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/SWG to mm/mm2, inch/inch2 & kcmil Calculator & Conversion
Dreams Come True :.: Amarican Wire Gauge (AWG) to mm2 convertion
AWG conversion table
Denkabel - Tabla de equivalencia entre AWG - MM2, para nuestros cables europeos. AWG (American Wire Gauge). MM2 (Sección en milímetros cuadrados). | Facebook
Convertidor Cable AGW - mm2 APK للاندرويد تنزيل
Electrical wire sizes & Diameters table of Electrical Service Entry | Electrical wiring, Wire, Chart
AWG To mm Chart (0-40 AWG Gauge Wires To mm) - LearnMetrics