The neutral conductor under third-harmonic loading — why "it's unloaded anyway" is wrong

The harmonics & THD guide covers the THD limits at the point of connection from the grid operator's perspective (Netcode Elektriciteit). This article covers a different, installation-internal consequence of that same harmonic distortion: the effect of third-harmonic currents on the sizing of the neutral conductor itself.

Why the neutral is unloaded under symmetrical linear load

Under a symmetrically loaded, linear three-phase installation (three equal, purely sinusoidal phase currents, displaced 120° from each other), the instantaneous phase currents sum to zero at the node at every moment — the neutral carries, in theory, no current. This is the classic assumption behind allowing a neutral conductor thinner than the phase conductors.

Why that assumption no longer holds under non-linear load

Non-linear single-phase loads — LED drivers, switch-mode power supplies of IT equipment, energy-efficient lighting — draw current in short, repeated pulses rather than a smooth sine wave. In addition to the fundamental 50 Hz component, this current waveform also contains harmonic components, including the third harmonic (150 Hz) and its multiples (9th, 15th, ...) — the so-called triplen harmonics. These specific harmonics have a peculiar property in a symmetrical three-phase system: instead of cancelling out between the three phases (like the fundamental component), they add up in the neutral conductor. The result: with sufficient third-harmonic content, the neutral current can become larger than the current in each individual phase conductor — even though the load is perfectly symmetrically distributed across the three phases.

The practical threshold values

Under IEC 60364-5-52 (Annex E), the correction based on the third-harmonic proportion of the phase current is applied as follows:

Below approximately 15% third-harmonic content: no special correction is needed, the neutral is not significantly loaded by harmonics.
Between approximately 15% and 33%: the neutral is significantly loaded; the permissible current of the cable as a whole is reduced by a correction factor (on the order of 0.86) compared with the current-carrying capacity without harmonic correction.
Above approximately 33%: the neutral current can exceed the phase current — from this point on, it is no longer the phase current but the neutral current that determines the sizing of the entire cable (including the neutral itself, which must then be sized at least equal to the phase conductors, instead of the classic, smaller neutral cross-section).

Note: these are the common practical thresholds from the IEC 60364-5-52 context for third-harmonic correction; the precise table values and the exact calculation method for a specific load composition are set out in the full, current standard text (Annex E) and must be verified per project against the actually measured or calculated harmonic content of the connected load.

When this becomes relevant

This effect is particularly relevant for circuits with a high concentration of single-phase, non-linear load distributed across the three phases of the same circuit or main distribution board:

Office floors with many IT workstations (switch-mode power supplies).
Large-scale LED lighting installations with many drivers.
Data centres and server rooms.

A circuit with predominantly linear, three-phase load (motors, heating elements) shows this effect little or not at all.

Practical relevance

When sizing a circuit with a lot of single-phase, non-linear load (IT workstations, LED lighting), the third-harmonic proportion of the connected load must be estimated or measured, after which it must be established whether the neutral conductor needs to be sized separately on that basis — an installation that runs the neutral simply as "half the phase cross-section" can, under high third-harmonic content, have a structurally overloaded neutral, without this being apparent from the regular phase-current calculation.

Common mistakes

Assuming that a symmetrically loaded three-phase circuit automatically has an unloaded neutral — that only holds for linear load; under non-linear single-phase load, the neutral can in fact be more heavily loaded than each phase conductor individually.
Sizing the neutral as standard thinner than the phase conductors without checking the third-harmonic proportion of the load — with a high proportion, an equal or larger neutral cross-section is needed.
Assessing harmonic correction only at grid level (THD, see the harmonics & THD guide) and overlooking the installation-internal neutral loading — these are two separate, complementary checks at different levels of the installation.