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,
Tabla de-equivalencias-awg-a-mm2
6 Gauge Wire Vs 8 Gauge Wire: mm, mm2, Amps Comparison - LearnMetrics
AWG TO SQUARE MILLIMETER (MM2) CONVERSION | Electrical Engineering – A Learning Platform