Background: I'm considering installing a secondary battery, battery charger, and 120v inverter in my car, to provide power when I'm in remote locations, without having to worry about killing my car's main starting battery. (Specifically, eyeing the Renogy 12V 20A DC-to-DC charger, and configured to only charge the secondary battery while the engine is running.)
For this project, I'll quite likely need to run new (fused) wires from my car's battery (or the fuse box itself, if I can add a new one there) and I want to ensure that I size the wires appropriately.
From reading other questions on this site, with regards to household wiring, "amps is amps" regardless of voltage. For example, this fascinating answer indicating that you can go up to 600v (I'm guessing the limit of the insulation rating) to move a lot of watts, but keeping the amperage at 20 amps on 12 gauge wire. This information lines up table 310.16 in the NEC (2020 edition), stating that 12AWG copper wire is required for 20A circuits.
However, from searching online, most sites (such as this and this) claim that you need larger wires (such as 10AWG or even 6AWG) to supply 20 amps at 12V DC. (Looking at the 10-15 foot length, from the engine bay to the trunk.)
Why do these sites recommend larger wires for the same amperage at 12V DC? Are there reasons that the NEC wire size requirements are valid for higher voltages, but not lower voltages?