Grounding-Dedicated drain wire for ACU-100

[daknecht]

I installed an EV-100 (ACU100) last year and was confused by the ground drain wire.
1. There is also a ground drain wire on the Seatalkng network, but that is a tiny wire. So how to join this to a #12 drain wire to the ACU-100 or do those need to be separate?
2. Why can't these be tied to the negative wire since that also goes back to the engine/battery ground?