Standby vs. Prime Vs

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The Importance of Rating a Generator Engine Correctly

When looking to purchase a generator
(http://www.dieselserviceandsupply.com/Used-Generators/) the
rst 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:
Inde nite Running Time
The prime power rating (http://www.dieselserviceandsupply.com/Prime_vs_Standby_Power.aspx) 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 con gured 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

(http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=28330), speci cally
BS 5514 (http://www.datasheetarchive.com/BS5514*-datasheet.html)and DIN 6271
(http://www.macallisterpowersystems.com/405/generator-set-ratings.htm). It is always best to consult an
electrical contractor or a generator specialist to determine what generator and rating best suits your
speci c needs.
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Articles & Other Information

The Importance of Rating a Generator Engine Correctly

Standby Power Rating

Standby power rated generators are the

Standby power rating should be applied to

Prime Power Rating

Inde nite Running Time

The prime power rating (http://www.dieselserviceandsupply.com/Prime_vs_Standby_Power.aspx) is the

Limited Running Time

Prime power is accessible for a limited number of

Continuous Power Rating

Continuous power rating is used in applications where

Elevations and Temperature’s Effect on Power Rating

More information on power ratings is available under guideline ISO 3046

>>Back to Articles & Info<< (http://www.dieselserviceandsupply.com/Articles_info.aspx)

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