As suggested above, length of wire affects this discussion. Wire does have a capacity, based on AWG, but what's being discussed here is voltage drop. Consulting a wire sizing chart will show you that length is part of the equation. The longer the wire run length, the more resistance that will be added to the circuit. This is why your long run of wire back to the rear mounted second battery should be something like 1/0, while your short run of wire from the rear batt to the rear amplifier can be 4awg.