How to Calculate Diesel Fire Pump Fuel Tank Capacity (As per NFPA 20-2025 Edition) Many fire safety professionals ask: "How much diesel fuel is required for a standby fire pump?" NFPA 20 gives a clear and simple answer. 1 Standard Rule - When Fuel Consumption Data Is Available NFPA 20-Clause 11.4.1.3.1 The diesel fuel tank must be sized for: Minimum 12 hours of continuous engine operation Based on the engine's hourly fuel consumption Add: 5% for fuel expansion, 5% for sump (unusable fuel) Simple