Gidday Andy!, [Linked Image]
Is this cabling to supply a Star or a Delta load?.
Because if it is to supply a Star(Wye) load, the Neutral will only carry the Out of Balance current, between the 3 Phases.
As a rule, the Neutral current is the Phasor sum of the Line currents.
A Delta load never has a Neutral, because it works on circulating line/phase currents.