fuse

how do we calculate the sizes of the fuses and on what base .

You times 1.5 of the maximum Amp draw on the circuit, then you find the the next available fuse which must be the next highest then what you calculated. For example say you're running two cooling fans on the same circuit which each draw 10 amps and both share the same fuse you would 10x2=20 for the amp drawn by then fans then you would do 20x1.5=30 you would then use a 30 amp for that circuit. Hope that helps