"Standpipe systems are an important fire fighting tool for manual suppression. They must deliver a minimum flow at a specific pressure so fire fighters can have confidence their nozzles will operate properly and there is enough water to control and extinguish a fire. Where higher than normal operating pressures (175 psi or 12 bar) are expected, standpipe systems may be equipped with pressure regulating devices. Class I and III standpipe systems that have pressure regulating device on the hose outlets must have a means for conducting flow tests every five years to verify the pressure regulating device is operating properly and the minimum required flows are available. According to the National Fire Protection Association (NFPA) 14, 'Standard for the Installation of Standpipes and Hose Systems', the drain must have a 3-inch (76 mm) minimum inside diameter to accommodate the required flow. At every other floor level or intermediate landing where the hose outlet is located, the drain should have a 3-in. x 2-1/2-in. (76 mm x 65 mm) tee with a female-threaded, swivel fitting having National Hose Standard threads and a plug."
Coffee Break Training - Fire Protection Series No. 2008-53
United States Fire Administration: http://www.usfa.fema.gov/