UFC 3 -520-01
June 10, 2002
full load. This is why transformer k -factor ratings must be based on full-load conditions.
Nationwide surveys indicate average loading levels for dry-type transformers of
between 35 percent for commercial facilities and 50 percent for industrial facilities. With
such a light loading, a general purpose transformer will provide acceptable
performance. A k -4 rating will provide acceptable performance in all but the most
extreme harmonic distortion environments.
4-4.3.5 In almost all applicati ons, the service entrance transformer will be acceptable
if it is a general purpose dry-type transformer rather than a k -rated transformer. An
individual lower-voltage transformer within the facility might need a k-factor rating (or
derating if it is a general purpose transformer) under the following conditions:
It supplies a large concentration of nonlinear electronic equipment, and
It is operating near full load or there is a reasonable expectation that it will eventually
be fully loaded.
4-4.3.6 Equipment suppliers can provide bundled power distribution systems that
contain k-rated transformers or otherwise address power quality issues. Evaluate the
applicability of this equipment before selecting a k -rated transformer.
Specialty Transformers. Specialty transformers include control, industrial
control, Class 2, signaling, ignition, and luminous tube transformers. Select these
transformers using National Electrical Manufacturers Association ( NEMA) ST 1,
Specialty Transformers (Except General P urpose) , as a guide.
TRANSFORMER INSTALLATION CRITERIA.
4-5.1.1 NEC Article 450 (2002 Edition) provides specific criteria applicable to
transformers and transformer installations. The NEC location and installation criteria
are applicable as described in this manual.
4-5.1.2 For each of the specified criteria in the NEC, exceptions are often provided.
As part of any installation design, review the NEC to ensure that applicable criteria,
including allowed exceptions, are satisfied. Regardless of the location, ensure
transformers have adequate ventilation to avoid overheating. Comply with clearances
specified by NEC Articles 110.26 and 110.34 (2002 Edition) for installations below 600
volts or above 600 volts, respectively.
4-5.2.1 Dry-type transformers, available at voltage ratings of 15 kV and below, are
cooled primarily by internal air flow. The three principal classes of dry-type transformers
are: self-cooled (AA), forced -air cooled (AFA) and self-cooled/forced -air cooled (AA/FA).
Self-cooled transformers require adequate room ventilation to ensure proper