Water Supply and Distribution
Water supply principles
Water supply design balances pressure, flow, and velocity. The service must provide enough residual pressure at the most remote fixture; the code sets minimum pressures for fixtures and flushometer valves, and excessive pressure requires a regulator. Pipe sizing uses water supply fixture units (WSFU), developed length, and available pressure loss. Velocity is normally kept below about 8 feet per second in small copper and plastic lines to limit noise and erosion. Valves must be accessible, and hotels and similar occupancies need shutoffs for groups of fixtures.
Backflow and cross-connections
A cross-connection is any actual or potential link between potable water and a contaminant. Protection must match the hazard: an air gap for severe hazards, a reduced-pressure zone assembly for high hazards, and a double-check or vacuum breaker for lower hazards. Hose bibbs need vacuum breakers; boiler feed lines, irrigation, and chemical dispensers require approved protection. Backflow assemblies must be tested and maintained.
Worked example
A meter supplies 40 psi. A 20-foot vertical rise to a second-floor fixture consumes 20 / 2.31 = 8.7 psi, leaving about 31.3 psi before friction losses. If the fixture needs 8 psi residual and the pipe and fittings lose 20 psi, the design has only 11.3 psi available and is marginal; increasing pipe size reduces friction loss.
Traps
Do not size water pipe by the fixture count alone without using the WSFU and pressure tables. Do not install a vacuum breaker downstream of a shutoff where it can be isolated. Do not use a double-check assembly for a high-hazard connection.