UFC 3 -520-01
June 10, 2002
CHAPTER 3
GENERAL POWER SYSTEM CRITERIA
3-1
CHARACTERISTICS.
3-1.1
Voltage.
3-1.1.1 Select electrical characteristics of the power system to provide a safe,
efficient, and economical distribution of power, based upon the size and types of loads
to be served. Use distribution and utilization voltages of the highest level that is
practical for the load to be served. The following guidelines apply:
3-1.1.1.1 Generally, single -phase, three-wire 120/240 volt systems are used to serve
single-phase lighting and power loads less than 50 kilovolt-amperes (kVA). This voltage
level is usually provided to small-scale and residential facilities.
3-1.1.1.2 Three-phase, four -wire, 208Y/120 volt systems are normally used for lighting
and power loads less than 150 kVA. A 208Y/120 volt system is usually most
economical when most of the load consists of 120 volt lighting and utilization equipment,
and the average feeder length is less than 61 meters (200 feet). This voltage level is
usually provided to commercial-type facilities that do not have extensive fluorescent
lighting or motor loads. Although there are many existing 208Y/120 volt systems, the
480Y/277 volt system is preferred wherever possible because the higher voltage is
more energy-efficient and has more potential for future load growth.
3-1.1.1.3 Three-phase, four -wire, 480Y/277 volt systems should normally be used for
lighting and power loads greater than 150 kVA. This voltage level should also normally
be used if large motors are a significant portion of the total load or if most of the load
can be served by 480Y/277 volts. In this case, design lighting to operate at 277 volts
unless specifically prohibited by NEC criteria (such as dwelling units). All three-phase
motors should be served at 480 volts. Utilize dry-type transformers as needed to serve
smaller 208Y/120 volt loads.
3-1.1.1.4 Perform an economic analysis for 208 volt systems larger than 300 kVA or
serving motors larger than 25 horsepower (18,650 watts).
3-1.1.2 Justify exceptions to the above guidelines based on the existing facility design
or on unique requirements of the proposed design.
3-1.1.3 Use the standard available voltages of the host nation for facilities located
outside of No rth America; refer to TM -5-688 for additiona l information. Apply the
highest distribution and utilization voltage level that is practical for the load to be served.
Use power conversion equipment as necessary to satisfy voltage requirements of
specific equipment.
3-1.1.4
Other voltage levels can be used as necessary to serve specific loads.
3-1
Previous Page
