Overview#
The Cummins C125 N6 is a 125 kW standby natural gas generator set built on the QSJ8.9G engine -- an 8.9-liter, inline-6-cylinder, turbocharged and aftercooled spark-ignited platform. Sold as the Quiet Connect RS125 series, it ships in a sound-attenuated aluminum enclosure rated at 71 dB(A) at 23 ft -- quieter than the smaller RS80 due to the larger enclosure volume and enhanced baffling. The C125 N6 is one of the most commonly specified units in the 100-150 kW natural gas class for light commercial standby applications where acoustic performance and compact footprint both matter.
The generator is UL 2200 listed, EPA certified for stationary emergency use, IBC Seismic compliant, and NFPA 110 capable when installed with the appropriate accessories. It is controlled by the PowerCommand 2.3 system, which provides isochronous governing, digital voltage regulation, auto start/stop, alarm and status display, and NFPA 110 Level 1 compliance capability. The enclosure is wind-rated to 180 mph per ASCE 7-10.
The C125 N6 fits naturally in professional office buildings, medical clinics, restaurant facilities, retail anchor tenants, and small data closet applications where 125 kW covers life safety and critical operational loads. Its 8.9-liter engine provides meaningful headroom above the 100 kW class for motor starting and transient load acceptance.
Technical Specifications#
| Parameter | Value |
|---|---|
| Standby Rating | 125 kW |
| Frequency | 60 Hz |
| Engine Model | QSJ8.9G |
| Displacement | 8.9 liters (543 in³) |
| Configuration | Cast iron, in-line 6 cylinder |
| Aspiration | Turbocharged and aftercooled |
| Bore x Stroke | 114.1 mm x 144.5 mm (4.49 x 5.69 in) |
| Rated Speed | 1800 rpm |
| Starting System | 12 VDC, negative ground |
| Battery Capacity | 850 CCA at 0 F to 32 F ambient |
| Battery Charging Alternator | 100 amps |
| Lube Oil Filter | Spin-on with relief valve |
| Cooling | Set-mounted radiator, 50 C ambient design |
| Control | PowerCommand 2.3 |
| Enclosure | Aluminum, Sound Level 2 standard |
| Weight | 3,915 lbs (1,776 kg) average |
| Dimensions (L x W x H) | 143.0 x 40.0 x 72.0 in (3,621 x 1,016 x 1,836 mm) |
| Sound Level | 71 dB(A) at 23 ft with SL2 enclosure |
Natural gas fuel consumption (CFH):
| Load | CFH | m³/hr |
|---|---|---|
| 25% | 652 | 18.5 |
| 50% | 981 | 27.8 |
| 75% | 1,306 | 37.0 |
| 100% | 1,666 | 47.2 |
Fuel inlet pressure: 6.0 to 13.0 in H₂O (1.5 to 3.5 kPa) at the engine connection.
Available voltage configurations:
- 1-phase: 120/240V
- 3-phase: 120/208V, 277/480V, 120/240V
Derating guidelines: Engine power available up to 1,800 m (5,900 ft) at ambient temperatures up to 40 C (104 F). Above these conditions, derate at 4.25% per 300 m (985 ft) and 2% per 10 C (18 F).
Our Service Experience#
The QSJ8.9G is a larger and more capable engine than the QSJ5.9G used in the C80 and C100, and this shows in its fuel consumption -- 1,666 CFH at full load versus 1,290 CFH for the C100. The 100-amp battery charging alternator (versus 52 amps on the smaller units) keeps the 12V battery in better condition between exercises, which reduces fail-to-start events in low-exercise-frequency installations.
The PowerCommand 2.3 control on the C125 N6 is more sophisticated than the 1.1 on the C80 -- it includes AmpSentry alternator protection, which monitors overcurrent, short circuit, and voltage conditions and can be a source of nuisance shutdowns if the alternator thermal model is not properly configured for the actual installation. The InPower service tool is required to access fault history and adjust thresholds. We recommend scheduling a Cummins-certified commissioning visit after installation to verify settings.
Frequently Asked Questions#
What is the standby rating of the C125 N6? 125 kW at 60 Hz, 0.8 power factor, per Emergency Standby Power (ESP) rating in accordance with ISO 8528 and NFPA 110. No sustained overload capability is available at this rating.
What engine does the C125 N6 use? The Cummins QSJ8.9G -- an 8.9-liter, in-line 6-cylinder, turbocharged and aftercooled spark-ignited natural gas engine. Bore is 114.1 mm (4.49 in), stroke is 144.5 mm (5.69 in). This is a larger engine than the QSJ5.9G used in the C80 and C100 N6 models, providing greater motor-starting capability.
How much natural gas does the C125 N6 consume? At 100% standby load: 1,666 CFH. At 75%: 1,306 CFH. At 50%: 981 CFH. At 25%: 652 CFH. Fuel inlet pressure must be maintained between 6.0 and 13.0 in H₂O under all operating conditions at the engine connection.
What control system does the C125 N6 use? PowerCommand 2.3, providing isochronous governing, digital voltage regulation, AmpSentry protective relay, automatic remote start/stop, output metering, and NFPA 110 Level 1 compliance capability. The InPower service tool connects via RS-485 for diagnostics and configuration.
What does the C125 N6 weigh and how large is it? Average weight is 3,915 lbs (1,776 kg). Dimensions are 143.0 in long x 40.0 in wide x 72.0 in tall (3,621 x 1,016 x 1,836 mm) with the standard Sound Level 2 enclosure. Actual weight varies with configuration options.
Is the C125 N6 quieter than the C100 N6? Yes -- the C125 N6 Quiet Connect RS125 is rated at 71 dB(A) at 23 ft with its Sound Level 2 enclosure, versus 73 dB(A) for the C80 N6. The larger enclosure volume with enhanced acoustic baffling accounts for the improved sound performance despite the larger engine.
