Number 2 AWG welding cable is used for all high-current wire (anything in the drive circuit).  Welding cable is used in virtually all electric motorcycle conversions because it is flexible and well-insulated.  It contains very fine strands of copper.
welding cable

Welding Cable

The connectors are tin-plated copper lugs.  The copper lugs have holes of varying sizes, including 1/4″, 5/16″, and 3/8″. They are crimped to the welding cable using a hex crimper.

cable lugsCable Lugs

EVDrives has the following recommendation for cable size:
200-300A controllers = 6 AWG
300-400A controllers = 4 AWG
400-500A controllers = 2 AWG
500-650A controllers = 1/0 AWG

Ordinary copper wire ranging from 10 to 18 gauge is used for the low current applications.  10 gauge is only used on the charger high-voltage output wires.  The rest of the wires are much smaller since they carry very little current.  Ring terminals and female quick disconnect terminals are crimped to the wires for most of the connections.  Shrink wrap tubing is used on the crimped sections.

Next:  Diodes