Power suppl

[Rousey75]

Hi folks,
I upgrading my ROV doing mods on the bilge pumps, using relays and an usb interface for control. I understand the main supply is 16 gauge wire seperate to the control wires but still part of the tether as a whole. Is the common positive better being directly wired to positive supply if not where does the common supply come from also does the main positive, negative 12v dc supply come directly from the battery or is somehow done through the usb interface control box.