UFC 3 -520-01
June 10, 2002
service entrance voltage rating. Generally, ratings above 2,000 kVA will necessitate the
use of a medium voltage system. Base the choice of the main supply service voltage
on an economic analysis of the various electrical system options for any particular
facility under design. If the total facility load is large, consider the impact of transformer
failure on the facility's mission and consider if dual transformers or backup power might
be needed.
Figure 3-1. Single Power Feed to Facility
Commercial Power
480Y/277V
Service Entrance
Facility Loads
3-2.2
Mission-critical facilities should normally use secondary-selective
configurations, consisting of double -ended transformer installations with normally open,
interlocked bus -ties, and either open or closed switchgear lineups (refer to Figure 3 -2).
Design double -ended transformer installations with incoming sections, transformers,
and primary bus in the middle, and the secondary distribution sections on the outside to
allow for future load growth. Size each transformer of a double -ended system to serve
approximately 60 percent to 80 percent of the total demand load served and ensure
each transformer and electric lines are capable of carrying greater than 100 percent of
the critical loads. This arrangement effectively provides a spare transformer for the
critical loads. Separate the two feeds from commercial power to the degree practical
consistent with the overall facility design to minimize the possibility of simultaneous
damage to both feeds by lightning, nearby excavation, or other reasons.
3-3
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