jojotide - Yes it adds a new fuse, but the POWER to that fuse comes off the same ONE ONLY plug that the original one plugged into.
So basically it's like one of those extension cords that instead of having one female end, has 2 ends so you can plug 2 items in, but they STILL come off the ONE same male plug.
Given this, the wire that brings the power to that ONE ORIGINAL plug that everything, the old and your new addition is being powered with, was designed with the rating of that ORIGINAL fuse in mind.
So lets say the original fuse was 30 amps, it's probably 10 Ga, maybe 12 Ga wire to it. Just because you split it and have 2 fuses that total 45 amps does not mean it's safe because there are 2 fuses in there now. The WIRE can handle the 30 amps of the original setup, NOT necessarily 45 amps, the new setup. You'll burn it up.
Let me try to reword this.
Whatever fuse you add to the second circuit you put in there. Honestly, Id not go above 15 to 20 percent of the rating of the original fuse. So if your original fuse was 30 amps, Id not go above 5 or 6 amps with the new fuse.
Whatever you end up doing, watch for heat CAREFULLY, especially on hotter days when you don't have a crisp wind to help dissipate it. Wiring harness don't like a lot of heat and when you burn one of them up,you are not just replacing one bad socket, you'll be replacing the entire harness / box. Got a few grand laying around?
Aaron