when u replace all the old wire can u just leave it there or even just leave it still connected. what i meen is say there is already a 4 gauge grounded wire from the battery, but you want to use 0 gauge wire. can you just leave the older wire still connected to the battery and grounded or do u have to remove the old wie and replace it. if so wil the 0 gauge now take priority of the 4 gauge?
Yes, you can leave the old wiring there. Power will take the path of least resistence, which will be the bigger wire. But it won't hurt to leave the small stuff there.