Residual pressure used in available water supply calculations is typically expressed in psi as

• A. 10
• B. 20
• C. 25
• D. 50

Answer: (B)

Residual pressure is the pressure that remains at the most remote point in the system while water is flowing. It serves as a minimum pressure baseline used in available water supply calculations to ensure there is enough pressure to push water through the farthest parts of the piping network, despite friction losses, valves, fittings, and elevation changes.

The typical value used is 20 psi because it provides a conservative, practical minimum that keeps the remote point from dropping to unusable pressure, without overestimating the system’s capability. Ten psi would be too low for reliable fire streams, while fifty psi would overstate what most municipal systems can consistently deliver. Twenty psi strikes a standard balance and is the common default used in general calculations.