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The Importance of Rating a Generator Engine Correctly

When looking to purchase a generator the first thing to consider is the intended use. It may seem obvious what power rating is required for an application but there are a number of factors to consider before purchasing a generator. The guidelines set forth in this article will better explain the proper application of generator drive engines in AC Generator Set installations.


Standby Power Rating

Standby power rated generators are the most commonly rated generator sets. Their primary application is to supply emergency power for a limited duration during a power outage. With standby rated generators there is no overload capability built into the units. It is important to note that standby rated generators, under no circumstances, should run in conjunction with a public utility source.

Standby power rating should be applied to the unit where public utility power is available. The typical rating for a standby engine should be sized for a maximum of 80% average load factor and roughly 200 hours per year. This includes less than 25 hours per year of running time at the standby rating. Standby power ratings should never be applied except in true emergency outage situations. Predetermined outages with the utility company, under UL guidelines, are not considered emergency outages. Manual load shifts for testing purposes can be performed with most automatic transfer switches.


Prime Power Rating

Prime power rated generators should be used in applications where the user does not purchase power from a public utility. Prime power applications fall under two distinct categories:

Indefinite Running Time

The prime power rating is the maximum power accessible at the variable load for an unlimited number of hours per year in a variable load setting. It is not advisable that the variable load exceed 70% average of the prime power rating during any operational period of 250 hours. If the engine is running at 100% prime power, yearly hours should not exceed 500. Overload situations should be avoided however a 10% overload capability is available for a 1 hour period within a 12 hour cycle of operation.

Limited Running Time

Prime power is accessible for a limited number of hours in non-variable load situations. Limited prime power is intended for circumstances where power outages are expected, such as a planned utility power reduction. Engines in generator sets may operate up to 750 hours per year at power levels less than the maximum prime power rating. In these situations it is important to never exceed the prime power rating. The end user should be aware that constant high load use will reduce the life of any engine. It is recommended that any application requiring over 750 hours per year that the engine be continuous power rated.


Continuous Power Rating

Continuous power rating is used in applications where supplying power is at a constant 100% load for an unlimited number of hours each year. Continuous power rated units are most widely used in applications where the power grid is unreachable. Such applications include mining, agriculture or military operations.


Elevations and Temperature’s Effect on Power Rating

Elevation and temperature are factors to consider before rating the engine. The engine may be operated at 3,000 ft. of altitude and at a temperature of 100° F without deration for standby power rating. For prime power rating the engine may be operated at 5,000 ft. of altitude and at a temperature of 100° F without power deration. For continuous duty operations at higher altitudes, the engine should be configured to limit performance by 3% per 1,000 ft. of altitude and 1% per 10° F inlet air temperature.

More information on power ratings is available under guideline ISO 3046, specifically BS 5514 and DIN 6271. It is always best to consult an electrical contractor or a generator specialist to determine what generator and rating best suits your specific needs.

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