[axsaxoman]

the reason why the fuse is higher value than the calculated load is the surge you get when the circuit is first turned on ,but to keep same saftey margin as the maker decided it needed then fuse will be too small +therefore so will the wiring size ,and once you start to overload it has the effect of heating up the wire cos current draw rises when you try to push too much current through wrong size wire which over a period of time and melt the conductor through the wall of the insulating outer cover --nex step is to melt through other wire tied tight together in a the loom --which by its nature will get no cooling +then comes the possiblity of the full loom short +then fire
as cars get older connections get corroded and again current draw rises --more heat etc etc -- even a poor earth in the circuit will lift the load -the car maker does not waste money on over size wiring --so doubling the load you just don,t do it -fit a relay and seperate feed for the new load
you wouldn,t fit a pair of spot lights without going through a relay --big fuel pump is similar draw