition of approval.
  • High-elevation sites. Sites at higher elevations in the water system's pressure zone may have inadequate static pressure. If the static pressure at the hydrant is less than 40 psi, the available fire flow may be insufficient even if the main is adequately sized.
  • Fire flow vs. sprinkler demand. These are separate calculations. Fire flow is the water supply at the hydrant for fire department operations. Sprinkler demand is the water supply inside the building for the automatic sprinkler system. Both must be available simultaneously. The fire code requires the water system to deliver both the sprinkler demand and the hose stream allowance (the outside fire flow that the fire department uses in addition to the sprinklers).