# Understanding Generator Set Ratings

Generator set ratings may seem complex, but their basic purpose is simple: fit the customer’s needs at the optimum reliability, performance, and cost. An improper rating means a customer either buys more capacity than needed or risks shorter life to overhaul, more repairs, and more downtime. Read more to learn how smart rating choices combined with the latest in onsite power generation systems can help engineers match equipment to their customers’ applications for optimal long-term performance.

**UNDERSTANDING EQUIPMENT APPLICATIONS AND CAPABILITIES**

Available ratings have changed in recent years, and more sophisticated switchgear can be integrated with generator sets. That means more flexibility to specify generating systems that closely match users’ requirements. The key to choosing the right rating is to understand the application in detail. That means knowing the type of duty and answering several key questions:

- What is the average load factor?
- What is the maximum required load?
- How many hours per year will the generator sets run?
- Will the generator sets be run isolated from or in parallel with the utility?

**RATINGS DEFINED**

Caterpillar defines five basic generator set ratings:

- Standby
- Emergency Standby Power (ESP)
- Mission Critical Standby
- Prime
- Continuous

Cat generator set ratings differ in certain respects from those defined by the industry standard ISO 8528-1. Basic descriptions of the Caterpillar genset ratings are offered below.

**STANDBY RATING**

In this application, the generator set is capable of providing emergency backup power at the nameplate rating for the duration of an outage.

The average load factor of a Standby rated generator set should be no more than 70% of the nameplate rating and applied to varying loads. A Standby generator set can run for a maximum of 500 hours per year. The normal standby rating is not for use in utility paralleling applications. For example, a 3 MW standby rated generator set will provide power for the duration of an outage. It should be run for up to 500 hours per year and have an average load factor of 2.1 MW.

**EMERGENCY STANDBY POWER (ESP) RATING**

The ESP rating differs from the Standby rating only in the number of running hours allowed per year. ESP ratings allow a maximum running time of 200 hours per year at a 70% average load factor with varying load.

**MISSION CRITICAL STANDBY RATING**

In this application, the generator set is capable of providing emergency backup power at the nameplate rating for the duration of an outage. The average load factor of a mission critical standby rated generator set should be no more than 85% of the nameplate rating with varying loads. A mission critical standby generator set can run for a maximum of 500 hours per year.

Typical peak demand is 100% of the rating for maximum of 5% of the operating time. The mission critical standby rating is not for use in utility paralleling applications. For example, a 3 MW mission critical standby rated generator set will provide power for the duration of an outage. It should be run for up to 500 hours per year and have an average load factor of up to 2.55 MW.

**PRIME RATING**

In this application, the generator set is capable of providing power to a varying load for an unlimited number of hours per year. A Prime rated generator set is capable of providing full nameplate rating for a period of time, but must have an average load factor of no more than 70% of the Prime rating. Ten percent overload is allowed for emergencies for a maximum of one hour in 12, and for no more than 25 hours per year. The standard prime rating is for use in either utility paralleling or isolated applications.

For example, a 2.7 MW rated unit may provide the full nameplate rating for a short duration, but should have a maximum average load of 1.89 MW (not including generator set non-running time per ISO8528-1). The generator set can also provide 3 MW of power in emergencies as defined above. An example of the Prime rating is shown in Chart 3.

**LOAD MANAGEMENT RATING**

The Load Management rating is simply a special application of the Prime rating. A Prime rated generator applied under load management guidelines allows for a Prime rated generator set to be used in parallel with the utility.

A Prime rated generator set under load management guidelines can run for a maximum of 500 hours per year. This generator set has the same nameplate rating as a Prime rated unit, but allows for an average load factor of up to 100%. The Prime rating with load management guidelines does not allow for a 10% overload capability. For example, these guidelines state that a 2.7 MW unit (same nameplate rating as the Prime rated unit) can be run at 2.7 MW for a maximum of 500 hours.

There are two basic load management practices:

**base loading**and

**peak shaving**. In base loading, the generator set operates at a fixed kW output, and the utility provides power for any peaks above that level. In this scenario the end user may export power to the grid if more power is being generated then is required by the facility loads, and the appropriate agreements with the utility are in place.

**CONTINUOUS RATING**

In this application, the generator set is able to provide power to a non-varying load for an unlimited number of hours per year. The average power output of the generator set is 70% to 100% of the rating. The rating is designed to provide 100% of the rating for 100% of the operating hours.

Typical Continuous rating applications include base loading in parallel with the utility and cogeneration operations. An example of a Continuous rated generator set is shown in Chart 6.

**PERFORMING DUAL DUTY**

Intelligent use of ratings also can help your customers use power systems for the added purpose of load management. Here, advanced switchgear is part of the equation.

Customers often want to use backup generators for load management to produce an additional return on their investment. However, standby rated generator sets are not intended for operation in parallel with the utility. To perform utility paralleling applications a prime rating with load management guidelines or a continuous rated unit are appropriate.

Paralleling switchgear combined with the correct prime rated units is required when operating under load management guidelines. The switchgear controls provide increased system flexibility allowing generator sets to operate in parallel with the utility. The switchgear is able to control the load on the generator sets and monitor the power supplied from the utility. This ensures that the generator sets are providing the proper load and are not operating outside of their rating guidelines.

**WANT TO DISCUSS THIS SOME MORE?**

Regardless of the application, generator set ratings help ensure that your customers’ power needs are met and that generating equipment is protected from premature wear. Choosing the right rating means making the proper tradeoffs between run hours, peak load, and average load. The proper rating means the customer receives the optimum combination of installed cost and long-term cost of ownership.

Stowers' Power Systems sales staff understands these generator set ratings and is well positioned to assist with rating choices and act as a safe source of information for supporting the entire power system.

Contact your

**Stowers Power Systems Sales Representative**for more information on choosing the best rating for your project.

**CALL NOW TO LEARN MORE**

Neal Reinhardt

**865-595-1034 or 865-771-1036**

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## CATERPILLAR GENERATOR SET RATINGS GUIDE

For Caterpillar diesel and gas-powered generator sets, 60Hz and 50Hz. (PDF 672KB)

## CATERPILLAR GENERATOR SET RATINGS DEFINITIONS

Get more details about the ratings definitions for Caterpillar diesel generator sets. (PDF 28KB)

## POWER SYSTEMS WHITE PAPER: A HOLISTIC VIEW ON GENERATOR SET RATINGS

Learn more about the factors to be considered when selecting a particular rating for a given application. (PDF 80KB)

## CATERPILLAR GENERATOR SET SPECIFICATION SHEETS

View specification sheets for your diesel generator set applications at Cat's website.

## CONTACT A SALES REP

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