Diesel Fuel Storage and SPCC Plans at Data Centers

Most data centers still rely on diesel generators for backup power, and each generator carries fuel in a base (belly) tank, often supplemented by day tanks or bulk tanks. Fuel storage is usually treated as a design detail, but it has site consequences: tank pads and containment take space in the equipment yard, drainage must be designed around possible spills, and setbacks from streams, wells and property lines can constrain where the generator yard goes.

The scale is no longer small. In 2025, Google sought an Indiana permit modification for a Fort Wayne data center to add 179 diesel generators with 179 fuel tanks of 6,000 gallons each, more than 1 million gallons of storage, and generators capable of up to 527.52 MW if all ran.⁸ The Indiana Department of Environmental Management approved the request in April 2026, taking the site from 34 to 179 backup generators.⁹ At that size, oil spill rules, state tank programs and fire codes all apply at once, and the air permit for the same generators is a separate track covered in air permits for data center generators.

How much fuel to store is an engineering and contract decision, set by runtime targets, redundancy such as N+1 or 2N, and refueling assumptions. See backup generators and fuel storage sizing. This guide covers the rules that follow from that choice.

EPA’s SPCC rule, at 40 CFR part 112, applies to non-transportation facilities that meet two tests. The first is capacity: more than 1,320 U.S. gallons of aboveground oil storage, or more than 42,000 gallons of completely buried storage.¹ Containers smaller than 55 gallons and permanently closed containers are left out of the count, and completely buried tanks that are subject to the federal underground storage tank technical rules are excluded.¹

The second is location: whether the facility could reasonably be expected to discharge oil in harmful quantities to navigable waters or adjoining shorelines. EPA’s guidance says this judgment must be made as if manmade features such as dikes or other structures that might contain oil were absent.¹ In practice, a site with drainage ditches, storm sewers or streams nearby will rarely be able to rule itself out, and the determination should be documented by the engineer.

Oil-filled electrical gear matters too. EPA defines oil-filled operational equipment as equipment that includes an oil storage container whose oil supports only the function of the device, and it offers an optional alternative to general secondary containment for qualified equipment, conditioned on an inspection or monitoring program and a contingency plan.¹⁰ Ask the engineer how on-site transformers are counted and contained in the plan.

The rule lets small facilities prepare their own plans. A qualified facility has total aboveground oil storage of 10,000 gallons or less and, in the three years before certification, no single discharge to navigable waters over 1,000 gallons and no two discharges over 42 gallons each within any 12-month period.² If no aboveground container exceeds 5,000 gallons, it is a Tier I facility and can use the rule’s template; otherwise it is Tier II and self-certifies a full plan.² EPA notes that some states limit plan certification to professional engineers.²

A campus with dozens of generators will usually exceed 10,000 gallons, so its plan must be reviewed and certified by a licensed professional engineer familiar with the facility.⁵ The plan must be reviewed at least once every five years, and amended within six months of a change in design, construction, operation or maintenance that materially affects the potential for a discharge; technical amendments also need PE certification.⁵ For a phased campus, every new building and generator block is likely such a change.

A separate Facility Response Plan applies at 1 million gallons or more when the facility also lacks adequate secondary containment, has had a reportable discharge of 10,000 gallons or more in the past five years, or could harm a drinking water intake or sensitive environments; a lower 42,000-gallon trigger applies only to facilities that transfer oil over water to or from vessels.¹¹ Few data centers approach that volume, but the largest AI campuses now can.⁸

For bulk storage containers, 40 CFR 112.8(c)(2) requires a secondary means of containment for the entire capacity of the largest single container plus sufficient freeboard to contain precipitation.³ Containers can be protected individually or share a common collection area designed into site drainage, and permanently manifolded tanks can be treated as one container for sizing.³

The familiar 110% sizing rule is not in the regulation. EnSafe notes it is a remnant of design practice from before the 2002 rule amendments, and that EPA considered but did not adopt a 25-year, 24-hour storm as the freeboard standard; in high-rainfall regions such as the Gulf Coast, a 110% basin may still lack enough freeboard.⁶ Engineers should size uncovered containment to local rainfall, which ties into the campus stormwater design and any NPDES permit for releasing accumulated rainwater.

Most generator belly tanks are factory-built double-wall steel units. UL 142 covers standard double-wall tanks for flammable and combustible liquids, while UL 2085 tanks add fire-resistant construction that earns a two-hour rating and are treated as protected tanks.¹² Installation then follows the fire code and the NFPA 30 and NFPA 110 editions the local authority has adopted, which govern items such as venting and placement. UK permit filings show the same approach at hyperscale: an Amazon application for a data center at Didcot describes containerized, self-bunded belly tanks sized to hold 110% of their contents.¹³

The SPCC rule is only the federal floor. Three other layers often apply to the same fuel.

Underground storage tanks

EPA’s 1988 underground storage tank rule deferred tanks storing fuel solely for emergency generators from release detection, because they were often at remote, rarely visited sites. The 2015 revisions removed that deferral, since improved monitoring methods made release detection practical.⁷ Buried tanks therefore bring federal or state UST obligations, while aboveground tanks fall under SPCC and state AST rules.

State aboveground tank rules

States vary widely. Virginia, for example, requires registration of facilities with more than 1,320 gallons of oil in aggregate or any individual tank over 660 gallons, and applies pollution prevention and oil discharge contingency plan requirements at 25,000 gallons in a tank or in aggregate.⁴ The University of Virginia’s compliance office notes that emergency generator base or belly tanks are excluded from Virginia’s aggregate calculation, which can keep a generator-only site below the higher tiers.¹⁴ Check the specific state program early, because thresholds, exclusions and inspection duties differ.

Some operators are replacing fossil diesel with hydrotreated vegetable oil (HVO), a renewable diesel. Kao Data moved the generators at its Harlow, U.K. campus to HVO, starting by replacing 45,000 liters of diesel and planning for more than 750,000 liters at full build-out.¹⁵ A 2025 Rolls-Royce and Microsoft paper on Singapore data centers found HVO two to three times as costly as fossil diesel and cited supply and feedstock consistency as barriers.¹⁶

For spill and tank rules, HVO is still an oil stored in the same tanks, so the SPCC and state requirements do not go away. Its main effects are on Scope 1 emissions reporting and on fuel management, such as supplier claims about reduced microbial growth during long idle periods.¹⁵ For the reporting side, see data center emissions and ESG reporting.

1. 01Estimate total aboveground fuel and oil capacity at full build-out, not just phase one, including belly tanks, day tanks, bulk tanks and oil-filled equipment.
2. 02Map drainage paths from the generator yard to the nearest stream, ditch or storm sewer, assuming no containment, to inform the navigable waters determination.
3. 03Confirm the state tank program’s registration and plan thresholds and any exclusions for generator tanks.
4. 04Decide between aboveground and buried bulk storage early, since buried tanks bring UST release detection duties.
5. 05Size containment and freeboard to local rainfall and route accumulated water through the stormwater design.
6. 06Coordinate tank locations with the fire code official, the air permit and the site plan review.

A Phase I environmental site assessment of the land will also look for old tanks and past releases, which can complicate a new tank farm. If you want a parcel screened for drainage, setbacks and permitting factors before design starts, you can get a site reviewed.