Table 52.3 has the minimum for aluminium wire as 16mm
Generally you don't want to use aluminium unless it is really necessary, say for cost reasons on big SWA style cables, as you have far more difficulties with terminating it and preventing galvanic corrosion.
The adiabatic limit is one you should be careful with, as the simple "rule of thumb" is half the phase conductor for conductors above 35mm, 16mm CPC down to 16mm phase conductors, and same CPC as phase below that (Table 54.7 but also scaled by adiabatic k-factors if CPC and phase are different materials).
So if your results are well below that you need to be very sure that the end-of-circuit Zs and OCPD really meet the disconnection times and fault currents you based the computation on. Then you get on to the regulations citing tables such as 54.1 for buried earth conductors, and 54.8 if it is TN-C-S supply, etc, that present further limits on how small your CPC/earth can be irrespective of the adiabatic result.
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