Quantity of water supplied must be adequate for the needs of occupants and processes to be carried out in the building. The total water demand may be calculated by adding the maximum flows at all points of use and applying a factor less than unity to account for the probability that only some of the fixtures will be operated simultaneously (Art. 14.8).
In addition, the pressure at which water is delivered to a building must lie within acceptable limits. Otherwise, low pressures may have to be increased by pumps and high pressures decreased with pressure-reducing valves. Table 14.1 lists minimum flow rates and pressures generally required at various water outlets. The pressure in Table 14.1 is the pressure in the supply pipe near the water outlet while the outlet is wide open and water is flowing.
In delivery of water to the outlets, there is a pressure drop in the distribution pipes because of friction. Therefore, water supplied at the entrance to the distribution system must exceed the minimum pressures required at the water outlets by the amount of the pressure loss in the system. But the entrance pressure should not exceed 80 psi, to prevent excessive flow and damage to system components. Velocity of water in the distribution system should not exceed 10 ft / s.
A separate supply of water must be provided for fire-fighting purposes. This supply must be of the most reliable type obtainable. Usually, this requirement can be met with water from a municipal water supply. If the municipal water supply is not adequate or if a private water supply is utilized, pumps or storage in an elevated water tank should be provided to supply water at sufficient quantities and pressures.
Generally, such water should be provided at a pressure of at least 15 psi residual pressure at the highest level of fire-sprinkler protection for light-hazard occupancies and 20 psi residual pressure for ordinary-hazard occupancies. Acceptable flow at the base of the supply riser is 500 to 700 gpm for 30 to 60 minutes for light-hazard occupancies and 850 to 1500 gpm for 60 to 90 minutes for ordinary-hazard occupancies.
If a building is so located that it cannot be reached by a fire department with about 250 ft of hose, a private underground water system, installed in accordance with NFPA 24, Installation of Private Fire Service Mains and Their Appurtenances, may have to be provided. Many municipalities require that the water system for a building site be a type generally called a loop-to-grid system. It consists of pipes that loop around the property and has a minimum of two municipal-watersystem connections, at opposite sides of the loop, usually at different water mains of the municipal system. Hydrants should be placed so that all sides of a building can be reached with fire hoses. The requirements for fire hydrants should be verified with the local code officials or fire marshal.
