How do you size a diesel fire pump fuel tank?
A diesel-driven fire pump is chosen so that water supply survives a mains power failure. That promise only holds if enough fuel sits next to the engine to run the pump through the fire and beyond. The two main standards define the quantity in different ways: NFPA 20 uses a fixed ratio to engine power, while EN 12845 sets a full-load running time that depends on the hazard class.
NFPA 20: one gallon per horsepower
Under NFPA 20 the fuel tank must hold at least 1 US gallon for every horsepower of the engine. On top of that, 5% is added for expansion and 5% for the sump, where sediment and water collect at the bottom of the tank. In SI units 1 gal/hp is 3.7854 L ÷ 0.7457 kW = 5.076 L/kW.
Vtank = P [kW] × 5.076 L/kW × 1.10Example: a 100 kW (134 hp) engine needs a base volume of 507.6 L, or 558 L (147.5 gal) with the expansion and sump allowances; the next standard tank is 750 L. The rule is generous: at a typical 0.25 L/kWh, 558 L runs the engine at full load for about 22 hours.
EN 12845: full-load running time
EN 12845 defines fuel by how long the engine must run continuously at full load. The duration depends on the hazard class of the installation:
| Hazard class | Full-load running time | Fuel for 100 kW · 0.25 L/kWh |
|---|---|---|
| LH (light) | 3 hours | 75 L |
| OH (ordinary) | 4 hours | 100 L |
| HHP / HHS (high) | 6 hours | 150 L |
Specific fuel consumption (SFC) varies by engine. Modern fire pump diesels typically burn 200–230 g/kWh at full load; with diesel at 0.83–0.85 kg/L that is roughly 0.25 L/kWh. The calculator uses this as an assumption — always replace it with the full-load consumption from the engine manufacturer's data sheet. Because EN 12845 defines usable fuel, add an allowance for the dead volume below the outlet and the ullage above; 10% is a common choice on site.
Design note: if the building may later become a storage occupancy (HHS), sizing the tank for 6 hours from day one is far cheaper than replacing it. For the same engine the NFPA 20 rule usually gives several times the EN 12845 volume; where both standards apply, the larger governs.
Tank position and fittings
- Gravity feed: the tank outlet must be above the engine's fuel injection or lift pump, so no air lock forms in the fuel line and the engine picks up fuel on every start attempt.
- Not above the engine: EN 12845 does not allow the tank directly over the engine, because leaks would drip onto a hot surface.
- Dedicated tank and line: each engine needs its own tank and fuel line; do not share the tank with a generator or boiler.
- Vent and filling: run the vent pipe to outside the building and provide fill and overflow connections plus a level gauge. Do not use soldered joints in the fuel line.
- Double wall and bunding: whether the tank needs a double wall or a bund depends on local environmental and fire regulations; confirm with the authority having jurisdiction.
Choosing a standard tank
The calculated volume is rounded up to the common market sizes of 200, 300, 500, 750, 1000, 1500, 2000 and 3000 litres. The level gauge in the calculator shows what percentage of that standard tank the requirement fills; above 95% the next size up is advisable. For requirements over 3000 L, consider a purpose-built tank or a multiple engine and tank arrangement.
Fuel may sit in the tank for a long time, so quality matters too: do not miss the seasonal switch between summer and winter diesel, drain water from the tank bottom regularly, and check the level at every weekly run test.
Frequently Asked Questions
How is a diesel fuel tank sized to NFPA 20?
Allow 1 US gallon per engine horsepower (5.076 L/kW), then add 5% for expansion and 5% for the sump. A 100 kW (134 hp) engine needs 507.6 L × 1.10 ≈ 558 L (147.5 gal).
How many hours of fuel does EN 12845 require?
Full-load running time by hazard class: 3 hours for LH, 4 hours for OH and 6 hours for HHP and HHS. Fuel = engine power × specific consumption × time; for 100 kW at 0.25 L/kWh that is 75 L for LH and 100 L for OH.
What specific fuel consumption should I use?
Use the full-load consumption from the engine manufacturer's data sheet. Without it, 0.25 L/kWh (about 210 g/kWh) is a reasonable starting point for preliminary sizing.
Where should the fuel tank be located?
The tank outlet must be above the engine's fuel lift or injection pump so fuel arrives by gravity. Do not place the tank directly above the engine, run the vent outside, and give each engine its own tank.
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