K-Factor Transformers

Underwriters laboratory (UL) recognized the potential safety hazards associated with using standard transformers with nonlinear loads and developed a rating system to indicate the capability of a transformer to handle harmonic loads. The ratings are described in UL1561 and are known as transformer K-factors.

K-factor transformers are designed to reduce the heating effects of harmonic currents created by loads like those in the table below. The K-factor rating is an index of the transformer’s ability to withstand harmonic content while operating within the temperature limits of its insulating system.

Load K-Factor

  • Electric discharge lighting – K-4
  • UPS with optional input filtering- K-4
  • Welders- K-4
  • Induction heating equipment- K-4
  • PLCs and solid state controls (other than variable speed drives)- K-4
  • Telecommunications equipment (e.g. PBX) K-13
  • UPS without input filteringK-13
  • Multiwire receptacle circuits in general care areas of health care facilities and classrooms of schools, etc. K-13
  • Multiwire receptacle circuits supplying inspection or testing equipment on an assembly or production line K-13
  • Mainframe computer loads K-20
  • Solid state motor drives (variable speed drives) K-20
  • Multiwire receptacle circuits in critical care areas and operating/recovery rooms of hospitals K-20

To help get around the problem of successfully applying derating factors to conventional transformers, the K-factor is used by transformer designers to develop transformers made especially for non-linear loads and the extra heating caused by the harmonic currents. Transformers come in basic K-factors such as 4, 9,13, 20, 30, 40, and 50.

The strategy is to calculate the K-factor for your load and then specify a transformer with a K-factor of an equal or higher value. In this way, the transformer can be sized to the load without derating.

The advantage of using a K-factor transformer is that it is usually more economical than using a derated, oversized transformer.