What Is a Belly Tank Generator?

A belly tank generator is a generator set with its fuel tank integrated into the base frame directly beneath the engine and alternator, providing extended runtime in a compact, self-contained package. Often called a sub-base or skid-base tank, it is most commonly paired with diesel generators for standby, prime, or mobile power where space, quick deployment, and simplified fuel plumbing are priorities.

How It Works and What It Includes

A belly tank generator places the fuel reservoir inside the generator’s structural base. The engine draws fuel from this under-slung tank and returns unused fuel to it, while the enclosure and skid provide support, spill protection, and access for service. This integrated approach reduces external piping and keeps the footprint small.

The following components are typically part of a belly tank generator’s sub-base assembly:

• Double-wall steel tank (UL 142 or UL 2085) with integral secondary containment
• Emergency and normal vents, fill and return ports, and overfill prevention
• Fuel level gauge or sender, low-fuel alarm/shutdown, and leak detection for interstice
• Drain and sump with water draw-off to manage condensation and sludge
• Anti-siphon/solenoid valves and flexible, vibration-resistant fuel lines
• Skid frame that supports the generator and may include forklift pockets and tie-downs
• Optional fuel polishing ports, heaters (cold climates), and spill containment fittings

Together, these features make the generator largely “plug-and-play,” reducing site fuel infrastructure while meeting safety and environmental requirements when correctly specified and installed.

Where Belly Tank Generators Are Used

These systems appear wherever compact, self-contained power is advantageous or fuel lines are impractical or undesirable. They’re prevalent across both stationary and mobile deployments.

• Construction sites and rental fleets (towable/mobile gensets with undercarriage tanks)
• Telecom shelters and remote facilities with limited space or access
• Commercial standby power for retail, light industrial, and municipal sites
• Data rooms, edge computing, and small healthcare facilities needing quick installs
• Temporary events and disaster response where rapid setup is essential

Large hospitals, data centers, and high-availability campuses often favor day-tank plus bulk-storage systems, but belly tanks are still used for smaller capacity or interim power.

Standards, Codes, and Compliance

Belly tank generators must satisfy both fuel tank and engine/generator requirements. Exact obligations vary by jurisdiction and application (emergency, legally required standby, optional/prime).

• UL 142 (steel aboveground tanks) or UL 2085 (protected tanks) listings for sub-base tanks
• NFPA 30 (Flammable and Combustible Liquids Code) and NFPA 37 (engines and turbines)
• NFPA 110 (Emergency and Standby Power Systems) for EPS design, testing, and fuel supply
• NFPA 70/NEC Articles 445 (Generators), 700–702/708 for emergency/standby/critical operations
• International Fire Code (IFC) and local fire codes, including indoor fuel quantity limits
• U.S. EPA rules: SPCC (40 CFR 112) if aggregate aboveground storage exceeds 1,320 gallons; NSPS IIII/Stationary CI engines and NESHAP ZZZZ compliance for emissions; local air district permits
• Seismic anchorage (e.g., California) and, for towables, DOT trailer and fuel system requirements

Project engineers should confirm tank listings, emergency vent sizing, secondary containment, and emissions permitting with the authority having jurisdiction (AHJ) early in design.

Advantages and Trade-offs

A belly tank design offers compelling benefits for many sites, especially where installation speed and a small footprint matter.

• Compact footprint: fuel is within the generator base—no separate tank yard or day-tank room
• Simplified install: fewer penetrations and shorter fuel runs reduce complexity and leak points
• Mobility: common in rental/towable units for quick deployment and retrieval
• Predictable runtime: standardized 8-, 12-, or 24-hour “at full load” configurations
• Spill control: double-wall tanks provide integrated secondary containment

These strengths make belly tanks a practical default for small-to-mid-size standby sets and remote or temporary power, particularly with diesel fuel.

However, there are limitations that may steer owners toward different fuel storage strategies in larger or highly regulated installations.

• Capacity and weight: very large tanks increase base weight/height and may exceed floor ratings
• Inspection access: under-slung tanks can be harder to inspect and maintain
• Indoor volume limits: codes may cap on-floor diesel quantities without special protections
• Fuel aging: diesel stored long-term risks water, sludge, and microbial growth; polishing/biocides may be needed
• Heat/ventilation: large sub-base tanks inside enclosures can complicate thermal management

For high-demand or mission-critical sites, a day tank with bulk storage often provides easier maintenance, longer runtimes, and centralized fuel management.

