Cal. Health & Saf. Code § 13025.5
Any fire department maintained by the City and County of San Francisco using fire hydrant outlets with other than two-and-one-half-inch (2 1/2-inch) threaded fittings shall cause any vehicle used for firefighting purposes and designed to pump water from those hydrants, that is normally used in areas of the city and county bordering the boundaries of any other public entity, as defined in Section 13050.1, providing any fire protection and suppression service, to carry a minimum of eight adapters, consisting of four increasers and four reducers, that enable the vehicle to couple its equipment and apparatus to fire hydrant outlets having two-and-one-half-inch (2 1/2-inch) threaded fittings, and that enable fire equipment vehicles from other public entities using two-and-one-half-inch (2 1/2-inch) threaded fittings to couple their firefighting equipment and apparatus to fire hydrant outlets maintained by the city and county.
Ca. Health and Saf. Code § 13025.5