Suddenly no juice

Ok
I used 12 gauge (rated for 30 amps) ATO fuse holder (FHAC2BP) from O'Reilly with 40 amps. After everything was in place, I hooked up battery, turned on head light, and started engine and it started without hesitance. Everything is back to normal for now. Next step is to replace 40amps fuse with 30amps, then 25amps and so on.........i just want to ensure when it going to fail .........
Thanks guys