A combined standpipe must have a minimum size pipe of what?

The minimum size for a combined standpipe is set to ensure that it can adequately serve both fire department operations and building occupants during a fire emergency. A pipe size of 6 inches allows for sufficient water flow to meet the needs of firefighting efforts while also accommodating the demands of a building's sprinkler system if it is equipped with one.

This size is critical because it strikes a balance between capacity, pressure, and flow rate, which are all essential for effective fire suppression. Inadequately sized pipes can lead to insufficient water supply, which can endanger lives and property in a fire situation.

Standpipes are an integral part of fire protection systems, and the standards for their construction, including pipe sizing, are based on extensive research and testing from the National Fire Protection Association (NFPA) and other relevant bodies. The decision to specify a minimum size of 6 inches derives from the need to ensure that a combined standpipe system remains functional under high-demand conditions, providing effective and responsive fire protection.