The following table lists the recommended minimum sizes of earthing conductors.
Note: This serves only as a guide, calculations are still required for verification.

Minimum Copper Earthing Conductor Size
Nominal size of active conductor mm² Nominal size of copper earthing conductor
mm²
With copper active conductors With aluminium active conductors
1 1* -
1.5 1.5* -
2.5 2.5 -
4 2.5 -
6 2.5 -
10 4 -
16 6 4
25 6 6
35 10 6
50 16 10
70 25 10
95 25 16
120 35 25
150 50 25
185 70 35
240 95 50
300 120 70
400 ≥120+ ≥95+
500 ≥120+ ≥95+
630 ≥120+ ≥120+
>630 ≥25% of active size+ ≥25% of active size+

* - May be used only when incorporated in a multicore cable
+  - A larger conductor may be required.

Source: AS/NZ 3000-2007, Table 5.1

Update (29-Jan-2011):
Calculating the fault loop impedance of a circuit will provide a more precise selection of the size of the earth conductor. See the article on fault loop impedance for more details.

Author

  • A Filipino Engineer, Registered Professional Engineer of Queensland (RPEQ) - Australia, and Professional Electrical Engineer (PEE 2574 - 1st Place April 1991) - Philippines with extensive experience in concept selection, front-end engineering, HV & LV detail design, construction, and commissioning of Hazardous and Non-Hazardous Area electrical installations in water and wastewater pipeline and pumping facilities, offshore platforms, hydrocarbon process plants and pipelines including related facilities. Hazardous area classification and design certification (UEENEEM015B, UEENEEM016B, UEENEEM017B).

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