from above:
From Kencove fencing, one of the largest and best known electric fence companies in the US:
Use at least one 12½ gauge double insulated cable as your leadout. Two or more joined in parallel are better, as this lowers the resistance of the wire.
https://kencove.com/fence/100_Fence+Construction_resource.php
From Premier:
Energizer Installation & Operation Instructions
1. Keep energizer ground system 30 to 40 ft away from any other electrical ground source. This includes house ground systems and wells.
2. Use only galvanized lead-out wire and galv. ground rods. Do not use copper lead-out wire or copper ground rods.
3. Use only insulated wire designed for electric fencing. Do not use wire rated at less than 10,000 volts.
https://www.premier1supplies.com/img/instruction/35.pdf