📊 ELECTRICAL

Free Online Total Harmonic Distortion (THD) Calculator

Calculate total harmonic distortion (THD) from non-linear loads and check against IEEE 519 limits. Free online electrical power-quality tool, no sign-up.

📐 Standard: IEEE 519
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Harmonic Distortion (THD) Calculator
Reference: IEEE 519
📊 ELECTRICAL
Calculate total harmonic distortion (THD) from non-linear loads and check against IEEE 519 limits. Free online electrical power-quality tool, no sign-up.
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ℹ About This Calculator

Non-linear loads - VFDs, UPS, LED drivers, computers - draw current in pulses that distort the supply waveform, adding harmonics. This calculator estimates the total harmonic distortion (THD) from the harmonic currents and checks it against IEEE 519 limits.

THD is the ratio of the combined harmonic content to the fundamental, as a percentage. High current THD overheats transformers and neutrals, trips breakers and distorts the voltage for other equipment. IEEE 519 caps voltage THD at 5% for general LV systems and sets current-distortion limits that tighten as the load's share of the supply grows.

THD Formula

IEEE 519

THD = √(I₂² + I₃² + I₄² + …) / I₁ × 100% THD = total harmonic distortion, %
I₁ = fundamental (50 Hz) current
I₂, I₃… = harmonic current magnitudes
Voltage THD uses the same form with voltages.

Worked Example

Example: A load draws 100 A fundamental with 30 A of 5th, 18 A of 7th and 12 A of 11th harmonic. THD = √(30² + 18² + 12²) / 100 = √(900 + 324 + 144) / 100 = √1368 / 100 ≈ 37% - well above the IEEE 519 target, so harmonic filtering or a low-harmonic drive is needed.

IEEE 519 Distortion Limits

IEEE 519 sets limits at the point of common coupling. Voltage distortion limits for systems ≤1 kV:

QuantityIndividualTotal (THD)
Voltage (≤1 kV)5.0%8.0%
Voltage (1-69 kV)3.0%5.0%

Current-distortion limits depend on the ratio of short-circuit to load current (Isc/IL) - the larger the supply relative to the load, the more harmonic current it tolerates (from ~5% TDD for weak supplies up to ~20% for strong ones).

Reducing Harmonics

Options include line reactors and DC chokes on drives, passive tuned filters, active harmonic filters, 12- or 18-pulse rectifiers, and low-harmonic (active front-end) drives. Oversizing the neutral and using K-rated transformers manages the effects where harmonics cannot be eliminated.

Simplified estimate - a power-quality survey measures the actual spectrum.

Frequently Asked Questions

How do I calculate total harmonic distortion? +
Take the square root of the sum of the squares of all the harmonic current (or voltage) magnitudes, divide by the fundamental, and express as a percentage. For 30 A, 18 A and 12 A harmonics on a 100 A fundamental, THD is about 37%.
What is an acceptable THD level? +
IEEE 519 limits voltage THD to about 5% for systems 1-69 kV (8% below 1 kV). Current distortion limits (TDD) depend on how strong the supply is relative to the load, ranging from about 5% to 20%.
What causes harmonic distortion? +
Non-linear loads that draw current in pulses rather than smoothly - variable-frequency drives, UPS systems, LED and electronic lighting drivers, computers and switch-mode power supplies. They inject harmonic currents that distort the supply.
Why is harmonic distortion a problem? +
High harmonic current overheats transformers, cables and especially neutrals (where triplen harmonics add), causes nuisance breaker tripping, derates equipment and distorts the voltage seen by other loads, which can disrupt sensitive electronics.
What is the difference between THD and TDD? +
THD references distortion to the present fundamental current, which can look high at light load. Total Demand Distortion (TDD) references it to the maximum demand current, giving a fairer measure for compliance. IEEE 519 current limits are stated as TDD.
How do I reduce harmonic distortion? +
Fit line reactors or DC chokes on drives, add passive tuned or active harmonic filters, use 12/18-pulse or active-front-end drives, and where harmonics remain, oversize the neutral and use K-rated transformers.
Why do harmonics overload the neutral? +
Third-order (triplen) harmonics from single-phase non-linear loads do not cancel in the neutral - they add. In heavily loaded IT and lighting circuits the neutral current can approach or exceed the phase current, so it must be oversized.
What standard governs harmonics? +
IEEE 519 is the recommended practice for harmonic control in electrical power systems, setting voltage and current distortion limits at the point of common coupling. IEEE 519 series covers equipment emission and immunity.

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⚠️ Disclaimer: For preliminary engineering design only. Verify all results with a licensed engineer before use. Full disclaimer →