SSC JE Electrical Engineering 24 Jan 2018 Evening Paper

Nutral wire carry the differential current of the 3-phase when there is unbalanced loading on the 3-phase.
Mathematically, In = ${\mathit{I}}_{\mathit{a}}+{\mathit{I}}_{\mathit{b}}+{\mathit{I}}_{\mathit{c}}$, which is a phasor addition and not to be read as arithmetic addition.
The nutral current can at the most can be equal to the phase current when only a single phase is loaded. Practically this never happen and the unbalance in the load is small current comparison to phase current and so they have a less cross section then that of phase wire. Practically cross section of nutral is half then phase wire.