Reduced Neutral Cross-Section (Three-Phase Loads)

A design rule under NEN 1010 §524.2 that, under specific conditions, permits the cross-section of the neutral conductor (N) to be smaller than that of the line conductors in a multi-phase cable or wiring system, provided the load is predominantly linear and evenly distributed across the phases, so that the neutral current in practice does not exceed the current in a line conductor. The standard attaches an important exception to this, however: for loads with a significant third-harmonic content (and multiples thereof) — typical of, for example, large quantities of LED lighting, switch-mode power supplies or variable-frequency drives — the third harmonics from the three phases can add up in the neutral conductor instead of cancelling out, which can result in a neutral current even higher than the phase current. In that case NEN 1010 instead requires a neutral conductor with the same or even a larger cross-section than the line conductors, rather than the reduction otherwise permitted for a purely linear, balanced load.