Main (battery to starter) wire size is determined by actual total current load + 20% (to protect wire from damage)
Main fuse (when used) is rated at actual total load + 10% (10% less than wire capacity)
Alternator wire size is determined by maximum output + 20%
Alternator fuse is rated at maximum output + 10%
Ignition/Fuse box wire size is determined by total equipment load + 20%
Ignition/Fuse box fuse is rated at total equipment load + 10%

~Fuses should be placed as close to source as practical, i.e. Main at battery, Ignition/Alternator at starter
~Often, fusible link wires or breakers are used in-place of fuses
*Batteries mounted in engine bay with short cables do not always have fuses at Main
*Battery's remote mounted (trunk exc.), should always have a Main fuse and disconnect

TOTAL LOAD = everything electrical, including starter.
EQUIPMENT LOAD = everything electrical that comes from fuse box, excludes starter.

Wire sizing is typically sized large enough to safely carry more than the maximun load of the circuit without causing damage to the wire or its insulation (insulations are temperature and environment rated for their applications).
Fuse sizing is typically sized large enough to safely carry more than the maximun load of the circuit but less than the maximum rating of the wire's safe current capacity.

Circuit Wizard - Blue Sea Systems