A conventional transformer has restricted use in an environment where non-linear loads are utilized, unbalanced input voltage or load current exceeds 5% total harmonic distortion.
K-factor rating is a value used to determine how much harmonic current a transformer can handle without exceeding it's maximum temperature rise level. K-factor values range from 1 to 50. K-factor of 1 is used for linear loads only, and a K-factor of 50 is used for the harshest harmonic environment possible.
The K-factor is a number derived from a numerical calculation based on the summation of harmonic currents generated by the non-linear load. The higher the K-factor, the more significant the harmonic current content. Details of the calculation method can be found in IEEE Standard 1100-1992.