Grounding-Dedicated drain wire for ACU-100

[marchem]

The installation manual for the ACU-100 says to install a dedicated drain conductor (screen) to the vessel RF ground point (which in my case would be the negative battery terminal). I'm trying to figure out just what it is for, and why such a large conductor is called for (>#8 AWG for run >1 m). It seems to me that the actual connection at the ACU-100 won't take wire larger than #12!