Max Current through Neutral in wye-wye 3phase circuit
If the three-phase currents are unbalanced, the neutral current can exceed the phase currents.
If there are 3rd, 6th, 9th, ... harmonics present - the "triple-N harmonics" - then these harmonics are in phase between all three phases. Therefore they all add up in the neutral. These harmonics are drawn by things like switched-mode power supplies (i.e. personal computers) and fluorescent lights.


Problem No.2 is common enough in modern office buildings, with a high concentration of desktop PC's and fluorescent lights, that the neutral conductor often needs to be larger than the phase conductors.

The Australian Wiring Rules, AS 3000, section 3.5.2 Neutral Conductor, refer: