Generac · SD Series (Industrial Diesel)

Generac SD400

400 kW standby / 360 kW prime · Diesel · Liquid-cooled

400 kW diesel standby generator. 12.5L turbocharged-aftercooled inline-6, EPA Certified Stationary Emergency.

Generac Generac SD400 generator — specifications and details
Standby power
400 kW / 360 kW prime
Voltage options
120/208V, 120/240V, 277/480V, 346/600V
Frequency / Phase
60 Hz · 3-phase
Engine
Perkins 12.5L · 12.5L
Alternator
Generac 520
CommercialManufacturingHealthcareData CentersGovernmentcritical-facilities
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Overview#

The Generac SD400 is a 400-kilowatt diesel standby generator powered by a 12.5-liter turbocharged and aftercooled inline-six engine. Rated at 400 kW standby and 360 kW prime, it serves the upper range of Generac's SD Series industrial diesel lineup, targeting large commercial, industrial, healthcare, and critical facility applications.

The SD400 pairs the 12.5L engine with Generac's Model 520 alternator featuring Class H insulation, permanent magnet excitation, and sealed bearings. The Digital H control panel provides NFPA 110 Level I and II programmability, Modbus protocol, and a predictive maintenance algorithm. At 400 kW, this unit competes with the Caterpillar C15, Cummins C400D6, and Kohler 400REOZK.

Stepping into the 24 VDC class#

The SD400 is the first model in Generac's SD Series to use a 24 VDC starting system with dual Group 8D batteries in series. This reflects the higher cranking demands of the 12.5-liter engine compared to the 12 VDC systems on the SD100 through SD300. The fuel pump lift also increases substantially to 12 feet (versus 3 feet on smaller models), with total fuel pump flow of 159 gph -- important considerations for installations with remote fuel storage.

Exhaust temperatures reach 1,256 degrees F at full load, the highest in the sub-500 kW SD range. The 5-inch exhaust outlet and 3,044 CFM exhaust flow require properly rated exhaust piping and silencer sizing.

Our service experience#

The 400 kW class serves large commercial buildings, hospitals, and data centers across Northern California. The SD400's thermal output is significant -- at over 1 million BTU/hr heat rejection to coolant and 18,685 CFM radiator airflow, undersized generator rooms are the most common installation issue we encounter. We recommend verifying the 24 VDC battery bank voltage at every service visit, as a single weak battery in the series pair can cause intermittent starting failures. Load bank testing annually at full 400 kW is essential to verify the turbocharger and aftercooler system are functioning properly under load.

Engineering specifications

Physical

Length
137.5 in
Width
57.6 in
Height
68 in
Dry weight
6,649 lb

Fuel system & runtime

Fuel use @ full load
27.8 GPH
Fuel use @ 75% load
23.1 GPH
Fuel use @ 50% load
16.7 GPH

Thermal & ventilation

Heat to coolant
1,067,520 BTU/hr
Exhaust flow
3,044 CFM
Exhaust temp
1256 °F
Combustion air
1,180 CFM
Ventilation air
18,685 CFM

Documents & downloads

Spec sheets & manuals

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Estimate runtime for the Generac SD400

Adjust load percent and tank size to estimate runtime. Pre-filled with this model's spec where available.

Estimate runtime on this tank

Estimated runtime

9.3 hours(0.4 days)

Fuel consumption ≈ 21.45 GPH at 75% load. Estimate based on industry-typical 1800 RPM standby curves (≈0.07 GPH/kW at full load). Actual consumption varies by engine, ambient temperature, fuel quality, and tuning.

Service intervals

Manufacturer-recommended intervals for the Generac SD400 under standby duty. Field intervals may differ based on load profile, ambient conditions, and fuel quality.

Oil & filter
Every 500 hours or 12 months
Air filter
Every 1000 hours
Fuel filter
Every 500 hours
Load bank test
Every 12 months

Common failure modes

What we've seen fail on this platform. Use as a service-planning reference, not a diagnostic — actual failure modes depend heavily on duty cycle and maintenance history.

ComponentSymptomTypical hoursSeverity
Fuel quality / degradationHard starting, filter clogging, reduced power output4,380+moderate
TurbochargerPower loss, excessive smoke, elevated exhaust temperatures15,000+moderate
Coolant systemOverheating, coolant leaks, water pump bearing wear12,000+moderate
Starter motorSlow cranking, failure to engage (24 VDC system)15,000+moderate
Battery bankFailed to start, low voltage alarm (dual 12V batteries in series)8,760+minor

Frequently Asked Questions

What engine does the Generac SD400 use?
The Generac SD400 uses a Perkins 12.5-liter, six-cylinder in-line turbocharged and aftercooled diesel engine running at 1800 RPM. The engine has a bore of 130 mm (5.12 in) and stroke of 157 mm (6.18 in) with a 16.3:1 compression ratio. It operates on a 24 VDC electrical system with dual 12V Group 8D batteries in series. The engine is EPA Certified for Stationary Emergency applications.
What is the fuel consumption of the Generac SD400?
At standby rating, the SD400 consumes 27.8 gallons per hour at full load (100%), 23.1 gph at 75% load, 16.7 gph at 50% load, and 9.8 gph at 25% load. Total fuel pump flow (combustion plus return) is 159 gph. The fuel pump lift is rated at 12 feet, significantly higher than the smaller SD models.
What are the dimensions and weight of the SD400?
The open-set SD400 (no base tank) measures 137.5 x 57.6 x 68 inches and weighs approximately 6,649 lbs. Sub-base fuel tank options provide runtimes from 6 to 48 hours. Enclosure options include standard, Level 1 acoustic, and Level 2 acoustic configurations in steel or aluminum.
What is the ventilation requirement for an indoor SD400 installation?
The SD400 requires 18,685 CFM of radiator cooling airflow and 1,180 CFM of combustion air. Heat rejection to coolant is 1,067,520 BTU/hr. Exhaust exits at 1,256 degrees F at 3,044 CFM through a 5-inch outlet. These are substantial thermal loads that require careful mechanical room design.

Compatible equipment

Transfer switches

More in the SD Series (Industrial Diesel)

Generac's broadest diesel standby lineup — 10 to 1500 kW across four engine families for commercial and industrial applications.

View all 20models in this series →

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