Neutral conductor dimensioning 1

[electpower]

"Preferred neutral conductor options include 50% or 100% of the phase conductor. In limited cases, it is also possible
to select a conductor rating up to 200%."
The above lines were stated in the ABB- MNS panels documents.
Why is the neutral conductor dimensioned sometimes lesser than the phase conductors.

 

[dpc]

In theory, if all phases are balanced and there are no harmonic currents, there is zero current in the neutral. Codes have generally allowed a reduced neutral in three-phase systems.

However, with the increased use of non-linear loads, the neutral can carry a substantial harmonic current, primarily third harmonic currents in addition to the imbalance current. For many applications, such as office buildings, high tech facilities etc, it has become common to oversize the neutral to 200% to account for the harmonic (triplen) currents.

 

[Adamito]

As a 3 phase system is designed, all currents of a resistive load cancel as the sum of all 3 voltages is 0. So for linear, 3 phase loads or a big amount of linear and consistent 12 phase loads, the total current on the neutral should be very low compare to phase currents.

Those office buildings dpc is talking about are mostly lighting and electronic loads. As you probably know, their power supply are made of capacitors charging only on top of the voltage sinewave, creating high current inrushes for only very short amounts of time. If you look on a 3phase plot, you`ll see that very short bursts of currents on peak of each phase will add up instead of cancel with each other.