Under common wiring rules such as IEC 60364-5-54, where the earth (protective) conductor is made of the same material as the phase conductors, the usual minimum sizes are as follows. For phase conductors up to 16 sq mm, the earth conductor is the same size as the phase conductor. For phase conductors above 16 and up to 35 sq mm, the earth conductor is 16 sq mm. For phase conductors above 35 sq mm, the earth conductor is half the phase size.
These are minimum values. The earth conductor must also withstand the prospective earth-fault current for the time the protective device takes to clear it, so the figures should be checked against the fault level of the installation and against the local wiring code. In an armoured cable, the armour can serve as the protective conductor only if its cross-section and resistance have been verified for the purpose.