Constant wires, amps?

I think what you are referring to is that the circuit is fused at that rating. They are fused to a particular limit based on what devices they supply power to and what the total current draw would be if all were using their maximum current. Typically circuits fuse above 20 amps are for large power consuming devices such as power windows, rear defoggers, amplifiers. The 40 amp and up circuits usually are fuses for entire circuits like everything on the accessory or igntion circuit and are underhood close to the battery. They will supply power to other fuse boxes which split the power into smaller fused cicuits.sparky