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,
American Wire Gauge (AWG) Cable / Conductor Sizes - Meters UK
Motorola - Standard Mobile Power Cable (10ft, 14 AWG, 15A)
14 awg Silicone Electrical Wire 2 Conductor Parallel Wire line 50ft [Black 25ft Red 25ft] 14 Gauge Soft and Flexible Hook Up oxygen free Strands Tinned copper wire : Amazon.co.uk: DIY & Tools