Generator Derating for Heat and Altitude

A generator’s nameplate power applies under specific reference conditions. At high ambient temperatures, for example 48 °C in the shade, a set with 500 kVA on its nameplate may raise a high coolant temperature alarm at a load of 420 kVA and shed the load. This is not a fault; it happens because the site temperature was not taken into account when the set was selected.

The same thing happens at high altitude. As the air gets thinner, the engine takes in less air, can burn less fuel and produces less power. This reduction in power is called derating. To choose the right generator, you need to know the site’s maximum temperature and altitude, and then check the manufacturer’s derating data.

Under what conditions does the nameplate rating apply?

Engine ratings are given for standard reference conditions. The standard reference conditions of ISO 3046-1 (which ISO 8528-1 also refers to for generating sets) are:

Parameter	Reference value
Total barometric pressure	100 kPa
Air temperature	25 °C
Relative humidity	30%

In addition, manufacturers state in the data sheet up to what temperature and altitude the power can be delivered without reduction. For example, one engine may deliver full power up to 40 °C and 1,000 m, while another may start losing power at a lower limit. Once the limit is exceeded, the derating curve or table applies.

Three sources of power loss

• Engine: In hot, thin air, the mass of air entering the cylinder falls. Turbocharged engines compensate for this up to a point; beyond that point, power drops. Because the turbocharging system, fuel system and emissions tuning differ from engine to engine, so does the loss.
• Cooling: The radiator is designed for a specific ambient temperature. At high altitude, air density falls, so the mass of air moved by the fan also falls. If cooling is not enough, a temperature alarm limits the set before its power does.
• Alternator: Alternators are usually rated to the conditions in IEC 60034-1, that is, 40 °C cooling air and 1,000 m altitude. Above these, the windings run hotter and the kVA rating falls according to the alternator manufacturer’s table.

The power the set can deliver on site is limited by whichever of these three is most restrictive.

How to read an example derating table

The table below is entirely hypothetical; it has been prepared to illustrate the method and does not belong to any engine or brand. The real values are in each engine’s own data sheet.

Altitude / Temperature	25 °C	40 °C	45 °C	50 °C
≤ 1,000 m	100%	100%	96%	92%
1,500 m	97%	95%	91%	87%
2,000 m	93%	90%	86%	82%
2,500 m	88%	85%	81%	77%

How to read it: find the site’s maximum temperature and altitude; the percentage where they intersect is the ratio of the power the engine can deliver under those conditions to its nameplate rating.

Worked example (with hypothetical values)

Site: 1,500 m altitude, maximum 45 °C in summer. Power required: 400 kW (500 kVA).

1. Engine: The ratio in the table is 91% → required nameplate rating ≈ 400 ÷ 0.91 ≈ 440 kW.
2. Alternator: In the alternator manufacturer’s table, say 94% for this condition → required alternator rating ≈ 500 ÷ 0.94 ≈ 532 kVA.
3. Result: A set that meets both conditions, in this example one in the 550 kVA class approximately, is considered as the preliminary selection.
4. Cooling: It is verified separately that the radiator is adequate at 45 °C ambient and 1,500 m.

General formula:

Required nameplate rating = power required on site ÷ derating ratio

To work out your power requirement from scratch, you can use the power calculator and our article what kVA generator do I need.

Common mistakes on site

• Selecting on the average temperature. The calculation is done not with the yearly average, but with the afternoon of the hottest day.
• Forgetting the temperature at the air intake. The air inside a canopy or generator room can be a few degrees hotter than outside, and much hotter with poor ventilation. What counts is the temperature of the air entering the air filter and the radiator. See generator room and installation and generator canopy.
• Hot air recirculation. If hot air leaving the radiator is drawn back in, the set runs on air much hotter than the ambient temperature.
• Looking only at the engine. The alternator and radiator can also be the limiting factor.
• Mixing up frequencies. The same engine can have different derating curves at 50 Hz and 60 Hz; see 50 Hz vs 60 Hz generators.
• Skipping the power rating. Derating is applied to the selected power rating (standby or prime); see standby, prime and continuous power.

Checklist

• Is the site altitude known?
• Was the calculation done with the maximum temperature of the hottest period?
• Has the real air temperature at the air filter and radiator inlet been estimated?
• Has the derating table or curve in the engine data sheet been checked?
• Has the alternator derating value been checked?
• Has the radiator’s capacity at that temperature and altitude been verified?
• Was the calculation based on the correct power rating (standby/prime) and frequency?

Next step

When selecting a set for a hot climate or a high-altitude site, share the site altitude, maximum temperature and load information with the Terex Diesel team. We will assess the right engine and alternator combination against the data sheets together: contact us.