Earth cable size can be determined using Table 5.1 in AS/NZS 3000:2018 or by calculation.
Table 5.1 works by specifying a minimum earth cable size based on the active conductor size and the material. The method below is conservative and often over-sizes the earth cable.
The following are important notes about earth conductor sizing from AS/NZS 3000:2018.
The earthing conductor cross-sectional area must meet the following criteria (Section 18.104.22.168.1):
- Adequate short-circuit rating for the prospective earth fault current and fault clearing time of the associated protective device. Refer to this article about short circuit rating calculations.
- Appropriate earth fault loop impedance. Refer to this article about understanding fault loop impedance.
- Adequate mechanical strength and resistance to the external environment.
- To allow for deterioration in conductivity of elements (other than earth cables) part of the earth loop impedance.
Section 22.214.171.124.2 states any copper protective earthing conductor can be determined either:
- from Table 5.1 in relation to the cross-sectional area of the largest active conductor; or
- by calculation based on short-circuit rating (and fault loop impedance)