I'm guessing there a step-down transformer on the AC side of the rectifier.
By the wire-current charts, #8 gauge should not give you any problems on the line side for the distance your going. Just get the appropriate cable insulation for the environment.
The output side I would look at heavy welding cables.
Other considerations, proper sized circuit breaker, proper connections "direct wiring" or "correct electrical plugs/sockets for use", grounding, proper insulating/jack protection of cable unless run in conduit.
Without more info on your setup/needs these are only best practice guides I can think of for now. Others on here may add to this.
Metal is the material, The forge is life, The anvil and hammer bring character and soul.