about the relay, I should use >45A relay? Is there a problem if I use for example a 60A Relay? or Should I use a 45 A relay?I am guessing that 45 amps Max refers to inrush current which is the initial spike in current when a circuit is turned on. Your fuse will need to be slightly higher than this. If lower, it may blow when turned on. @pagrey suggestion of a breaker is a good option as well.
For the relay, the fuse should be on the supply wire as close to the power source ( battery) as possible.
The coil wire on the relay should be fused as well but this should be small - around 3 to 5 amps should be sufficient.
Breakers also make troubleshooting much easier since they have a visual indicator.Personally I'd use a marine circuit breaker rated right about 45A. This way if you make a mistake it just pops and you press a button and move on. It sounds like you're the type that is going to make a few mistakes, buy a breaker and not a fuse. For the relay you can go right at the peak current or slightly under. T-max is giving you an absolute worst case and you don't need to worry too much about that. Personally I'd trust a 40A but that's a safety issue you should decide for yourself.
Edit: I use these Del City breakers.
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