Generator altitude and temperature derating is model-specific: there is no single percentage-loss table that applies to every generator. Start with the generator model, operating mode, site elevation, and the temperature measurement specified by its manufacturer. The tables below return published output multipliers for one documented configuration—not universal correction factors for home standby, portable, or diesel generators.
What the Derate Factor Means
A derate multiplier expresses the fraction of the applicable rated output remaining under the stated conditions. A factor of 0.90 means 90% remains, equivalent to a 10% reduction. Cummins instructs users of the data sheet below to multiply the nominal maximum rated load by the applicable altitude-and-temperature factor. Confirm the correct base rating for the supplied configuration before calculating available kW.
[
P_{\text{site}}=P_{\text{base}}\times f
]
[
\text{Output reduction (%)}=100(1-f)
]
Where:
- (P_{\text{site}}) is the preliminary site-adjusted active-power limit, in kW.
- (P_{\text{base}}) is the manufacturer-confirmed base output to which the chart applies, in kW.
- (f) is the dimensionless altitude-and-temperature multiplier.
These equations do not convert kW to kVA or establish motor-starting capability.
Generator Altitude and Temperature Derate Tables
The following values are selected entries from Table A of the Cummins C1350N6 generator set data sheet, D-6453, January 2020. That document identifies a 60 Hz, pipeline-natural-gas generator set with a turbocharged, charge-air-aftercooled QSK60G engine. Use these factors only when that document applies to the equipment and configuration being evaluated.
The temperature columns refer to air-filter inlet temperature, not automatically the local weather-station temperature. Elevation in feet is rounded as published in the source. Each factor already accounts for both altitude and temperature; do not apply another altitude percentage to it.
On-Grid, Soft-Start Operation
Use this table only for the operating mode labeled “On Grid, Soft Start” in the manufacturer’s document.
| Barometric altitude, m (ft) | 25°C / 77°F | 30°C / 86°F | 35°C / 95°F | 40°C / 104°F | Source and revision |
|---|---|---|---|---|---|
| 0 (0) | 1.00 | 1.00 | 1.00 | 1.00 | D-6453, Jan. 2020, Table A—On Grid |
| 500 (1,640) | 0.99 | 0.97 | 0.96 | 0.95 | D-6453, Jan. 2020, Table A—On Grid |
| 1,000 (3,281) | 0.92 | 0.91 | 0.90 | 0.89 | D-6453, Jan. 2020, Table A—On Grid |
| 1,500 (4,921) | 0.86 | 0.85 | 0.84 | 0.82 | D-6453, Jan. 2020, Table A—On Grid |
| 2,000 (6,562) | 0.80 | 0.79 | 0.77 | 0.76 | D-6453, Jan. 2020, Table A—On Grid |
Off-Grid Operation
The same data sheet publishes a separate “Off Grid” table. Do not substitute the on-grid factors when evaluating this operating mode. “Off Grid” is the chart’s operating-mode label; it does not change the generator’s approved duty rating into a continuous or prime-power rating.
| Barometric altitude, m (ft) | 25°C / 77°F | 30°C / 86°F | 35°C / 95°F | 40°C / 104°F | Source and revision |
|---|---|---|---|---|---|
| 0 (0) | 1.00 | 1.00 | 1.00 | 0.99 | D-6453, Jan. 2020, Table A—Off Grid |
| 500 (1,640) | 0.97 | 0.96 | 0.94 | 0.93 | D-6453, Jan. 2020, Table A—Off Grid |
| 1,000 (3,281) | 0.91 | 0.90 | 0.88 | 0.87 | D-6453, Jan. 2020, Table A—Off Grid |
| 1,500 (4,921) | 0.85 | 0.83 | 0.82 | 0.81 | D-6453, Jan. 2020, Table A—Off Grid |
| 2,000 (6,562) | 0.78 | 0.77 | 0.76 | 0.74 | D-6453, Jan. 2020, Table A—Off Grid |
Conditions Attached to These Values
The source bases altitude on the SAE standard pressure–altitude relationship and instructs users to add 150 m (500 ft) to site altitude for low-barometric-pressure conditions. Its temperature-and-altitude calculation assumes the low-temperature cooling-circuit return temperature is 10°C above the air-filter inlet temperature, with a maximum low-temperature circuit temperature of 50°C. If that circuit temperature exceeds 50°C, Cummins directs users to consult the factory.
The data sheet also calls for application-engineering guidance on air-cleaner restriction when intake temperature exceeds 35°C. Consequently, a published factor in the 40°C column does not, by itself, establish that the complete installation can operate at that output. Fuel quality and cooling conditions require their own checks under the same document.
How to Use the Table
Consider the documented C1350N6 configuration at a barometric altitude of 1,000 m and an air-filter inlet temperature of 35°C:
- Select the operating mode. The on-grid, soft-start factor is 0.90; the off-grid factor is 0.88.
- Calculate the reduction. These factors represent 10% and 12% reductions, respectively.
- Apply the selected factor to the base kW rating confirmed for that equipment and application.
For example, if Cummins confirms that 1,350 kW is the applicable base rating for the off-grid calculation:
[
P_{\text{site}}=1{,}350\ \text{kW}\times0.88=1{,}188\ \text{kW}
]
The arithmetic gives a preliminary site-adjusted limit of 1,188 kW under that assumption. It does not establish that a 1,188 kW load can start successfully or that the alternator, cooling system, fuel supply, and installed equipment configuration support that load.
For conditions between the displayed entries, consult the full manufacturer chart and obtain an approved calculation method where interpolation is needed. Do not extrapolate beyond published conditions or treat an unlisted condition as zero derating.
Why a Fixed Percentage Rule Cannot Replace the Model Chart
Some manufacturers publish approximate percentage rules instead of a combined chart. For example, the Generac owner’s manual for 10–26 kW, 60 Hz air-cooled generators, under “Specifications—Engine,” states that engine power decreases approximately 3.5% per 1,000 ft above sea level and approximately 1% per 10°F above 60°F. Those are engine-power statements for the equipment covered by that manual, not universal electrical-output factors.
Do not apply that rule to the Cummins tables above, combine it with their multipliers, or transfer it to an unrelated generator. For another model, use its own manual or site-rating chart, including its altitude threshold, temperature reference, fuel, and rating category.
Checks Before Using the Result for Sizing
Before comparing the adjusted output with a load list, confirm:
- The exact model, fuel configuration, and standby, prime, or continuous rating to which the manufacturer’s data applies.
- Whether the quoted output is already site-rated, so the same reduction is not applied twice.
- The alternator’s applicable kVA, current, and temperature-rise limits—not just the engine’s available kW.
- Motor-starting demand, load-step sequence, and acceptable voltage and frequency response.
- Cooling-system and air-inlet conditions at the actual installation.
The C1350N6 data sheet identifies separate alternator configurations and cooling requirements. Its altitude-and-temperature multiplier is one input to equipment evaluation, not a replacement for those checks or for installation requirements assessed by qualified professionals.