UFC 3 -520-01
June 10, 2002
13-4.3.2 Depending on the electrical dist ribution system, determining the required UPS
size can be difficult, especially if a number of noncontinuous loads are present. When
the actual load can vary, size the UPS for 125 percent of the maximum expected load to
provide margin for any uncertainty in the total load. The UPS should be sized for the
following:
125 percent of the total real power load.
125 percent of the total reactive power load.
0.8 lagging power factor unless an actual power factor is known.
13-4.3.3 The above sizing criteria a ssume that no single load represents either a large
portion of the total load or has a high inrush current. Some static UPS systems have
very little overload capability and it is usually not practical or economical to oversize the
UPS so that it is capable of satisfying inrush or short-circuit current requirements. On
some UPS designs, the output voltage will collapse as the UPS is overloaded.
13-4.3.4 If high inrush starting currents loads can exceed the UPS rating, evaluate the
following design options :
Selective loading of individual loads.
UPS with guaranteed overload capability sufficient for inrush currents.
Static switch bypass when starting inrush loads.
Rotary UPS design to obtain transient load-change capability.
13-4.3.5 The preferred method of establishing the expected UPS loading is to perform
a site survey, if a UPS is to be added to an existing system. The UPS should not be
sized based on the total connected using nameplate information if a site survey
demonstrates that the actual loading is substantially less than this amount. For each
load or panel, record the following information to document the actual facility loads.
Ensure that measurements capture peak loading periods. Determine future facility load
requirements by estimating the following information for each location.
Reading location, such as a panel or specific load.
Phase currents (record each phase separately).
Neutral current.
Load type.
Voltage.
13-9
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