UFC 3 -520-01
June 10, 2002
CHAPTER 4
POWER DISTRIBUTION AND UTILIZATION --TRANSFORMERS
4-1
RATINGS.
4-1.1
Introduction.
4-1.1.1 The choice of the main supply service voltage should be based on an energy
economic analysis of the various electrical system options for any particular facility
under design. This is an integral part of determining the transformer primary voltage
rating.
4-1.1.2 An economic analysis of the selected transformer can be performed using the
analysis tools provided by the Energy Star program at http://www.energystar.gov.
4-1.2
Voltage and Current.
4-1.2.1 The transformer primary voltage rating must be determined by the voltage
available to the transformer from the facility electrical distribution system. Select the
transformer secondary voltage based on the required voltage on the secondary as
determined by the most economical facility distribution voltage, the largest loads with
previously fixed voltages, and energy usage density.
4-1.2.2 The rated secondary voltage is the voltage at which the transformer
secondary is designed to deliver the rated kVA capacity. Transformer windings are
connected in either series or parallel to obtain the desired output (secondary) voltage.
Common nominal voltages are shown in Table 4 -1.
Table 4-1. Common Primary and Secondary Transformer Voltages
Single-Phase Transfo rmer
Three-Phase Transformer
Primary
Secondary
Primary
Secondary
240 480
120
208
208Y/120
120/240
240
240/120
120/208/240/277
480
480Y/277
480
4,160
4,160
13,800
Note: Table 4 -1 does not include all possible combinations of transformer ratios.
Refer to Table 5 -1 for additional primary and secondary voltages that have been
used. Also, overseas facilities will commonly be d esigned around a 400Y/230
volt, 50 Hertz system. Contact the transformer vendor for non-standard voltage
and frequency ratings.
4-1
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