Tool
THD calculator
Enter the RMS value of each harmonic and the fundamental, and get total harmonic distortion plus the dominant order.
THD is the RMS sum of all harmonic components divided by the fundamental component, expressed as a percentage. The formula is √(H₂² + H₃² + … + Hₙ²) ÷ H₁ × 100, where each H is an RMS magnitude.
Total harmonic distortion
Enter magnitudes in any consistent unit — amps or volts. Leave unused orders blank.
Calculation follows IEC 61000-4-7 grouping: each order is taken as a single RMS value rather than split into sidebands. For compliance work, use the analyser's own aggregated output.
# M = RMS magnitude of each order, M1 = fundamental
True RMS = √( M1² + Σ Mh² )
A worked example from a real drive
A six-pulse drive with a DC reactor draws 100 A fundamental. The analyser reports 32 A at the 5th, 18 A at the 7th, 9 A at the 11th, and 7 A at the 13th. Smaller orders are negligible.
| Order | RMS (A) | Square |
|---|---|---|
| 5th | 32 | 1,024 |
| 7th | 18 | 324 |
| 11th | 9 | 81 |
| 13th | 7 | 49 |
| Sum | 1,478 |
The RMS sum of harmonics is √1,478 = 38.4 A. Divided by the 100 A fundamental, THD is 38.4%. The true RMS current is √(100² + 1,478) = 107.1 A, which is 7.1% more heating than the fundamental alone suggests.
That last figure is the one that matters for cable and transformer sizing. A 38% THD does not mean 38% more heating — it means 7% more, because the squares add.
Why THD rises when load falls
THD is a ratio with the fundamental in the denominator. When production stops and the plant runs at 15% load, the fundamental current drops sharply while the harmonic current from switch-mode supplies and standby UPS units drops less. The result is a higher THD percentage with fewer harmonic amps.
This produces two opposite mistakes. A compliance reading taken at light load may show a violation that does not exist at full load. A filter sized from a light-load reading will be undersized, because filters are rated in amps and the amps are highest at full load.
Always express harmonic content in amperes before sizing equipment. Percentages are for reporting, amperes are for engineering.
Reading the result against a limit
| System voltage | THD limit | Individual harmonic limit |
|---|---|---|
| ≤ 1 kV | 8.0% | 5.0% |
| 1–69 kV | 5.0% | 3.0% |
| 69–161 kV | 2.5% | 1.5% |
Current limits are not a single THD figure — they depend on the short-circuit ratio. See IEEE 519 limits for the full table and the TDD definition.
Sizing a filter from your analyser export?
Send the harmonic spectrum in amps and the load profile. We will confirm the filter rating and flag any resonance risk with existing capacitors.