THD and TDD calculator

Work out voltage or current total harmonic distortion, or current total demand distortion, from the RMS magnitude of each harmonic.

Electrical power and current Updated Free, no sign-up

Example values
What do you want to calculate?
Distortion
Measured values

RMS, 50 or 60 Hz component

V

RMS, 50 or 60 Hz component

A

At the point of common coupling

A

One order and RMS magnitude per line, such as 5: 12. Orders 2 to 50, in the same unit as the fundamental.

Current THD

14.97%

From harmonic orders 5, 7, 11.

Harmonic content, root-sum-square
14.97 A
Total RMS, fundamental and harmonics
101.1 A
Largest harmonic
5th, 12 A
0%5%10%15%20%3579111315

Orders not entered count as zero.

How it’s calculated
  1. IH = √(Σ Ih²)=√(12² + 8² + 4²)=14.97 A
  2. THD = IH ÷ I1 × 100=14.97 ÷ 100 × 100=14.97%

Harmonics totalling 14.97 A RMS against a fundamental of 100 A give a current THD of 14.97%. The largest is the 5th.

Tip: TDD divides by the maximum demand current, so a lightly loaded site shows a high THD but a low TDD. IEEE 519 current limits are written in TDD.

How to calculate THD and TDD

Total harmonic distortion (THD) compares the harmonic content of a voltage or current with its fundamental. Square each harmonic’s RMS magnitude, add them, take the square root, and divide by the fundamental. Harmonics of 12, 8 and 4 A on a 100 A fundamental give √(12² + 8² + 4²) = 14.97 A, a current THD of 14.97%.

Harmonic content

XH = √(X22 + X32 + … + X502)

Harmonics add as the root of the sum of squares, not as a plain sum, because they are at different frequencies. Orders you do not enter count as zero.

THD

THD = XH ÷ X1 × 100

X1 is the RMS fundamental, the 50 or 60 Hz component. The same formula serves for voltage and current.

TDD

TDD = IH ÷ IL × 100

Total demand distortion divides the harmonic current by the maximum demand load current IL at the point of common coupling instead of the present fundamental. IEEE 519 expresses its current distortion limits in TDD; IL is a demand-current benchmark established by the applicable method, not an equipment nameplate rating.

Total RMS

XRMS = √(X12 + XH2)

The true RMS value a meter reads, which is what heats cables and transformers.

Total harmonic distortion examples

Voltage THD on a 230 V supply

A power quality meter shows 2.3 V of 3rd, 6.9 V of 5th and 4.6 V of 7th harmonic on a 230 V fundamental. The harmonic content is 8.606 V, a voltage THD of 3.742%. That one reading is below the 8% low-voltage figure in EN 50160, but does not establish conformity: the standard applies at supply terminals over a defined week of 10-minute values, includes harmonics through order 40 and checks individual harmonics too. IEEE 519 assesses the point of common coupling under its own measurement rules.

Voltage THD 3.742 % Open in the calculator

The same current, as THD and as TDD

A feeder carrying 40 A of fundamental has 14.97 A of harmonics: a current THD of 37.42%, which looks alarming.

Its established maximum demand load current at the point of common coupling is 400 A, though, so the TDD is 14.97 ÷ 400 = 3.742%. TDD normalizes these harmonics to that demand-current benchmark rather than the 40 A fundamental measured at light load; neither one reading nor this ratio alone determines IEEE 519 conformity.

Current THD 37.42 % Open in the calculator

Why harmonics do not simply add

Adding 12, 8 and 4 A would suggest 24 A of harmonics. They are at 250, 350 and 550 Hz on a 50 Hz supply, so they combine as √(144 + 64 + 16) = 14.97 A. The THD is 14.97%, not 24%.

Current THD 14.97 % Open in the calculator

THD from the 5th and 7th harmonics

The two largest harmonics of a six-pulse rectifier are the 5th and the 7th. The table shows the THD they produce together, each as a percentage of the fundamental.

THD from the 5th and 7th harmonics, values in % THD
5th (% of fundamental)7th at 0% (% THD)7th at 5% (% THD)7th at 10% (% THD)7th at 15% (% THD)
55.07.111.215.8
1010.011.214.118.0
1515.015.818.021.2
2020.020.622.425.0
2525.025.526.929.2
3030.030.431.633.5
4040.040.341.242.7

Download this table (CSV)

Questions about total harmonic distortion

What is an acceptable THD?

For low-voltage supply terminals, EN 50160 gives an 8% voltage THD figure for harmonics through order 40 under its weekly 10-minute measurement conditions, alongside individual-harmonic limits. IEEE 519-2022 applies at the point of common coupling with its own voltage and current assessment rules; its current TDD limits depend on the short-circuit to maximum-demand load-current ratio. This calculator alone cannot establish conformity to either standard.

What is the difference between THD and TDD?

THD divides by the fundamental present at the time; TDD divides by the established maximum demand load current at the point of common coupling. They are equal only when those denominator currents are equal. IEEE 519 uses TDD for current distortion limits at that point.

Which loads cause harmonics?

Anything that draws current in pulses rather than smoothly: variable speed drives, rectifiers, LED drivers, switch-mode power supplies, UPS systems and EV chargers without active front ends. Six-pulse drives are dominated by the 5th and 7th.

Why do harmonics matter for power factor correction?

Capacitors lower the network impedance at high frequencies and can resonate with the supply at a harmonic. Where THD is significant, correction needs detuned banks, and the power factor correction calculator should use displacement power factor.

Limits of this result

  • Every entered magnitude must be RMS, use the same unit and represent the same measurement window and point.
  • Interharmonics, subharmonics, phase angles, aggregation rules, instrument bandwidth, windowing and measurement uncertainty are outside this arithmetic.
  • IEEE 519 voltage and current limits apply at the relevant point of common coupling and require system context; this tool does not classify compliance.
  • For current TDD, the maximum demand load current is not the instantaneous fundamental current and must be established by the applicable measurement method.

Measure it continuously

A ZEM records circuit current and voltage trends alongside the load. Measure harmonic magnitudes with a suitable power-quality instrument; RMS trends alone do not show THD or prove which load caused it.

Related guides

Sources

  1. IEEE 519-2022: Standard for Harmonic Control in Electric Power Systems (opens in a new tab) IEEE Standards Association, published 2022-08-05
  2. Total harmonic distortion (opens in a new tab) IEEE Technology Navigator, accessed 2026-09-14