1. How can you tell what number of amp fuse you should use inbetween you amp and distibution block(it has no fuse)?
2. Do I need a fuse between my power caps and battery?
https://www.bcae1.com/fuses.htm should answer your basic fusing questions. I don't think it has information re: caps on that page. I have no experience with caps and can't offer any assistance there. :-)
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New Project: 2003 Pathfinder
1. Fuse this wire same as the amp's fuse rating that is connected on that wire.
2. Fuse the power wire less than 18" from the battery positive terminal, (you want it as close to the battery as possible). The fuse rating is determined by the gauge of power wire from battery to equipment. 1/0 gauge will handle a lot of amperage, so just fuse the wire with a fuse rated at the sum total of all three amps. This fuse protects the car from fire if the wire gets grounded on the chassis.
So, you will have a fuse in the power wire at the battery, and a fuse on each of the three 4 ga. wires going out of the distro block.
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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.