K-Factor Transformer

Standard distribution transformers are designed for sinusoidal 50/60 Hz current. Harmonic currents from non-linear loads — computers, LED drivers, VFDs, UPS systems — increase transformer winding losses in two ways: I²R losses increase proportionally to the total RMS current, and eddy current losses increase with the square of the harmonic order (a fifth harmonic component causes 25 times more eddy current loss than the same magnitude at fundamental frequency). The K-factor is defined as the sum of (Ih/I1)² × h² for all harmonics, where Ih is the harmonic current, I1 is the fundamental current, and h is the harmonic order. A purely sinusoidal load has K = 1; typical office loads with many switch-mode power supplies might have K = 4–9; heavy industrial VFD installations might reach K = 13–20. K-factor transformers have reinforced windings with lower eddy current losses, oversized neutral connections to handle triplen harmonics, and enhanced cooling to manage the additional heat. Common K-factor ratings are K-4 (light harmonic loading), K-9 (moderate), K-13 (heavy), and K-20 (very heavy). Specifying the correct K-factor prevents premature transformer failure, overheating, and the associated fire risk in harmonic-rich environments.