Also in: Disaster Response and Recovery
Generator Output Derate for Altitude and Temperature
A rental generator shipped to a mountain town in a heat wave does not make its nameplate.
Example
You enter
- Rated output, standby or prime (kW) 300
- Site elevation (ft) 5000
- Site design ambient temperature (degF) 110
- Data sheet altitude threshold (ft) 1800
- Altitude step (ft) 1312
- Derate per altitude step (%) 7
- Data sheet temperature threshold (degF) 104
- Temperature step (degF) 18
- Derate per temperature step (%) 7
You get
- Altitude derate 17.07%
- Temperature derate 2.33%
- Combined derate, added 19.41% (58.2 kW lost; mostly altitude)
- Available output (derates added) 241.8 kW (80.6% of 300 kW rated)
- Available output (derates multiplied) 242.976
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
ScopeA diesel makes power by burning air, and thin or hot air carries less of it, so every generator data sheet states full output up to an altitude and an ambient temperature and then a percentage off for each step beyond. Those thresholds differ by model, by rating, and by fuel -- the same frame can derate from 1,800 ft on standby and 1,640 ft on prime -- so the calculator takes them from the unit's own data sheet. When both thresholds are exceeded the derates are combined by ADDING them, the conservative reading, and the multiplied figure is shown beside it. A 300 kW standby unit (full output to 1,800 ft and 104 degF, 7% per 1,312 ft and 7% per 18 degF beyond) sent to a 5,000 ft site on a 110 degF day loses 17.07% to altitude and 2.33% to heat: 241.8 kW added, 243.0 kW multiplied. A fifth of the unit is gone, almost all to altitude, and a 260 kW load that fit at sea level no longer fits; at 7,000 ft and 115 degF it is 203.9 kW. Feed the result to generator-sizing and the Generator Sizing for Motor Starting calculator as the real capacity. It does not supply derate rates, or address humidity, fuel quality, cooling capacity at altitude, or alternator derates. The manufacturer's data sheet, NFPA 110 as adopted, and the AHJ govern.
Formula or tablealtitude derate % = rate per step x max(0, site elevation - threshold) / step; temperature derate % = rate per step x max(0, site ambient - threshold) / step; combined (added, conservative) = altitude % + temperature %; available kW = rated kW x (1 - combined %); the multiplied combination (1 - a)(1 - t) is shown beside it.
Source and editionThe generator manufacturer's published derate statement for the specific model and rating (worked example from the Cummins DQDAC data sheet: full output to 1,800 ft and 104 degF, 7% per 1,312 ft and 7% per 18 degF beyond), named as the method and as governing; NFPA 110 cited by number for the requirement that the EPSS be rated for its site conditions.
Free accessArithmetic on thresholds and rates the user enters from the unit's own data sheet; no manufacturer derate curve or code table is reproduced.
What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
API field namesrated_kw, site_elevation_ft, site_ambient_f, altitude_threshold_ft, altitude_step_ft, altitude_rate_pct, temp_threshold_f, temp_step_f, temp_rate_pct, altitude_derate_pct, temperature_derate_pct, total_derate_pct, available_kw, available_multiplied_kw
- The unit's own derate rules thresholds, steps, and rates are entered from the data sheet for the model and rating in usegenerator manufacturer data sheet
- Derates combined by addition the conservative reading of 'must be combined'; the multiplied figure is shown for comparisonmanufacturer data sheets
- Site rating required the emergency power supply must be rated for the conditions at its siteNFPA 110