UFC 3 -520-01
June 10, 2002
Note: Derating applies to the full-load capability of the t ransformer when applied
in an environment containing significant harmonic distortion. If the transformer is
not fully loaded, the derating process might have little or no practical significance
unless it is expected that the transformer will eventually be fully loaded.
Nationwide surveys indicate average loading levels for dry-type transformers of
between 35 percent for commercial facilities and 50 percent for industrial
Refer to ANSI/IEEE C57.110 for additional information regarding t he
evaluation and derating process.
If it is determined that a transformer will require derating because of harmonic
distortion, perform the following additional reviews:
12-4.4.1 Determine if the harmonic distortion environment can be improved by design
changes for the most offending loads.
12-4.4.2 If the transformer requires more than 10 percent derating, evaluate the
feasibility of installing a new transformer designed for a harmonic distortion environment
(often referred to as a k -factor transformer). Include delivery and replacement time
scheduling as well as cost in the evaluation.
12-4.4.3 If transformer derating is the selected option, annotate the percent derating
on the applicable design drawings and install a label near the transformer nameplate
indicating that the transformer has been derated. The purpose of these actions is to
prevent inadvertent overloading of the transformer in the future.
If it is determined that an installed transformer requires derating, perform a
transformer inspection to confirm whether the transformer has been operating in an
overloaded condition. Because of the method by which flux is produced in a
transformer, current heating paths can develop along any magnetic material. Signs of
overheating include the following:
General excessive heating of the enclosure.
Bubbled, peeling, discolored, or burnt paint on the enclosure.
Evidence of overheating at the ends of internal components.
Premature gasket failure.
Methane gas generation.
Heat-damaged b ayonet fuse holders.