What residual pressure value is required when computing available water supply for a specific area?

• A. 5 psi
• B. 10 psi
• C. 15 psi
• D. 20 psi

Answer: (D)

In hydraulic calculations for available water supply, the residual pressure is the pressure that must remain in the system at the most hydraulically remote point when the design fire flow is being drawn. Using a residual pressure of 20 psi provides a safety margin so that, after friction losses from pipes, fittings, and elevation, there’s still enough pressure to operate sprinklers, standpipes, and hose streams. Values like 5, 10, or 15 psi would leave too little pressure once losses are accounted for, increasing the risk of inadequate flow to the protected area. Therefore, 20 psi is the standard residual pressure used when computing available water supply.