Water Supply and Distribution: Pressure, Demand, and Sizing
What actually drives sizing
A water system runs service, meter, main, risers, and branches. Size it from fixture units converted to probable demand, never from the sum of every fixture flowing at once. Convert water-supply fixture units (WSFU) to gallons per minute using the code's conversion table or Hunter curve, and note that flushometer fixtures demand far more flow at higher residual pressure than tank-type fixtures. Then check velocity (roughly 5–8 fps; keep hot water at the lower end to limit erosion and noise) and check residual pressure at the most remote fixture.
Pressure you must be able to compute
Static pressure is no-flow; residual is what remains while water is moving; flow pressure is what the fixture sees. Available pressure must cover elevation (0.433 psi per foot of rise), meter loss, device losses, and pipe friction over the developed length - which includes fittings and valves, not just measured pipe. A pressure-reducing valve is required where supply pressure exceeds 80 psi; once a PRV, check valve, or backflow preventer creates a closed system, an expansion tank is required to absorb thermal expansion.
Common traps
Using building total fixture units when the question gives only one branch; forgetting elevation loss; sizing a flushometer branch off tank-type figures; omitting the PRV on an 85 psi service; and sizing on velocity alone without confirming terminal pressure.
Worked example
Meter delivers 60 psi static. The highest fixture is 30 ft above the meter. Elevation loss = 30 × 0.433 = 13 psi, leaving 47 psi static at the top before any flow loss. Add friction and meter losses, and confirm the fixture still sees its required flow pressure - typically about 8 psi minimum, more for flushometer valves.