The previous owner of my truck for some reason had put deadman switches on both batteries in my truck and the cables going to them were getting nasty. So I built my own today at work. the switches were on the negative side so I didnt have to deal with the big ass positive cable and I didnt want the dumb things under the hood anyway. I was going to use 2/0 but all we had left was 3/0, so thats what I used, its over kill but I can for sure tell a difference in the cranking speed!