Overview#
The Cummins RS125 is a 125 kW liquid-cooled residential standby generator from the QuietConnect RS Series. It steps up to the QSJ8.9G -- an 8.9-liter turbocharged and aftercooled inline 6-cylinder engine -- providing a significant power increase over the 5.9L platform used in the RS50-RS100 models. The RS125 delivers 520.8 amps at 120/240V single phase, making it suitable for ultra-large estates, multi-structure properties, and small commercial facilities.
The RS125 runs on natural gas or propane vapor (field selectable) at 1,800 rpm and produces 71 dB(A) at 23 feet with the Sound Level 2 powder-coated aluminum enclosure. The enclosure is wind-rated to 180 mph per ASCE 7-10 and the unit is equipped for cold weather starts down to -40 F. An 850-amp CCA battery rated to 0 F is required.
At 3,915 lbs and 143 x 40 x 72 inches, the RS125 is the largest residential enclosure in the QuietConnect lineup. PowerCommand 2.3 controls provide advanced diagnostics, exercise scheduling, and remote monitoring capability. Fuel supply pressure must be maintained between 1.5-3.5 kPa (6.0-13.0 in H₂O) at the engine connection under all operating conditions.
Technical Specifications#
| Parameter | Value |
|---|---|
| Standby Rating | 125 kW (156 kVA) |
| Voltage (1-phase) | 120/240V |
| Voltage (3-phase) | 120/208V, 120/240V, 277/480V |
| Frequency | 60 Hz |
| Rated Amps (1-phase 120/240V) | 520.8 |
| Rated Amps (3-phase 120/208V) | 433.6 |
| Rated Amps (3-phase 277/480V) | 187.9 |
| Engine Model | QSJ8.9G |
| Displacement | 8.9 L (543 in³) |
| Cylinders | 6 (inline) |
| Bore x Stroke | 114.1 mm x 144.5 mm |
| Aspiration | Turbocharged and aftercooled |
| Rated Speed | 1,800 rpm |
| Cooling | Liquid-cooled |
| Starting System | 12V negative ground |
| Battery Capacity | 850 CCA at 0 F |
| Controller | PowerCommand 2.3 |
| Sound Level | 71 dB(A) at 23 ft (7 m) |
| Dimensions (L x W x H) | 143.0 x 40.0 x 72.0 in |
| Weight | 3,915 lbs |
| Fuel Pressure | 1.5-3.5 kPa (6.0-13.0 in H₂O) |
| Wind Rating | 180 mph (ASCE 7-10) |
| Cold Weather | -40 F (with factory accessories) |
Our Service Experience#
The RS125 uses the QSJ8.9G engine platform, which is shared with the commercial C125 N6. This is a well-proven engine in the 100-150 kW class with excellent parts availability through the Cummins distributor network. The 8.9L platform provides significantly more displacement headroom than the 5.9L models, which means the engine runs at a lower percentage of its mechanical capacity for equivalent loads.
Service intervals are standard for the QuietConnect series: 500 hours or annually for oil and filter, 500 hours for air filter, 1,000 hours for all 6 spark plugs, 30,000 hours for major overhaul. The PowerCommand 2.3 controller provides more detailed diagnostic data than the 1.1, including fault code history, operational trends, and advanced exercise scheduling.
Installation of the RS125 requires careful site preparation. At nearly 4,000 lbs, the concrete pad and structural support must be engineered accordingly. Gas supply sizing is critical at this power level -- the gas meter, regulator, and supply piping must be verified for adequate flow at full load. We recommend a dedicated gas supply line from the meter to the generator.
Frequently Asked Questions#
What is the standby rating of the RS125? 125 kW (156 kVA) at 0.8 power factor. Available in single-phase (120/240V, 520.8 amps) and multiple three-phase configurations.
How does the RS125 compare to the RS100? The RS125 steps up from the 5.9L engine to the 8.9L engine, with a correspondingly larger enclosure (143 x 40 x 72 in vs 118.7 x 40 x 58.3 in) and heavier weight (3,915 vs 2,830 lbs). The RS125 uses PowerCommand 2.3 controls vs the 1.1 on the RS100.
What is the warranty on the RS125? 2-year standby warranty from Cummins. Extended options available through authorized dealers.
