Although I didn't find an official table for sizing DC cables, I found a precise explanation about the answer from @Li-aung Yip, "If you select a cable for DC current, using AC cable sizing guides / cable ampacities, your DC cable size will be conservative with respect to heating.":
This document explains how to calculate current in DC, AC/mono and AC/3 systems:
- DC: I = P/V
- AC/1: I = P/(V*cosF)
- AC/3: I = P/(V*1.73*cosF)
This means that, for same voltage, DC current is greater than AC/1 and AC/3 current; hence using wire sizing tables for AC current is safe also for DC systems, because cable heating is directly proportional to current carried by the cable, and the target is to prevent cable from overheating.
For my specific system, rated 70V/100A and enclosed into an electric scooter, I assume as applicable the table at p.22, column "posa interrata in tubo" (cables enclosed in pipe), sub-column "3 cavi unipolari" (three single-wire cables), as probably the ground thermal resistance is equal or greater than the thermal resistance of plastic case of the scooter.
For 100A I get a 25 mm2 section.
"AWG 3" is 26.7 mm2, so the final answer to my question is:
Cable section required for a 70V/100A DC system is:
AWG3 / 25mm2
But on an electric scooter there are actually both DC and AC: DC goes from battery to controller, but controller creates an AC current which drives the motor; so, for same voltage, I think that cable size from controller to motor (three cables) can be slightly less than battery-controller cable size.
Unfortunately I don't currently know the frequency of the motor current and the anount of CosF.
How do I know which current is in use on my system?
My scooter is rated 5000 W, but I also "manually" calculated the current needed to maintain a 90 km/h speed on flat road; I assumed 0.8m2 frontal area and 0.8 Cd for the scooter+driver system. This leads to 6000W needed (link).
Scooter uses a 60V LiFePO4 battery, with actual operating voltage between 56 and 66V; 6000W/66V gives 90A, rounded to 100A.