How this is calculated. Resistance is
ρL/A, where
L is twice the
run length — current has to come back. Copper's resistivity is 1.72×10⁻⁸ Ω·m at 20 °C and
climbs about 0.393 % per °C, so a cable in a hot engine bay or loom is materially worse than the
same cable on a bench. The recommended gauge is the smallest that satisfies
both
constraints: the voltage drop you allowed, and the ampacity of the wire. Which one binds depends
on the run — short high-current runs are limited by ampacity, long runs by voltage drop, and it
is normal for a long 12 V run to need far more copper than the current alone suggests. The 3 %
default is a common design target, not a rule.