You need to consider the application and the environment the cable will be installed in. Thicker cables carry larger current, but can be more difficult to install and are less discreet, smaller cables carry less current and are easier to hide, but the correct cable size must be selected for the application and circuit design. The installation method can have a derating factor on cables and may affect the load that a cable can safely carry. The selection of the cable and its size must be determined by a qualified electrician in accordance with the BS7671 regulations.