Sizing and Runtime Planning

Belly tanks are often sized to provide a specified runtime—commonly 8 to 24 hours—at full rated generator load. Actual runtime depends on the real load profile, fuel temperature, and engine efficiency. Manufacturers publish fuel-consumption curves; use those figures rather than rules of thumb whenever possible.

To size a sub-base tank, consider these basic steps:

1. Determine expected average load (percent of generator kW)
2. Find the engine’s fuel consumption at that load from the spec sheet (gallons/hour or liters/hour)
3. Multiply by desired runtime (hours) to get required usable fuel volume
4. Add reserve (typically 10–20%) to cover start/stop, temperature effects, and aging
5. Check code limits for on-site storage volume and structural support for added weight

Final selection should reconcile runtime goals with code limits, floor loading, enclosure size, and service accessibility.

Example Runtime Calculation

For a 150 kW diesel generator expected to run at ~75% load, typical consumption might be around 9–11 gallons/hour (consult the specific engine curve). For a 24-hour target: 10 gph × 24 h = 240 gallons. Adding 15% reserve suggests ~276 gallons; a 300-gallon UL 142 sub-base tank would be a common choice. Actual runtime will differ if average load or ambient conditions vary from assumptions.

Maintenance and Safety Best Practices

Even with double-wall containment, fuel system care is essential to reliability and compliance. Belly tanks concentrate fuel under the machine, so routine checks and documentation matter.

• Inspect weekly for leaks, corrosion, vent obstructions, and gauge accuracy; verify interstitial sensor function
• Drain water bottoms periodically; test fuel at least annually and treat with biocide as needed
• Exercise the generator per NFPA 110 and manufacturer guidance; keep logs of tests and alarms
• Verify proper emergency/normal venting and overfill prevention; maintain caps and spill kits
• Protect fill points from contaminants; use filtration on transfer and consider fuel polishing ports
• Confirm anchorage and clearances; ensure flexible fuel lines are undamaged and supported
• Train personnel on emergency shutdown, spill response, and notification procedures

Proactive maintenance preserves fuel quality, reduces failure risk during outages, and supports regulatory inspections.

Alternatives to Belly Tank Designs

Depending on runtime, space, and compliance needs, other fuel layouts may be preferable.

• Day tank plus remote bulk aboveground tank (AST) with transfer pump and controls
• Underground storage tank (UST) feeding a day tank for larger or space-constrained sites
• Natural gas generators that eliminate on-site liquid fuel storage (where pipeline gas is reliable)
• Hybrid systems with battery energy storage to reduce runtime and fuel consumption

These configurations can ease inspection, extend runtime, or satisfy stricter indoor fuel-volume limits, at the cost of more infrastructure and permitting.

Key Takeaway

A belly tank generator integrates a code-listed fuel tank into the generator’s base to deliver compact, self-contained power with predictable runtime—especially suited to diesel standby and mobile applications. When sized and maintained properly and installed to applicable codes, it offers a fast, space-efficient solution; for higher capacities or stringent maintenance and compliance needs, external day-tank-plus-bulk storage systems may be the better fit.

What is the difference between a generator belly tank and a day tank?

Third, while generator fuel day tanks typically only have one fill port, belly tanks typically have two fill ports available for use. This is due to the fact that the generator only needs to have its fuel tank replenished once per day.

What is a generator belly tank?

Generator base, sub-base, or belly tank is a UL 142 fuel tank that is typically built into a generator support frame as part of a packaged unit which includes an enclosure. The tank acts as a substitute for a freestanding day tank.

What is a diesel belly tank?

Diesel Fuel Sub-Base Tanks
Sub-base tanks are a popular choice for many commercial facilities because they’re compact and easy to install. Also called Belly Tanks, they are typically underneath the genset package and are used as the base for the unit.

What is a belly tank used for?

Belly tanks are used as external fuel storage tanks to supply emergency power generators for data centres. They are installed either beneath or near the generator, allowing for the storage of large fuel quantities needed to power a data center in case of a network power blackout.