Maximun Neutral and Earthing Cable Size

Unfortunately not very helpful or even relevant answers.

You ask a good question and deserve a proper answer.

The current carrying capacity of a cable refers to it carrying a continuous load. An earth cable normally carries no load, and under fault conditions will carry a significant instantaneous current but only for a short time - most Regulations define 0.1 to 5 sec - before the fuse or breaker trips. Its size therefore is defined by different calculating parameters.

Broadly speaking however the magnitude of earth fault current depends on:

(a) the external earth loop impedance of the installation (i.e. beyond the supply terminals)

(b) the impedance of the active conductor in fault

(c) the impedance of the earth cable.

i.e. Fault current = voltage / a + b + c

Now when the active conductor (b) is small, its impedance is much more than (a), so the earth (c) cable is sized to match. As the active conductor gets bigger, its impedance drops significantly below that of the external earth loop impedance (a); when quite large (here in NZ above 120mm^2, but in your region apparently 70mm²) its impedance can be ignored. At this point there is no merit in increasing the earth cable size

i.e. Fault current = voltage / a + c

(c) is also very small so the fault current peaks out.

The neutral conductor is a separate issue. It is defined as an active conductor and therefore must be sized for continuous full load. In a 3-phase system, if balanced, no neutral current flows. It used to be common practice to install reduced neutral supplies, and cables are available with say half-size neutrals (remember a neutral is always necessary to provide single phase voltages). However the increasing use of non-linear loads which produce harmonics has made this practice dangerous, so for example the current NZ Regs require full size neutrals. Indeed, in big UPS installations I install double neutrals and earths for this reason.