Current THD Calculator
Total harmonic distortion and RMS current from the fundamental and 3rd, 5th and 7th harmonics. With the formula, a worked example and typical values.
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Estimate only. Not engineering or safety advice. Check the result against the applicable standard and the equipment data, and have a competent engineer review it before it is relied on. Terms of use.
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About this calculator
This page calculates THD and total RMS current from fundamental current, 3rd harmonic, 5th harmonic and 7th harmonic. Change any value or its unit and the result updates. The steps below show the working for the default values, so you can check each line against your own method.
Limits of this calculator
Add further harmonic orders from a power-quality analyser for a complete figure.
Cross-check: 6 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review.
Verification status: Engineering reference
Engineering reference. Formula and method documented and tested against separately coded reference calculations. Not yet Verified: the evidence listed on this page is still needed.
What this calculator checks
- The arithmetic of the published formula for the values you entered, in the units shown.
What remains unverified
- That the inputs suit your application. Take ratings, resistances and constants from the manufacturer data sheet or project specification.
Verification register entry
| Calculation author | Rahul Sharma (author) |
|---|---|
| Technical review | No independent technical review has been carried out |
| Individual professional approval | Not recorded |
| Formula reference | not recorded |
| Standard and edition | IEEE 519; IEC 61000-4-7, IEEE 519-2022; IEC 61000-4-7:2002 + A1:2008 consolidated |
| Standard review state | Public-scope screened; licensed clause review still pending |
| Scope-screening note | Harmonic control at the point of common coupling and harmonic measurement instrumentation are relevant. The calculator only combines selected 3rd, 5th and 7th currents; it is not a full spectrum, TDD or compliance assessment. |
| Cross-check test cases | 6 reference vectors (tests/reference_vectors.json) |
| Review date | not recorded |
| Next review date | not recorded |
| Status | Engineering reference |
Needed before this calculator can be marked Verified: reviewer name, qualification and review date; licensed clause-by-clause standard review; exact formula reference; formula source (textbook or standard); next review date.
Formula
Worked example
| Input | Value |
|---|---|
| Fundamental current | 100 A |
| 3rd harmonic | 20 A |
| 5th harmonic | 12 A |
| 7th harmonic | 8 A |
- Harmonic RMS = 24.658 A
- THD = 24.658 / 100 × 100 = 24.658 %
- Irms = 102.995 A
Assumptions
- Only the harmonics entered are included
- Magnitudes are RMS
Add further harmonic orders from a power-quality analyser for a complete figure.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Fundamental current | A | greater than 0 A | |
| 3rd harmonic | A | 0 A or more | |
| 5th harmonic | A | 0 A or more | |
| 7th harmonic | A | 0 A or more |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| THD (%) | (number) | Shown to 2 decimal places | |
| Total RMS current | A | Shown to 2 decimal places |
How it is calculated
Each input is converted to the base unit of its quantity, the formula above is applied in the order shown in the worked example, and each result is shown in the unit you choose. Results are held at full precision and rounded only for display, except where a step selects a standard size, as the worked example shows. A value outside its allowed range is refused with a message instead of giving a result.
How the result changes with fundamental current
| Fundamental current (A) | THD | Total RMS current (A) |
|---|---|---|
| 10 | 246.58 | 26.61 |
| 25 | 98.63 | 35.11 |
| 50 | 49.32 | 55.75 |
| 100 | 24.66 | 103 |
| 200 | 12.33 | 201.51 |
| 400 | 6.16 | 400.76 |
| 1,000 | 2.47 | 1,000.3 |
Other inputs are the same as in the worked example above.
Standards and further reading
- IEC 60909-0 (short-circuit currents)
- IEEE Std 519 (harmonic limits)
Check results against your utility's connection requirements and the equipment data sheets. These are named as background reading. Elecmetric does not embed clause text or rating tables, and the reviewer record for this calculator is separate from this list.
Frequently asked questions
What do I need to enter?
Enter fundamental current, 3rd harmonic, 5th harmonic and 7th harmonic. Change the unit of a value with its selector and the result stays the same.
What does this calculator assume?
Only the harmonics entered are included. Magnitudes are RMS.
Which calculators should I use next?
Common next steps are Transformer Efficiency and Losses and Power Factor Correction. The related links at the bottom of this page open them.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. 6 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review. Check design values against the standard and the equipment data you work to.
Formula guides and reference
Longer explanations with worked examples and tables, written for the same topic.
- Harmonics Formulas: THD and True Power Factor: Harmonics formulas for current THD, RMS current, true power factor
- Single-Tuned Harmonic Filter Formulas: Single-tuned harmonic filter formulas: the reactor from the capacitor and the tuning order, the tuned frequency
- RMS, Peak and Average Value Formulas: RMS, peak, peak-to-peak and average value formulas for sine, square and triangle waves, form factor and crest factor
- Detuning Reactor Formulas: Capacitor Banks: Detuned capacitor bank formulas: reactor reactance from the percentage, resonance frequency and harmonic order
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