In short — sizing the neutral for harmonics
In three-phase circuits with non-linear loads (LED drivers, VFDs, IT power supplies), triplen (3rd, 9th…) harmonics add in the neutral instead of cancelling. The neutral current can approach or exceed the phase current. Per IEC 60364-5-52 Annex E: below 15% third-harmonic the neutral may be reduced; from 15–33% size the neutral equal to the phase and count it in the grouping factor; above 33% the neutral becomes a current-carrying conductor and may need to be larger than the phases. A useful estimate is I_neutral ≈ 3 × (h3% ÷ 100) × I_phase.
Standards & method
✓ Independently verified 12 July 2026- Governing standard
- IEC 60364-5-52
- Clauses applied
- §524.2 neutral conductor sizing · §523.6 and Annex E — triplen-harmonic current in the neutral of a three-phase circuit
- Core formula
I_N = I_L × THD factor; for 3rd-harmonic content > 33% the neutral carries more than the line conductors- Why this matters
- With non-linear loads the neutral can carry MORE current than the phases — triplen harmonics add arithmetically in the neutral rather than cancelling. A neutral sized equal to the line is then undersized.
- Independently verified
- 12 July 2026 — Re-derived from the governing standard and checked numerically against worked reference cases from the standard itself — not merely tested for “returns a number”.
Results are for guidance. Verify against the current edition of the governing standard and have a licensed engineer review before construction or installation.
Neutral Conductor Sizing — Equations
In a balanced three-phase system, power can be calculated using the line-to-line voltage and line current.
Frequently Asked Questions
How do you size a neutral conductor for harmonics?⌄
Estimate the neutral current as I_neutral ≈ 3 × (3rd-harmonic % ÷ 100) × phase current, because triplen harmonics add in the neutral. Then per IEC 60364-5-52 Annex E: ≤15% you may reduce the neutral; 15–33% size it equal to the phase; >33% treat it as a current-carrying conductor and it may need to be larger than the phases.
Why does the neutral carry current in a balanced system?⌄
Fundamental currents cancel in a balanced 3-phase neutral, but 3rd-harmonic (and other triplen) currents are in phase across all three lines, so they sum in the neutral instead of cancelling. With heavy non-linear loads the neutral can carry up to ~1.7× the phase current.
What loads cause high neutral currents?⌄
Switch-mode power supplies (IT/data centres), LED and fluorescent drivers, variable-speed drives and other electronic loads inject 3rd-harmonic current. Offices and data centres commonly exceed 15–33% third-harmonic, which is where neutral oversizing becomes necessary.
When must the phase conductors be derated too?⌄
When third-harmonic content exceeds 33%, IEC 60364-5-52 Annex E requires counting the neutral as a loaded conductor (apply the 4-conductor grouping factor, ~0.86) and sizing on the neutral current, so the phase conductors are effectively derated.
Can the neutral be smaller than the phase?⌄
Only for largely linear, balanced loads with low (<15%) third-harmonic content, and subject to the wiring rules. For any circuit with significant electronic load, use a full-size or oversized neutral.