I bought a roll of 13-14 AWG wire (2.5mm2). According to this chart, I would be safe running a wire from my battery (which is next to my firewall), through the firewall,
AWG conversion table
AWG Wire Gauge Chart 8 X 10 - Etsy New Zealand
Understanding Cable and Cable Sizes | Caravan Chronicles
How to calculate the suitable wire size for a 1500A AC load - Quora