When you operate electrical devices from a socket you may initially use a low power device such as a small wattage light bulb or soldering iron. But by the very nature of a socket at some point in the future you or someone else may unplug these devices and connect in higher powered devices such as a lawnmower or garden leaf sucker (this would then overload a cable of a low ratting).
So always use the cable which will handle the maximum loading possible. Protect the line with trip switch which protects the cable to the socket. When connecting in appliances use the lightest fuse in their plugs that will allow the appliance to work satisfactorily.
Either get advice from other answers here, or a leaflet from a Supermarket selling "add an outside socket" or an internet site.
best wishes Nyge C
March 2011