How to calculate the neutral wire size of a home (department)?
You appear to be talking initially about the neutral wire in a 3 phase distribution system and given certain constraints on load imbalances, the "star point" imbalance current is usually a lot smaller than either of the three individual phase currents.

For a single phase system (even if it is derived from a 3 phase system) you must make the neutral wire the same diameter as the live wire because it conducts the same current.