How common a practice is this?.

[Jack Shaft]

Its easy to troubleshoot without harness marking,each fuse protects a high amperage function,while one five amp fuse protects the milliamp control circuit to run the relays.The light switches,brake light switch and horn button only see the 150 milliamps it takes to operate the relay,which bears the load of the component,relays are overrated by a factor of 2,switches are way over.Have room for expansion for whatever..60a alternator.LED lamps pull very little load,conservatively have 40a available while night driving..

[Brendan]

its basic oms law, lower voltage higher resistance . think about filling a swimming pool with a 1/2 in garden hose, it will take a long time, and not much water flow. If you use a 3 in hose you will get a hell of a lot of water and fill the pool fast. lots of people use small battery cables with 6 volts, not a lot of juice use a bigger cable and get more juice