Water Supply and Distribution
Sizing and pressure
Water supply questions test four things: sizing, pressure, protection, and testing. Sizing uses the water supply fixture unit (WSFU) method: total the WSFUs, apply the developed length and meter loss, then select the building supply and branch sizes from the tables. Branches are sized by the fixture units they serve, not by the size of the main.
Pressure arithmetic is your best friend. One psi of pressure equals about 2.31 feet of head, so every foot of rise costs about 0.434 psi. The code cares about flowing (residual) pressure at the most remote fixture, not static pressure in the basement. Static pressure above roughly 80 psi calls for a pressure-reducing valve.
Protection details
Keep velocities moderate (roughly 8–10 fps) to limit noise and erosion. Provide accessible shutoff valves, unions, and supports; protect against freezing; install water hammer arrestors near quick-closing valves; and remember that a closed system (check valve or PRV) needs thermal expansion control. Hose bibbs, boiler makeup, and irrigation connections need backflow protection. New or repaired potable piping is tested before concealment and disinfected per local requirements.
Worked example
Static pressure at the meter is 62 psi. A shower valve is 18 feet above the meter. Elevation loss = 18 × 0.434 ≈ 7.8 psi, leaving about 54 psi static at the valve - before friction loss through the meter and piping. If the flowing pressure falls below the minimum residual (about 15 psi), upsize the piping or reduce losses.