Cummins · Cummins D6 Commercial Diesel

Cummins C125 D6

125 kW standby / 112.5 kW prime · Diesel · Liquid-cooled

125 kW EPA Tier 3 diesel standby generator with Cummins QSB5-G6 engine and PowerCommand 1.1 control. Sound-attenuated aluminum enclosure, UL 2200 listed, NFPA 110 Level 1 capable.

Cummins Cummins C125 D6 generator — specifications and details
Standby power
125 kW / 112.5 kW prime
Voltage options
120/240V, 120/208V, 277/480V, 347/600V
Frequency / Phase
60 Hz · 1/3-phase
Engine
Cummins QSB5-G6 · 4.5L
EPA / Emissions
Tier 3
Commerciallight-industrialResidential
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Overview#

The Cummins C125 D6 (model designation C125D6C) is a 125 kW standby diesel generator set built on the QSB5-G6 engine -- a 4.5-liter, 4-cylinder turbocharged and charge-air-cooled industrial diesel running at 1800 rpm. Rated at 156 kVA standby and 141 kVA prime at 60 Hz, the C125D6C represents the maximum output of the QSB5 4-cylinder platform. Cummins also offers the C125D6D variant using the larger QSB7-G5 (6.7L, 6-cylinder) engine at the same 125 kW rating.

Standard equipment includes the PowerCommand 1.1 control with integrated voltage regulation and a sound-attenuated aluminum enclosure. The unit is EPA Tier 3 certified for stationary emergency standby applications. The QSB5-G6 engine features Bosch high-pressure common rail fuel injection, a Holset HX35 wastegated turbocharger, and full-authority electronic controls.

The C125D6C is positioned as the top of the 4-cylinder QSB5 line, ideal for commercial buildings that need more than 100 kW but want the compact footprint of a 4-cylinder unit. For applications where engine longevity is prioritized over compactness, the C125D6D with its larger 6-cylinder QSB7 engine is the alternative.

Our Service Experience#

At 125 kW, the QSB5-G6 engine is at its rated maximum. We monitor these units more closely for charge air cooler integrity and fuel injection system health compared to the lower-rated C80 and C100 models on the same platform. The high-pressure common rail fuel system is sensitive to fuel quality -- we recommend fuel polishing for units that sit idle for extended periods. The Fleetguard dual spin-on fuel filter assembly with integral water separator should be serviced at every 500-hour interval without exception.

Frequently Asked Questions#

What fuel consumption should I expect from the C125D6C? Based on the QSB5-G6 engine specification sheet, fuel consumption at full standby load is approximately 10.3 gallons per hour (39 L/hr). At 50% load, consumption drops to approximately 5.6 gallons per hour (21 L/hr).

Is the C125 D6 NFPA 110 Level 1 compliant? The C125D6C is designed to meet NFPA 110 Level 1 requirements for emergency power supply systems, accepting full rated load in a single step. Verify the specific configuration with your Cummins distributor.

Engineering specifications

Physical

Length
119 in
Width
40 in
Height
58 in
Dry weight
2,600 lb

Fuel system & runtime

Fuel use @ full load
10.3 GPH
Fuel use @ 50% load
5.6 GPH

Documents & downloads

Spec sheets & manuals

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Estimate runtime for the Cummins C125 D6

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

25.2 hours(1.0 days)

Fuel consumption ≈ 7.95 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 Cummins C125 D6 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 500 hours
Fuel filter
Every 500 hours
Major overhaul
≈ 20,000 hours

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
QSB5-G6 high-pressure fuel injectionHard start, rough running, injector fault codes, reduced power—moderate
Charge air cooler coreCoolant in intake manifold, white exhaust smoke, engine overheat—severe
PowerCommand 1.1 battery monitoringWeak battery warning, slow crank, failed 10-second start—moderate
Spin-on lube oil filter sealOil seepage at filter base, low oil pressure warning—minor

Frequently Asked Questions

What engine does the Cummins C125 D6 use?
The C125D6C uses the Cummins QSB5-G6, a 4.5-liter, inline-4-cylinder, turbocharged and charge-air-cooled diesel rated at 1800 rpm. EPA Tier 3 certified. The QSB5-G6 is a higher-output variant of the QSB5 platform, pushing the 4-cylinder block to its maximum 125 kW standby rating. Cummins also offers the C125D6D variant using the larger QSB7-G5 (6.7L, 6-cylinder) engine for the same 125 kW output.
What are the standby and prime ratings of the C125 D6?
Standby rating is 125 kW (156 kVA) at 60 Hz, 0.8 power factor. Prime rating is 112.5 kW (141 kVA). Both at 1800 rpm.
What is the difference between the C125D6C and C125D6D?
The C125D6C uses the QSB5-G6 engine (4.5L, 4-cylinder) while the C125D6D uses the QSB7-G5 engine (6.7L, 6-cylinder). Both produce 125 kW standby. The C125D6D has a larger displacement engine operating at a lower percentage of its rated capacity, which generally means lower thermal stress and potentially longer engine life. The C125D6C is more compact.
What are the service intervals for the C125 D6?
Oil and filter changes are due at 500 hours or 12 months, whichever comes first. Air filter and fuel filter replacement at 500-hour intervals. Coolant flush at 4,000 hours. Major overhaul interval is 20,000 hours under standard standby duty conditions.
Can PowerGen service the C125 D6 in Northern California?
Yes. PowerGen serves the San Francisco Bay Area, Silicon Valley, and broader Northern California with authorized Cummins service. We carry QSB5 parts, diagnostic tooling, and provide annual maintenance, load bank testing, and emergency response for C125 D6 installations.

Compatible equipment

Transfer switches

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