Normal three-phase current cancels at the neutral. A specific family of harmonics — the triplens — does the opposite: it adds. That single fact can make the neutral the most heavily loaded conductor in the circuit.
Nonlinear loads — electronic ballasts, switch-mode power supplies, VFDs, LED drivers — don't draw current in a clean sinusoid. They draw it in sharp, non-sinusoidal pulses timed to the peaks of the voltage waveform. Mathematically, any repeating non-sinusoidal waveform decomposes into a fundamental sine wave plus a series of harmonics at integer multiples of that fundamental frequency. Most of those harmonics behave in three-phase systems exactly the way fundamental phase currentdoes — they cancel at the neutral. One specific family doesn't, and that's the one worth understanding in detail.