UFC 3 -520-01
June 10, 2002
3-1.2.1 Apply a frequency of 60 Hertz for distribution and utilization power. Other
frequencies, such as 400 Hertz, are used to serve specific loads or subsystems where
required by the using agency.
3-1.2.2 In locations in which the commercially-supplied frequency is other than 60
Hertz, such as 50 Hertz, use the available supplied frequency to the extent practical.
Where frequencies other than that locally available are required for technical purposes,
frequency conversion or generation equipment can be installed. The facility user will
normally provide this equipment.
3-1.3.1 Utilization equipment with an inductive load characteristic should have a
power factor of not less than 0.8 to 0.9 lagging under full load conditions as a design
goal. Generally, a load group of utilization equipment will have a power factor of
between 0.8 to 0.9 lagging if it is comprised of mostly motors, electromagnetic ballasts,
and incandescent lights. Electrical systems containing mostly motors might have power
factors closer to 0.8 lagging, while loads containing mostly electronic ballasts and
incandescent lamps will have power factors closer to unity.
3-1.3.2 Power factors lower than 0.9 lagging are not as energy efficient as desired.
Refer to paragraph 8 -7 if power factor correction is considered. Power factor correction
requires careful coordination with power quality design features.
Neutral Conductor Grounding. Solidly ground the neutral conductor of all
distribution systems operating at phase-to -phase voltages of 600 volts or less, except
for applications such as continuous processes for industrial systems where shutdown
would create a hazard, loss of materials, or equi pment damage. For those applications,
evaluate the use of a solidly grounded wye system with a backup power supply, or a
high-resistance grounded wye system. Use of other than solidly grounded systems
must be justified on the basis of the need for service continuity. If an ungrounded
system is used, include a ground detection system in the design to alert personnel of an
NORMAL POWER SOURCE.
Normal source systems should usually consist of radial distribution
configurations consisting of a single transformer for each building or group of buildings
for loads of 150 kVA or less at 208 volts, or 2,000 kVA or less at 480 volts. Figure 3 -1
shows an example of this arrangement. Higher kVA ratings are allowed provided that
the design analysis demonstrates that the system reliability and economic operation are
acceptable. The service entrance transformer size will establish interrupting rating and
coordination requirements for downstream equipment, which can alone necessitate
either dual transformers serving separate loads at the service entrance or a higher