Calculating the Required Fire Flow
Providing water to the responding fire department is a crucial aspect of the overall fire protection and life safety strategy of an entire community. When a new building is developed or an existing building is renovated, it is important to make sure that the proper amount of water is available to the responding fire department to allow for both suppression of the fire in the building, and protection of any exposed buildings. Because of this, NFPA 1, The Fire Code, requires a minimum amount of water be provided based on the type of construction of the building as well as fire flow area.
Fire flow is defined as the flow rate of a water supply, measured at 20 psi (137.9 kPa) residual pressure, that is available for the responding fire department for manual firefighting, typically this is water that is available at the surrounding fire hydrants, but it can be supplied with another approved source such as a static water supply like a tank or pond, or even using a fire department tanker shuttle service.
In addition to using the required fire flow water supply for manual suppression of the fire with hose lines, when responding to a fire at a sprinklered building or a building that contains a standpipe system, the fire department will also connect their pumper up to these systems through a fire department connection to use their pumper and the available fire flow water supply to supplement the water supply of these systems.
There is a difference between the required fire flow in NFPA 1 and the hose stream allowance that is required in NFPA 13, Standard for the Installation of Sprinkler Systems, to be added to the required sprinkler demand. The fire flow required by NFPA 1 is being provided for the fire department for both protection of exposures as well as the water required for manual suppression, while the hose stream allowance in NFPA 13 that is added to a sprinkler system demand is adding a safety factor to the calculations to account for the fact that the fire department will likely also be using water from the same water supply that the sprinkler system is being fed from, which will reduce the available water.
Because they are separate requirements that are trying to accomplish different goals, the fire flow, per NFPA 1, is not required to be added to the demand of an automatic sprinkler system in sprinklered buildings. However, the available water supply must be the greater of the two, either the sprinkler system demand, or the required fire flow. If the available water supply can support the greatest demand, it will also be able to support the other demand as well.
NFPA 1, provides requirements for fire flow in Section 18.4. The requirements are performance-based, which means they do not specify the type of system necessary to provide the required fire flow. The AHJ (typically the responding fire department) has the final authority to determine if the proposed water supply delivery method is appropriate.
Fire flow is calculated based on the fire flow area of the building. The flow area is the total floor area of all floor levels of a building, except for Type I (443), Type I (332), and Type II (222), in which case the fire flow area is the largest three successive floors. The fire flow area should be determined based on the area between the surrounding exterior walls of each floor and the fire separation walls used to create separate buildings.
Table 18.4.5.2.1 lists the minimum required fire flow and flow duration for buildings based on fire flow area and construction type. For more information on construction types, take a look at this blog. For example, a Type I (443) building with a fire area in the range of 0-22,700 ft2 (0-2108.83 m2) is required to provide the fire department with a fire flow of 1500 gpm (5677.5 L/min) for a flow duration of 2 hours (see below).
Paragraph 18.4.5.3.2 states that the required fire flow for buildings other than one and two family dwellings can be reduced by 75 percent when the building is protected by an approved automatic sprinkler system. However, the resulting fire flow cannot be less than 1000 gpm (3785 L/min) or 600 gpm (2270 L/min) where quick response sprinklers are used throughout. Sprinklered one- and two -family dwellings fire flows can be reduced by 75 percent with no minimum and the duration decreased to one hour.
Here is an example video stepping through the calculation of the required fire flow taken from our Certified Fire Plans Examiner Learning Path.
As mentioned, hydrants are the primary method for providing a fire flow. NFPA 1 requires that the flow capacity of all fire hydrants within 1000 ft (305 m) of a building not be less than the required fire flow. The distance should be measured as the fire apparatus would lay hose out on the fire department access road to the building. The distance should not be measured across adjacent lots or through fences, gates, or other obstructions that would prevent the normal movement of a fire apparatus performing a hose lay to a fire hydrant. Table 18.5.4.3 of NFPA 1 specifies the maximum capacity that each hydrant can be credited for when calculating the total available fire flow, based on the distance of the hydrant from the building.
Fire Hydrants must be installed to meet the requirements of NFPA 1, waterworks standards, and any local requirements of the jurisdiction. Where required by the Authority Having Jurisdiction (AHJ), the hydrant needs to be provided with a reflector and proximity flag. In some jurisdictions, the hydrants are also color-coded to indicate the available flow rate. Fire hydrants need to be located within 600 feet (183 m) from the closest point of the building in detached one- and two-family dwellings, with a maximum spacing of 800 feet (244 m). For buildings other than one- and two-family dwelling, hydrants need to be within 400 feet (122 m) of the building with a maximum spacing of 500 feet (152 m). Additionally, hydrants must also be located within 12 feet (3.7 m) of the fire department access road.
If you are interested in learning more about fire plans review, or want to become a Certified Fire Plans Examiner, take a look at our learning path explorer module, which provides some sample lessons.
