Free engineering tool
THD calculator
with IEEE 519 and EN 50160 checks.
Enter a voltage or current harmonic spectrum. Get THD, THD-R and TDD, and see which harmonics break the IEEE 519-2022 limits. No sign-up.
Harmonic spectrum
| Order (h) | Value |
|---|
Result
How the THD calculator works
Total harmonic distortion (THD) is the RMS sum of all harmonic components divided by the fundamental. Enter each harmonic as a percentage of the fundamental, or as an RMS value in volts or amps with the fundamental alongside it, and the calculator returns:
- THD relative to the fundamental, the definition used by IEEE 519 and EN 50160;
- THD-R relative to the total RMS value, which some instruments display instead and which is always lower;
- for current, TDD (total demand distortion), which is the harmonic current divided by the maximum demand current IL rather than by today's load.
The formula, with Vh the RMS value of harmonic order h and V1 the fundamental:
THD = √(V2² + V3² + … + V50²) / V1 × 100 %
Most standards sum harmonics up to the 50th order (EN 50160 up to the 40th). Leaving out small high-order values changes the result very little, because each one is squared.
THD or TDD: why current distortion can look alarming at light load
THD of current is measured against the fundamental current flowing now. At light load a drive can show a current THD of 80 % or more, yet the actual harmonic amps are small. That is why IEEE 519 limits current with TDD, which divides by the maximum demand current IL (normally the average of the monthly peak demands over 12 months). If you only have one reading, enter IL as a percentage of today's fundamental: for example, if the site's usual peak demand is twice today's load, enter 200.
IEEE 519 limits used by the calculator
IEEE 519 sets limits at the point of common coupling with the utility, not at each machine. Voltage limits depend on the bus voltage:
| Bus voltage at the PCC | Individual harmonic | THD |
|---|---|---|
| 1 kV and below | 5.0 % | 8.0 % |
| Above 1 kV up to 69 kV | 3.0 % | 5.0 % |
| Above 69 kV up to 161 kV | 1.5 % | 2.5 % |
| Above 161 kV | 1.0 % | 1.5 % |
Current limits depend on the short-circuit ratio ISC/IL: a stiff supply (high ratio) can absorb more harmonic current before the voltage distorts. For systems from 120 V to 69 kV, as a percentage of IL:
| ISC/IL | h < 11 | 11–16 | 17–22 | 23–34 | 35–50 | TDD |
|---|---|---|---|---|---|---|
| Below 20 | 4.0 | 2.0 | 1.5 | 0.6 | 0.3 | 5.0 |
| 20 to 50 | 7.0 | 3.5 | 2.5 | 1.0 | 0.5 | 8.0 |
| 50 to 100 | 10.0 | 4.5 | 4.0 | 1.5 | 0.7 | 12.0 |
| 100 to 1000 | 12.0 | 5.5 | 5.0 | 2.0 | 1.0 | 15.0 |
| Above 1000 | 15.0 | 7.0 | 6.0 | 2.5 | 1.4 | 20.0 |
The 2022 edition kept these values but relaxed the rule for even harmonics: orders 2, 4 and 6 are limited to 50 % of the value in the table, and even orders above 6 to 100 % (the 2014 edition used 25 % for all even orders). The calculator applies the 2022 rule. Summary of the change: IEEE 519-2022 update, powerquality.blog and MIRUS application note AN003.
These values are reproduced for screening. For a contract, a utility discussion or a compliance report, use the standard itself.
EN 50160 and voltage THD
For low- and medium-voltage public supply, EN 50160 expects voltage THD to stay at or below 8 % (harmonics up to the 40th) for 95 % of the 10-minute values over a week. A single reading above 8 % is therefore not a breach on its own, and a single reading below it does not prove compliance.
What to do with a high result
- Find where the harmonics come from. Measure at the incomer and on the largest non-linear loads (drives, rectifiers, UPS, LED lighting, EV chargers) at the same time.
- Check for resonance before adding capacitors. Power factor capacitors can amplify a harmonic instead of reducing it. Our harmonics and power factor guide explains the risk.
- Measure over time. Harmonic levels change with production, shift patterns and the network around you. IEEE 519 itself is assessed on 95th and 99th percentile values over days and weeks.
- Choose the mitigation on data. Line reactors, passive or active filters and multi-pulse drives each suit a different spectrum.
Frequently asked questions
What is a good THD value?
For voltage at low voltage, under 5 % is comfortable and 8 % is the common limit in both IEEE 519 and EN 50160. For current, judge TDD against your short-circuit ratio, not THD against a fixed number.
Why is my current THD so much higher than my voltage THD?
Non-linear loads draw distorted current; the voltage only distorts in proportion to that current times the source impedance. A stiff supply keeps voltage THD low even when current THD is high.
Is THD the same as THD-F and THD-R?
THD-F (or simply THD) uses the fundamental as the reference. THD-R uses the total RMS value and is always lower. IEEE 519 and EN 50160 use the fundamental.
Can I use this calculator for compliance?
No. It is a screening tool for one set of values. Compliance needs a measurement campaign with a suitable instrument at the point of common coupling. Talk to an Inspector engineer about measuring it properly.