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
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.
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.
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 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.
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.