Harmonics, neutral current and K-factor
THD, neutral current and K-factor from a spectrum.
Reads AS/NZS 3000:2018, AS/NZS 3008.1.2:2017 (NZ) and AS/NZS 3008.1.1:2017 (AU). Tables checked 2 Oct 2026.
Try a job 4
Each job fills in example values. Replace them with your values.
- Find the harmonic neutral current of an officeThe neutral current of an office floor full of computers, from its spectrum.
- Find the K-factor for a drive transformerThe K-factor of six-pulse drives, for the transformer that feeds them.
- Check an LED lighting circuit for harmonicsWhether a cable for LED drivers is picked on its neutral current.
- Neutral current with some electronic loadThe neutral of a board where electronics are 30 % of the load.
Inputs
Worked example with the default inputs. The live calculator replaces it when the page loads.
- Jurisdiction
- NZ · AS/NZS 3008.1.2
- Phase current, true rms
- 100 A
- Third harmonic
- 35 %
- Fifth harmonic
- 20 %
- Seventh harmonic
- 10 %
- Ninth harmonic
- 5 %
- Eleventh harmonic
- 4 %
- Thirteenth harmonic
- 3 %
- Fifteenth harmonic
- 2 %
- Share of the phase load that makes harmonics
- 100 %
- Out-of-balance current
- 0 A
Triplen neutral
97.88A
- Total harmonic distortion of the current
- 42.18%
- K-factor
- 3.59
- Table 2 reduction factor
- 0.86x
- Cable picked on
- The phase current
- Current to pick the cable on
- 116.28A
- Cl 3.5.2(b)(i) minimum for the neutral
- 38.86A
This result is a design aid. Verify it against the current standard.
Calculation steps
- Total harmonic distortion of the current42.18%
- Fundamental current92.14A
- Triplen harmonics, √(h3² + h9² + h15²)35.41%
- Triplen neutralResult97.88A
- Third harmonic, percent of the phase current32.25%
AS/NZS 3008.1.2:2017 Table 2
- Table 2 reduction factor0.86x
AS/NZS 3008.1.2:2017 Table 2
- Neutral current from the third harmonic alone96.75A
AS/NZS 3008.1.2:2017 Cl 3.5.9
- Current to pick the cable on116.28A
AS/NZS 3008.1.2:2017 Cl 3.5.9
- Harmonic current of one phase, third and higher38.86A
- Cl 3.5.2(b)(i) minimum for the neutral38.86A
AS/NZS 3000:2018 Cl 3.5.2(b)(i)
- K-factor3.59
About this calculator
How this is calculated
Electronic loads draw current in pulses, not as a smooth wave. A power quality analyser splits that current into the fundamental and its harmonics, each shown as a percent of the fundamental. The tool takes the true rms phase current and that spectrum, from the third to the fifteenth order.
The total harmonic distortion is the rms of all the harmonic percents. The fundamental current is the rms current divided by the square root of one plus the distortion squared.
The third, ninth and fifteenth harmonics are the triplens. In the three phases they are in step, so they add in the neutral instead of cancelling. With equal phases the neutral carries three times one phase's triplen current. That is the tool's main answer.
AS/NZS 3008.1 Table 2 bands the third harmonic as a percent of the phase current. So the tool converts the analyser's percent of the fundamental to a percent of the rms current. It reads the band through the cable size tool's own path, and gives the reduction factor, whether the cable is picked on the phase or the neutral current, and the current to carry to cable size.
AS/NZS 3000 Cl 3.5.2(b)(i) gives a minimum for the neutral of a substantial harmonic load: the out-of-balance current plus one phase's harmonic current. Size the neutral for the larger of it and the triplen figure. AmpSize reads the harmonic current as the rms of the third and higher orders. Note 1 sets what substantial means.
The K-factor weights each harmonic by its order squared, because eddy current loss in a transformer rises with frequency squared. A clean load gives 1.
| Step | Table or clause | What it decides |
|---|---|---|
| Distortion | The rms of the harmonic percents | The THD and the fundamental current |
| Triplen neutral | Three times one phase's triplens | The neutral current of three equal phases |
| Third harmonic band | Table 2 | The reduction factor and the basis the cable is picked on |
| Neutral of the third harmonic | Cl 3.5.9 | The current the cable is picked on above the middle band |
| Clause neutral | Cl 3.5.2(b)(i) | The out-of-balance current plus one phase's harmonic current |
| Substantial load | Cl 3.5.2(b)(i) Note 1 | Whether the clause figure applies |
| K-factor | Each order squared, weighted by its share | The transformer K rating to pick at or above |
Worked examples
- An office floor of 80 A with 40 % third harmonic puts 87.16 A in the neutral, more than a phase. The current has 49.34 % THD and a K-factor of 4.37.
- Six-pulse drives drawing 150 A put only 13.43 A in the neutral, as their harmonics are mostly fifth and seventh. The current has 35.14 % THD and a K-factor of 5.12.
- An LED lighting circuit of 40 A puts 64.28 A in the neutral. The current has 89.16 % THD and a K-factor of 10.01.
- A 60 A board where electronics are 30 % of the load puts 58.73 A of triplens in the neutral if every load had this spectrum. The current has 42.18 % THD and a K-factor of 3.59. Below 40 % the clause figure does not apply.
Limits
The tool assumes three equal phases with the same spectrum. A real board has a mix. Enter the worst phase's readings for a safe answer.
It does not work voltage distortion, resonance with power factor capacitors or filter design. Those need the supply's impedance and a harmonic study.
See also: Neutral current, Cable size and Transformer.
Questions
Each answer describes what this tool calculates. The result is a design aid. Verify against the current standard.
How do you calculate harmonic neutral current?
The third, ninth and fifteenth harmonics of the three phases are in step, so they add in the neutral. With equal phases the neutral carries three times one phase’s triplen current. The tool works it from the phase current and the spectrum.
What is THD of current?
Total harmonic distortion is the rms of all the harmonic currents over the fundamental, as a percent. An analyser shows each order as a percent of the fundamental. The tool takes those and gives the THD and the fundamental current.
What is a K-factor?
It weights each harmonic by its order squared, because eddy current loss in a transformer rises with frequency squared. A clean load is K-1. Pick a transformer K rating at or above the figure. The rating steps are not in a standard AmpSize holds.
When does the cable size go on the neutral current?
AS/NZS 3008.1 Table 2 bands the third harmonic as a percent of the phase current. Above 33 % the cable is picked on the neutral current. The tool gives the factor, the basis and the current to carry to cable size.
What does AS/NZS 3000 ask of a harmonic load?
Cl 3.5.2(b)(i) adds the third and higher harmonic current to the out-of-balance current for the neutral, taking 100 % of the highest phase’s harmonic current. Note 1 calls a harmonic load of 40 % or more of a phase’s load substantial.
Clauses and tables this tool reads
- AS/NZS 3000:2018 Cl 3.5.2(b)Neutral of a multiphase circuit
- AS/NZS 3008.1.2:2017 Cl 3.5.9Third harmonic current in the neutral
- AS/NZS 3000:2018 Cl 3.5.2Neutral conductor of a circuit
- AS/NZS 3008.1.2:2017 Table 2Reduction factors for harmonic currents in 4- and 5-core cables
- AS/NZS 3008.1.1:2017 Table 2Reduction factors for harmonic currents in 4- and 5-core cables
Every clause and table, with how each was verified
Tables last checked on 2 Oct 2026.
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