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Generating Insights: Overview of Rating Definitions for Alternators and Generator Sets

The kVA rating placed on an alternator is governed primarily by the thermal capabilities of the winding insulation system

The rating on an alternator defines its capability to deliver electrical power to a connected load. This is recognised in terms of kVA and is the starting point for sizing an alternator in a generator set application.

Whilst there are other factors that must be considered during the sizing & selection process, this article addresses the subject of rating and duty cycle definitions, the terminology used in the industry and how this informs the process of assisting the GOEM to select the right alternator for their generator set.

Alternator rating

The kVA rating placed on an alternator is governed primarily by the thermal capabilities of the winding insulation system and the required operational life expectancy. For a given operational life the higher the class of insulation, the hotter the alternator can run.

The table below lists the letters used and the corresponding nominal operating temperatures for classifying the performance of an insulation system.


The industry standard for modern winding insulation is class H for low voltage systems, (<1kV), and class F for medium and high voltage, (>1kV).

It should be noted however, that the letter assigned to the class of insulation system is not necessarily the same as the temperature rise class an alternator could be rated for. If the kVA rating placed on an alternator leads to a lower operating temperature than the insulation system’s nominal capability then operational life is extended. Conversely, if the kVA rating leads to a higher operating temperature than the insulation systems nominal capability then the operational life is reduced.

Either case is an acceptable operating condition, however the life expectancy of the alternator windings must be taken in to account when selecting an alternator for a certain application.

Example: Standby versus prime power applications

In a typical standby application such as a hospital, the operating hours will most likely be less than 200 hours per year. Over the life time of the installation, e.g. 15 years, that equates to a total of up to 3000 operational hours. Given this is relatively low, one can afford to run the alternator at a higher temperature and therefore a relatively higher rating.

For this reason customers will be offered a peak kVA rating on the alternator for standby applications.

However, in a continuously operating application such as an embedded generation scheme, the operating hours could be upwards of 8000 hours per year. This equates to upwards of 120000 hours over the same 15 year period. In this case the temperature of the windings must be reduced using a relatively lower kVA rating.

For this reason, customers will be offered a class F or even B kVA rating on the alternator.

The thermal damage curve illustrates the variation of insulation life with operating temperature and is used in conjunction with other tools to enable sizing and selection decisions to be made when assigning a rating to an alternator for a particular power generation application.

Generator set rating

The generator set rating philosophy introduces the concept of standby and prime power applications. Guided by ISO8528-1 the definition of a generator set rating is categorised by application in four ways.

■ Emergency Standby Power, (ESP rating)

■ Limited Time Prime, (LTP rating)

■ Prime Rated Power, (PRP rating)

■ Continuously Operating Power, (COP rating)

Each definition states the following criteria:

■ Load type

This is stated as a variable load, where load cycles vary in magnitude and duration or a constant load, where the load remains at a constant level but the duration of the load cycle may vary.

■ Number of operating hours per year

An average load level is compared to the assigned rating for the generator set and is stated as a percentage of the rated kVA. The generator set must be capable of operation at the average load level for the stated number of hours.

■ Overload requirement

The overload capability is stated as a percentage above the assigned kVA rating that can be applied for a stated duration.

The terminology and definition of alternator ratings varies across the industry and doesn’t align well with that used for generator sets. The definitions for alternators are guided by international standards ISO 8528-3, IEC 60034-1 and some national standards such as NEMA MG1-32 (North America) which in themselves introduce further variation.

The table below summarises the definitions according to ISO 8528-1 for the generator set and a combination of ISO 8528-3 and IEC 60034-1 for the alternator. Despite the differences the table shows how to align the rating and operating duty of the alternator with the generator set.

It is important to note that the GOEM may decide to offer higher levels of performance compared to the standards e.g. higher average loads or longer durations of operation.

This needs to be taken in to account when matching the alternator to the generator set rating and application.


For more information please contact the Applications team