The required residual pressure when computing available water supply in a specific area is ______ psi.

• A. 10
• B. 20
• C. 40
• D. 60

Answer: (B)

When evaluating whether the available water supply is adequate for fire protection, you look at the residual pressure—what’s left in the system once water is flowing and losses from friction and elevation are accounted for. This remaining pressure must meet a minimum to ensure the protection devices can function properly. In typical available water supply calculations, a residual pressure of 20 psi is required at the most hydraulically remote point. This minimum provides enough pressure to deliver the required flow for the system, even after accounting for pressure losses. If the residual pressure were only 10 psi, the system may not achieve the needed flow; values like 40 or 60 psi are higher than the standard minimum and aren’t the baseline used for these calculations. Therefore, 20 psi is the standard minimum residual pressure used to verify an adequate available water supply.