more important is not to leave it tightly coiled, a 13A cable coiled is derated to 5A as the coil makes it act like a big electromagnet, as well as trapping the heat.
if you are going to coil, use large loose coils, with plenty of air gaps around the cables, and it should be fine.
cable current carrying capacities are generally for a cable in free air, where it can dissapate the heat, when tightly coiled this heat goes into neighbouring coils and a hot spot forms (the hotter it gets the higher resistance - a standard 25m extension will dissapate about 85 watts as heat at full load), and a fire can occur, loose spaced coils avoid this