Water-Supply Evaluations

Forty percent of your community's Public Protection Classification (PPCTM) grading is based on your water supply. ISO looks at whether you have sufficient water for fire suppression beyond your community's daily maximum consumption for other purposes.

Minimum criteria
For your community to be eligible for a PPC of Class 8 or better, your water-supply system must be able to deliver at least 250 gpm for a period of two hours. That flow is in addition to the community's maximum daily rate of consumption for purposes other than firefighting.

If the fire department delivers the 250 gpm through tanker shuttle, large-diameter hose, or other alternative water supply, the water must be available within five minutes of the arrival of the first-due apparatus, and the department must maintain the flow, without interruption, for the two-hour duration.

Class 8B
If a community has superior fire-protection services and fire alarm facilities, but lacks the water supply required for a PPC of Class 8 or better, the community may be eligible for a Class 8B rating. For more information, see Minimum Criteria for Class 8B.

Evaluation of systems that exceed the minimum requirement
If your water supply meets the 250-gpm minimum, ISO will evaluate your system — together with factors relating to your fire department and your fire alarm and communications systems — to determine whether you qualify for a PPC of Class 8 or better.

Within the Fire Suppression Rating Schedule (FSRS), a section called "Needed Fire Flow" outlines the methodology for determining the amount of water necessary for providing fire protection at representative locations throughout the community. To evaluate your community's water supply, ISO calculates the needed fire flow for selected locations. We then determine the water-flow capabilities at those locations and calculate a ratio considering the need (needed fire flow) and the availability (water-flow capability). We use that ratio in calculating the credit points identified in the FSRS.

ISO calculates the needed fire flow for an individual building based on the building's area, construction, occupancy, and exposure. To get full credit, the water supply must be able to deliver water, at 20 psi residual pressure and at the specified rate of flow, for a specified period of time. Needed fire flows for individual buildings range from a minimum of 500 gpm to a maximum of 12,000 gpm. For more information, see the Guide for Determination of Needed Fire Flow.

Residential areas
For residential areas with one- and two-family dwellings, ISO determines the needed fire flows by considering the distance between buildings:

Buildings not considered in the community's PPC
For purposes of calculating your community's PPC, ISO does not normally consider the needed fire flow at certain high-demand properties. Those properties include:

• buildings graded and coded by ISO as protected by an automatic sprinkler system meeting applicable National Fire Protection Association standards
• buildings with a needed fire flow in excess of 3,500 gpm

ISO individually grades the protection of buildings with a needed fire flow in excess of 3,500 gpm, and their PPC can differ from that of the community or district that provides their fire protection.

Evaluation method
ISO analyzes three primary factors in the evaluation of the water supply at each representative site in your community:

• capacity of the supply works
• capacity of the water mains or distribution system
• distribution of hydrants

We analyze each of those factors according to applicable standards of the National Fire Protection Association (NFPA) and the American Water Works Association (AWWA).

We determine the capacity of the supply works for each service (pressure) zone through an analysis of such factors as water source (wells, springs, impoundments, stream-flow diversions, and the like); treatment facilities; transmission facilities (suction storage, booster pumps, transmission mains, pressure-reducing valves, and the like); and gravity storage. We calculate the flow — sustainable for the needed duration — from both suction storage tanks and gravity storage tanks, based on the capacity of the tanks at their average daily minimum levels.

We determine supply-works capacity, in gpm, for a duration of 2 hours for needed fire flows less than 3,000 gpm; for a duration of 3 hours for needed fire flows of 3,000 gpm to 3,500 gpm; and for a duration of 4 hours for needed fire flows in excess of 3,500 gpm. We credit only supply-works capacity in excess of the maximum daily consumption rate, based on records for the last 3 years.

We determine capacity of the water mains or distribution system by observing actual hydrant flow tests at each representative location.

Those tests produce results calculated in gpm at a residual pressure of 20 psi. Under certain circumstances, ISO may accept tests witnessed by outside agencies, if conducted according to industry-accepted practices, or flows determined from a valid and balanced computer model.

ISO evaluates hydrant distribution by examining the number and type of hydrants within 1,000 feet of each representative building. We also look at the distance from each such hydrant to the subject building, measured as apparatus can lay hose.

Hydrants with at least one large pumper outlet may receive credit for up to 1,000 gpm. Hydrants with at least two hose outlets, but no pumper outlet, may receive credit for up to 750 gpm. And hydrants with only one hose outlet may receive credit for up to 500 gpm.

Hydrants within 300 feet of the subject building may receive credit for up to 1,000 gpm (but not more than the credit that would apply based on the number and type of outlets). Hydrants from 301 feet to 600 feet from the subject building may receive credit for up to 670 gpm (but not more than the credit that would apply based on the number and type of outlets). And hydrants from 601 feet to 1,000 feet from the subject building receive credit for 250 gpm. Under certain circumstances, when all fire department pumpers carry sufficient large-diameter hose, ISO may allow maximum credit for hydrants up to 1,000 feet from the subject building.

For each representative location, ISO credits the least of the rates of flow determined for supply-works capacity, water-main or distribution-system capacity, or hydrant distribution. (The credit rate of flow cannot exceed the needed fire flow for that location.)

To determine the grading points that go into the calculation of your community's PPC, ISO considers the percentage of the needed fire flows that the credited rates of flow can provide. You receive that percentage of the available grading points. For example, if your community's credited flow could supply 75 percent of the needed fire flows at the representative locations, you would receive 75 percent of the available credit points.

In addition, ISO evaluates the type and installation of hydrants and/or suction points throughout your community or district, along with any program of hydrant inspections and maintenance. You receive additional credit points based on that evaluation.

For more information . . .
. . . on any topic related to the PPC program or the Fire Suppression Rating Schedule, click Talk to ISO Mitigation, or call the ISO mitigation specialists at 1-800-444-4554.