Depend upon surface area and type of soil.
PH value, acid or sulphuric.
Auckland is mostly clay and volcanic rock.
We have a quarry out Bethells which has an overhead twin 70mm² earth to get the earth resistance low enough, below 10 Ohms.
Upnorth in Kaitaia where i have done a few wiring jobs they have additional to the 1.8m earth pin a 15 meters of 25mm² bare strip earth laid in the ground which is mostly sand.
To do it properly overthere with the backfilling, peat and clean topsoil is added to make a good contact area for the strip earth wire.
Regards Ray
