"Fire pumps driven by diesel engines require an adequate fuel supply to perform regular weekly tests and maintain operational integrity during a fire. NFPA [National Fire Protection Association] 20, 'Standard for the Installation of Stationary Pumps for Fire Protection' requires a minimum of 1 gallon of fuel for each engine horsepower (HP). The fuel requirement is based on the premise that the engine will burn about 1 pint per horsepower per hour of run time, and the pump should have a total standby duration of 8 hours when the tank is full. The pump then can be tested for 12 weeks (6 hours of run time) and still have enough fuel to power the pump to meet the 2-hour requirements of a fire protection system."
Coffee Break Training - Fire Protection Series No. FP-2006-14
United States Fire Administration: http://www.usfa.fema.gov/