Neutral Conductor Sizing with Harmonics in Data Centers
In a balanced linear system the neutral carries nothing. With server power supplies, it can carry more than any phase. Sizing it like a phase conductor is a known mistake.
Why it happens
In a three-phase four-wire system, the fundamental currents of balanced phases cancel in the neutral. Triplen harmonics are in phase across the three phases, so they add together there. Switch-mode server power supplies produce strong third-harmonic currents.
Worked example
Illustrative example. Each phase carries a 100 A fundamental plus a third-harmonic current equal to 50% of it (50 A), and the phases are balanced.
- Phase RMS current = √(100² + 50²) = 112 A
- Neutral current = 3 × 50 A = 150 A, since the third harmonic adds arithmetically in the neutral
The neutral carries 150 A, about 34% more than each phase. A neutral sized for the phase current would be overloaded. Real harmonic content varies, so measure or take it from the equipment data.
What to do
- Estimate the triplen harmonic content of the loads and the resulting neutral current.
- Size the neutral for that current, and apply any phase conductor derating the code requires.
- Provide neutral protection or monitoring where appropriate.
- Consider transformer derating or a harmonic-rated transformer. See harmonics in data centers.
Related: cable sizing basics and the conductor sizing calculator.
Written by the Vision Matrix Institute editorial team. Worked examples use stated, illustrative assumptions; check them against your project data, the applicable standards and manufacturer datasheets before use.
Frequently Asked Questions
Why can the neutral carry more current than the phases?
Triplen harmonics (3rd, 9th, 15th) are in phase across all three phases, so they add in the neutral instead of cancelling.
How do I size the neutral?
Estimate the neutral current from the harmonic content of the loads and size the neutral conductor, and any derating of the phase conductors, accordingly. IEC 60364-5-52 gives guidance, with different treatment depending on the third-harmonic content.
Should the neutral be larger than the phases?
Where harmonic content is high, the neutral may need to be the same size as the phases or larger, and the phase conductors may need derating. Take the requirement from the code and a harmonic estimate.
How do I reduce neutral harmonic current?
By choosing equipment with low harmonic distortion, using filters or isolating transformers, and measuring the actual harmonic currents after commissioning.
Want to learn this properly?
This topic is covered in depth in our Electrical Design – Data Center Specialist program.
Ask us on WhatsAppLearn to do this on a full project in the Electrical Design – Data Center Specialist program, or read the data center design guide.