The size of the earth wire is determined by
Right Answer is:
The ampere capacity of the service wires
The minimum allowable size of Earthing conductors is determined principally by mechanical considerations, for they are more or less liable to mechanical injury and must therefore be strong enough to resist any strain to which they are likely to be subjected.
The general practice in electrical construction has been to place the minimum size at No. 8 copper, for service or system grounding.
For grounded secondary circuits, the hazard in case of loss of ground connection is less, and the size of the grounding wire is determined more by the amount of current it may be required to carry than by mechanical considerations. current, in turn, is determined by the rated capacity of the smallest fuse in the circuit supplying the equipment.