How is THD calculated?

THD is the root-sum-square of all the harmonic magnitudes divided by the fundamental, expressed as a percent: THD equals the square root of the sum of the squares of H2 through Hn, divided by H1, times one hundred. This is the THD-F form referenced by the fundamental that IEEE uses.

Can I enter harmonics as percentages or as actual values?

Both. If you enter percents of the fundamental, H1 is taken as one hundred percent and the result follows directly. If you enter absolute amps or volts, also enter the fundamental magnitude so the tool can divide by it. The THD value is the same either way.

What are the IEEE 519 voltage limits?

IEEE 519-2022 caps voltage THD at the point of common coupling at eight percent for buses up to one kilovolt, five percent up to sixty-nine kilovolts, two and a half percent up to one hundred sixty-one kilovolts, and one and a half percent above that. The tool checks your result against the class you select.

Why does current distortion use TDD instead of THD?

IEEE 519 limits current distortion as Total Demand Distortion, referenced to the maximum demand load current rather than the instantaneous fundamental. The allowed TDD also depends on the short-circuit to load current ratio, so current limits require short-circuit data and are not auto-evaluated here.

What causes high harmonic distortion?

Non-linear loads draw current in pulses rather than smoothly. Variable frequency drives, switch-mode power supplies, LED drivers, and rectifiers are common sources. High distortion overheats transformers and neutrals, trips breakers, and can disturb sensitive equipment, which is why IEEE 519 sets limits.

Total Harmonic Distortion (THD) Calculator

Total harmonic distortion measures how far a voltage or current waveform departs from a clean sine wave. Non-linear loads such as variable frequency drives and switch-mode power supplies inject harmonics that overheat equipment and disturb the grid. This tool computes THD from the individual harmonic magnitudes and checks it against the IEEE 519 limits.

How it works

THD referenced to the fundamental is the root-sum-square of every harmonic divided by the fundamental:

THD = sqrt(H2^2 + H3^2 + ... + Hn^2) / H1 x 100%

If you enter the harmonics as percent of the fundamental, H1 is one hundred percent and the math is direct. If you enter absolute amps or volts, the tool divides the harmonic root-sum-square by the fundamental you supply. Either way the result is the same percentage.

Limits and interpretation

The tool compares your voltage THD against the IEEE 519-2022 limits at the point of common coupling, which tighten as the bus voltage rises: eight percent at low voltage down to one and a half percent above one hundred sixty-one kilovolts. Current distortion is governed separately as Total Demand Distortion and depends on the short-circuit to load ratio, so evaluate current limits against the relevant table with your system’s short-circuit data. Persistent high distortion usually means a large non-linear load needs harmonic filters, a multi-pulse drive, or a line reactor.