K-factor rated transformers

K-factor rated transformers are capable of withstanding the heating effects caused by harmonic load currents. They are specially designed for non-linear harmonics generating loads. K-factor transformers are also known as K-rated transformers. It is defined in the ANSI/IEEE C57.110 standard.

Electronic circuits, switched-mode power supplies and semiconductor switches draw a large amount of non-linear current causing distorted current. This distorted current result in harmonics which have an adverse effect on transformers. It is necessary properly rate the transformers so that it can withstand the temperature rise cased by the harmonics generated by non-linear loads.

Definition of K-Factor rating

It is the value that determines how effectively a transformer can handle harmonic currents while maintaining the temperature rise well within the limits.

Formula of k-factor
Where, h is the order of harmonics and Ih is the fraction of total rms load current at h-order harmonics.

K-factor rating ranges from 1 to 50. Transformers having K-factor of 1 can handle linear loads only and those rated 50 can withstand harsh harmonics. For attained better K-factor ratings, transformers should be made capable of withstanding extra heats caused due to harmonics.

K-factor rating transformer

Design consideration

A K-factor transformer is designed to withstand core and conduction losses caused due to harmonics. Their construction varies slightly from the standard transformers.

  • The neutral conductor of a k-factor transformer is 200% oversized than the standard transformers. This can help in mitigating the effects of third-order harmonics that sums up in the neutral conductor.
  • The heat produced due to eddy current can be controlled by providing electrostatic shielding between primary and secondary windings.
  • Transformer leads are made bigger.
  • Ferromagnetic core with lesser inductance is used.
  • Multiple smaller conductors are used in the secondary winding of the transformer to reduce skin effect.

Typical load K-Factor ratings

The below table shows the typical load k-factor ratings.

K-1Motors, incandescent lighting, resistance heating
K-4Welders, Induction heating, HID lighting
Fluorescent lighting
K-13Telecommunications equipment, Classroom facilities, Healthcare facilities, Production lines, Uninterrupted power supplies
K-20Office buildings, Data processing equipment, Adjustable speed drives, Computer installations, Instrumentation
K-40Motors driven by variable frequency drive, soft starter


Harmonics are generated by all non-linear loads. If transformers are not capable of withstanding the heating effects of harmonics, they may fail. In order to overcome heating effects, transformers must be oversized, so that it will run at a fraction of its rated capacity or K-rated transformers have to be used. K-factor transformers are specially designed to withstand the harmonics and operate at full load without de-rating.

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