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Generator Sizing for Single-Phase Symmetra UPS Systems

Issue:
What is the recommended generator size for a single phase Symmetra UPS system?

Product Line:
Symmetra Power Array
Symmetra RM
Symmetra LX

Environment:
All serial ranges

Cause:
A generator must be properly sized to ensure that the UPS is able to operate online while the generator is providing power to a user's facility. If the generator is not sized properly, the UPS may run on battery or switch back and forth between line and battery operation while the generator is running.

Resolution:
A generator should be sized to 1.5 to 2 times the size of the total load to be attached to the generator in order to be compatible with a Symmetra. This includes load that is not attached to the UPS.

Many standard off-the-shelf Gensets are sized at 0.8 power factor for continuous duty operation ( > 30 mins of runtime ). When the Genset is sized < 1.5 times Symmetra frame size, the interaction of the Symmetra PFC (power factor corrected) circuitry with output impendence of the generator results in generator output waveform distortion. This distortion can lead to battery operation and/or alarms on the Symmetra related to input voltage & frequency.

For these reasons, the recommendation is to size the generator at 1.5 times the Symmetra's full capacity and not exceed 75% of the generator's load capacity. (This load includes other devices that may be powered by the generator but not by the UPS such as AC, heating, and lighting).