Guidance for the sizing diesel fuel tanks is quite straight forward due to the prescriptive requirements of the code. Just take your engine HP x 1.10 and the result in gallons is the minimum required diesel fuel storage tank size. The exact code reference from NFPA 20 (2010 edition) is provided below:

How the committee arrived at these simplified guidelines is as follows. First, lets look at the conditions for when we expect the fire pump to run:

11.4.2* Fuel Supply Tank and Capacity.

11.4.2.1* Fuel supply tank(s) shall have a capacity at least

equal to 1 gal per hp (5.07 L per kW), plus 5 percent volume

for expansion and 5 percent volume for sump.

A.11.4.2 The quantity 1 gal per hp (5.07 L per kW) is equivalent

to 1 pint per hp (0.634 L per kW) per hour for 8 hours.

Where prompt replenishment of fuel supply is unlikely, a reserve

supply should be provided along with facilities for transfer

to the main tanks.

- Quarterly Refilling of the Fuel Tank (approx 12-weeks)
- A weekly test run for 30 Minutes
- A minimum run time of 2-hour (or 4-hours during a fire depending upon your needs)

Multiply this out and you get basically

**8-hours of continuous run time**depending upon your run time during a fire. Take the NFPA 20 appendix guidance of 1 pint/hr/HP (0.125 gallons/hr/HP) x 8 hours and you get 1 Gallon per horse-power.
Lets compare this to the actual published data for a specific diesel engine. Take the smallest diesel engine Cummins makes a CFP5E-F10 which produces 95HP at 1760 RPM. The published fuel rate is 4.9 Gal/hr (18.5 L/hr). 4.9 Gallons/hr x 8 hours x 1.10 (sump/expansion) = 43 gallons minimum. If we use NFPA 20 guidance we would get 95 HP x 1 Gal/HP x 1.10 = 104.5 gallons minimum. As you can see the for this specific example NFPA 20 is much more conservative.

The other item you need to verify is that the fuel tank complies with UL 142 as required by NFPA 20 (2010 edition) paragraph 11.4.1.2.1. Fuel tank sizes are limited to 1320 gallons and the standard sizes available are as follows:

Nominal Tank Sizes (Gallons) |
Usable Volume (Gallons) |

119 | 105 |

187 | 165 |

300 | 270 |

359 | 320 |

572 | 515 |

849 | 766 |

1100 | 993 |