Harmonic
Calculator
Calculate Total Harmonic Distortion (THD) and K-factor from individual harmonic currents. Determine the correct K-rated transformer for your non-linear loads.
THD, K-Factor & Transformer Selection
VFDs, UPS systems, switch-mode power supplies, and electronic ballasts all draw non-sinusoidal current. These loads inject harmonic currents back onto the electrical system, distorting the voltage waveform.
Use a power quality analyser to measure individual harmonic currents as a percentage of fundamental. Alternatively, use the presets below for typical load types — or ask your drive manufacturer for a harmonic spectrum datasheet.
THD summarises total distortion as a percentage. K-factor weighs each harmonic by its order squared, reflecting the extra heat it causes in transformer windings — higher-order harmonics are disproportionately damaging.
Match or exceed the calculated K-factor with a standard K-rated transformer (K-1, K-4, K-7, K-13, K-20). K-rated transformers include oversized neutral conductors, electrostatic shielding, and special core designs to handle harmonic loads without overheating.
Total Harmonic Distortion (THD) is the ratio of all harmonic currents to the fundamental (60 Hz) current, expressed as a percentage. High THD causes transformer overheating, nuisance tripping of breakers, neutral conductor overloading (especially with triplen harmonics), and reduced equipment life. IEEE 519-2022 recommends THD below 5% at the point of common coupling.
K-factor quantifies how much harmonic content a transformer must handle. K-1 transformers are designed for purely linear loads. K-4 through K-20 are specifically designed with oversized neutral conductors, electrostatic shielding, and special core materials. Specifying the correct K-rating prevents premature transformer failure from eddy-current losses and hot spots caused by harmonics.
6-pulse VFDs (most common) produce predominantly 5th and 7th harmonics, with smaller amounts of 11th, 13th, 17th, and 19th. The 5th harmonic is typically 20–40% of fundamental. 12-pulse VFDs cancel the 5th and 7th through phase-shifting, leaving 11th, 13th, 23rd, and 25th as dominant harmonics — resulting in much lower THD.
For a single 6-pulse VFD serving less than 30% of a transformer's load, K-4 or K-7 is typically adequate. When VFD loads dominate (>50% of transformer capacity), specify K-13. For heavy VFD or UPS installations exceeding 75% of capacity, use K-20. In practice, K-13 is the most commonly specified rating for industrial facilities with multiple drives.
Need a K-Rated Transformer?
We supply K-4 through K-20 transformers in stock and custom configurations. Our engineers can review your harmonic profile and confirm the correct rating.