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Home / Technical Articles / Site Conditions Impact on Caterpillar Genset Ratings
Site Conditions Impact on Genset Ratings - Caterpillar
Site Conditions Impact on Genset Ratings - Caterpillar

Introduction to Genset Rating

An engine rating is primarily limited by structural and thermal limits. These limits include maximum cylinder pressure during combustion, turbocharger speed, exhaust gas temperature and in natural gas engines, fuel type. Where an engine operates relative to these limits will determine the maximum altitude and ambient temperature for a given rating.

When an engine exceeds the maximum altitude or ambient temperature, the engine must be derated. The environment or site conditions also impact an engine or generator rating, thus impacting the rating of the entire genset.

Site conditions which may affect a rating include altitudetemperature, corrosive atmosphereshumidity, and dust. Altitude and temperature most heavily influence engine ratings. The higher the altitude, the lower the air density. Clean dense air is needed for efficient combustion. Likewise, an increase in temperature lowers air density.

Therefore, a derate of the engine must occur in high altitude and/or high temperature conditions in order for the genset to meet performance expectations.


Generators operating at altitudes above 1000 M (3281 ft.) require temperature rise reduction of 1% for every 100 M (328 ft.) above base (1000 M or 3281 ft.). A derate chart is available in TMI for generators and each specific engine.

Table 1 shows a sample engine TMI derate chart.

Altitude Capability Data (Corrected Power Altitude Capability)

Ambient Operating Temp.50 F68 F86 F104 F122 FNormal
0 F2,876 hp2,876 hp2,876 hp2,876 hp2,876 hp2,876 hp
984 F2,876 hp2,876 hp2,876 hp2,876 hp2,876 hp2,876 hp
1,640 F2,876 hp2,876 hp2,876 hp2,876 hp2,876 hp2,876 hp
3,281 F2,876 hp2,876 hp2,876 hp2,876 hp2,808 hp2,876 hp
4,921 F2,876 hp2,876 hp2,816 hp2,726 hp2,642 hp2,876 hp
6,562 F2,835 hp2,737 hp2,647 hp2,563 hp2,484 hp2,763 hp
8,202 F2,663 hp2,572 hp2,488 hp2,407 hp2,333 hp2,624 hp
9,843 F2,500 hp2,415 hp2,335 hp2,261 hp2,190 hp2,492 hp
10,499 F2,437 hp2,353 hp2,277 hp2,203 hp2,135 hp2,441 hp

*** The powers listed above and all the powers displayed are Corrected Powers.

Impact of site conditions on genset performance
Impact of site conditions on genset performance


Where the temperature of the ventilating air to the generator exceeds 40°C (104°F), derating of the generator may be necessary. The altitude/temperature derating chart found in TMI can be used for proper derating information.

Corrosive Atmospheres

Salt and other corrosive elements can cause damage to the winding insulation which can lead to failure of the generator. Protection from these elements includes additional coatings of insulation on the windings during the manufacturing process and epoxy compounds as a final winding overcoat.


Condensation resulting from humidity will present a problem for all generators unless they are fully enclosed.

Temperature rise of the machine and the circulation of cooling air with sufficient load operation will usually prevent condensation. Space heaters should be used to raise the temperature to 5°C above the ambient temperature to prevent condensation in high humidity areas.


Conductive or abrasive dust drawn in through the cooling fan can be very harmful to the generator.

Examples of abrasive dust are: cast iron dust, carbon dust, sand, powdered graphite, coke dust, lime dust, wood fiber, and quarry dust.

When these foreign particles blow through the generator, they act as sandpaper scraping away the insulation. These abrasions can cause an electrical short within the generator. An accumulation of these materials in the crevices of the insulation system will act as an insulator or as a moisture attractor.

Filters which fit over the unit’s intake air openings or enclosure ventilation openings can prevent damage.

When using filters, it is important that they be regularly changed so as not to impede airflow. The use of a generator air filter will cause the generator to be derated due to higher temperature rise resulting from reduced cooling airflow. Differential pressure switches may be available as an option on many generator sets.

Resource: Electric Power Applications, Engine and Generator Sizing – Caterpillar

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. sudesh
    Jan 24, 2017

    hi does the fuel gas manifold pressure and temperature affect derating and maybe I am naïve does the generator winding temps also derate the engine. what parameters continuously affect derating and what are hard written into the operating software.

  2. Bojanola Nagaraju
    Jan 16, 2013

    Can anyone share the root cause of overspeed, over heat and coolant leakage from rocker arm gasket.

  3. Bojanola Nagaraju
    Jan 16, 2013

    What are the root causes of engine overheat and over speed.
    Could any one assist me and send mail to [email protected] please…..


  4. Dwivedi Sanjeev
    Aug 18, 2012

    can anybody tell me about the selection of cable size.

  5. Dwivedi Sanjeev
    Aug 18, 2012

    Actually i want to know about electrical designing of cables.

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