In short. Air density falls with altitude, so a diesel engine gets less oxygen per stroke, the alternator and radiator get less cooling, and the set cannot deliver its nameplate output. Most manufacturers rate turbocharged sets without derating to about 1,000 metres and then reduce output by a stated percentage per further 300 metres. Hyderabad sits near 500 metres, so altitude rarely limits a set there alone, but it compounds with high ambient temperature.
Why altitude reduces output
Atmospheric pressure drops by roughly 1.2 percent per 100 metres near sea level, and air density drops with it. A diesel engine is an air pump before it is anything else: the mass of oxygen it draws in sets how much fuel it can burn cleanly. At altitude a naturally aspirated engine ingests less air, so full fuel would mean smoke, high exhaust temperature and piston damage. A turbocharged engine recovers much of the loss because the compressor raises inlet pressure, but the turbocharger must spin faster to do so, and its speed and temperature limits set a new ceiling. Standard rating conditions under ISO 3046 and ISO 8528 are 100 kPa, 25 °C and 30 percent relative humidity, which corresponds to roughly 100 metres above sea level. Every published kVA figure assumes those conditions unless the data sheet says otherwise.
Three components, three different derates
A generating set is derated by whichever part reaches its limit first, and the three main parts behave differently.
Engine
Naturally aspirated engines typically lose in the order of 3 to 4 percent of output per 300 metres above the reference altitude. Turbocharged and aftercooled engines, which account for nearly every set above about 30 kVA sold in India today, usually hold full output to a threshold between 1,000 and 1,500 metres and then lose roughly 1 to 2 percent per 300 metres. Electronically governed CPCB IV+ engines measure barometric pressure and inlet manifold temperature and derate themselves through the ECM; the published curve tells you what the engine will do, not what it might be persuaded to do.
Alternator
The alternator is cooled by its own fan drawing room air through the windings. IEC 60034-1 rates machines for up to 1,000 metres, and above that the thinner air carries away less heat for the same volume flow. A typical allowance is around 3 percent output reduction per further 500 metres, with the alternative of accepting a higher winding temperature class if the insulation and expected life allow it.
Radiator and cooling
The radiator fan moves a fixed volume of air, but at altitude that volume contains less mass, so the heat it can carry falls in proportion to density. A set that is comfortably within its radiator’s ambient capability at 500 metres may run its coolant close to the alarm at 2,000 metres on the same afternoon. This is the derate most often forgotten because it does not appear on the engine power curve.
Working out the usable site rating
Psite = Prated × faltitude × ftemperature
Psite is the output available at the installation in kW or kVA, Prated is the nameplate output at reference conditions, faltitude is the manufacturer’s altitude factor for the engine, and ftemperature is the ambient factor for the same engine, both expressed as fractions. The engine result is then compared with the alternator and radiator limits and the lowest figure governs.
Worked example (illustrative). A 500 kVA, 400 kW set is proposed for a cement plant at 1,500 metres with a design ambient of 40 °C. The engine curve shows no derate to 1,000 metres and 1 percent per 100 metres beyond, so faltitude is 0.95 and the engine can give 380 kW. The alternator sheet allows 3 percent per 500 metres above 1,000 metres, which also leaves 97 percent of its rating, so the alternator is not the limit. The radiator is rated for 50 °C at sea level and the manufacturer’s cooling data at 1,500 metres and 40 °C still shows margin. The usable site rating is about 380 kW, and the load list should be checked against 380 kW, not 400 kW.
| Site altitude band | Naturally aspirated engine | Turbocharged, aftercooled engine | Alternator and radiator |
|---|---|---|---|
| 0 to 500 m (coastal Andhra, most of Telangana) | Usually within rating, confirm above 300 m | No derate | No derate |
| 500 to 1,000 m (Hyderabad plateau, Deccan sites) | Expect a few percent derate | Usually no derate | Usually within rating |
| 1,000 to 1,500 m (Western Ghats hill sites) | Significant derate, often 10 percent or more | Derate begins, typically 1 to 2 percent per 300 m | Alternator derate begins, cooling margin shrinks |
| Above 1,500 m (Himalayan and north-east sites) | Often impractical without oversizing | Manufacturer curve mandatory, cooling redesign likely | Both derate, larger radiator or remote cooling |
The table is a screening aid. The exact figures come from the engine and alternator data sheets for the specific build, and the derating assessment guide walks through the arithmetic for combined conditions.
Altitude never arrives alone
Altitude compounds with the other site conditions rather than replacing them. A hill station is usually cooler, which helps, but a plateau site in May is both higher and hotter than the reference, and the two factors multiply. Inside a canopy or a poorly ventilated room the air at the radiator face can be 10 to 15 K warmer than the weather report, which is the subject of ambient-temperature derating. High humidity in the monsoon slightly reduces oxygen content again, although most manufacturers fold this into the ambient factor. The practical rule is to collect the elevation, the design ambient at the equipment inlet and the ventilation arrangement before asking for a rating.
What to do when the derate matters
- Choose the next rating up only after confirming that the larger set is not itself derated into the same corner; two sets of different ratings may share an engine family and curve.
- Ask for the same engine with a higher boost turbocharger or an altitude-specific fuel setting where the manufacturer offers one.
- Specify the alternator with a higher temperature-rise margin or the next frame size, which is cheaper than a larger engine when the alternator is the limit.
- Check the radiator ambient capability at site density and, where the set is enclosed, the enclosure’s own airflow data, as covered in heat rejection.
Common mistakes
- Applying a single percentage per 100 metres from a textbook to every engine; the result is wrong for turbocharged engines below their threshold and optimistic for naturally aspirated ones.
- Derating the engine but not the alternator or radiator; the set then overheats electrically or thermally while the engine still has power in hand.
- Taking the altitude of the nearest city rather than the site; a plant on a ridge can be several hundred metres above the town named on the address.
- Checking the load list against the catalogue rating rather than the site rating; the generator trips on overload in the first real outage.
What to send us
Send the site elevation from a survey or a GPS reading, the design maximum ambient temperature and whether the set will sit in a room, a canopy or a container, and the load list in kW with the largest motor identified. If a particular engine and alternator have already been proposed, send the nameplate photographs or the data sheets so that the correct curves are used. Send your application details and the generator sizing guide explains how the derated rating is matched to the load.
Questions engineers ask
Does a generator in Hyderabad need altitude derating?
Hyderabad and Secunderabad lie at about 500 to 550 metres, which is below the altitude threshold of almost all turbocharged engines. A naturally aspirated small set may lose a few percent. The temperature at the equipment inlet in April and May matters far more at this elevation than the altitude itself.
Why do two sets of the same kVA derate differently at the same site?
They use different engines with different boost levels, thresholds and cooling capacity, and possibly different alternator frames. The curve belongs to the specific engine and alternator build, which is why the manufacturer data sheet, not the kVA figure, is the source for the factor.
Can the turbocharger be changed to recover output at altitude?
Some manufacturers offer high-altitude builds with a different turbocharger match or fuel setting, and electronically controlled engines may have an altitude calibration. It must be a factory or authorised engineering change, because an unmatched turbocharger risks overspeed, high exhaust temperature and loss of emission compliance.
Does altitude affect fuel consumption?
At a given load the specific fuel consumption changes little within the rated envelope, but a derated set runs at a higher load fraction for the same demand, and a set pushed beyond its derated limit smokes and wastes fuel. Plan fuel from the derated rating and the actual load.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.