|Some extension cords also incorporate safety features, such as a polarized plug and receptacle, grounded terminals, a 'power-on' indicator, a fusible link, or even a residual-current device (also known as a ground-fault circuit interrupter or GFCI).
Extension cords come in various lengths and thicknesses, and service duties. In general, the more power needed by the appliance, the thicker the cord should be (that is, larger wires inside). Cords to be used outdoors, in wet areas, around oils, or exposed to sunlight for long periods should be selected for such specific service.
In the USA where the domestic voltage is 120 V, the National Electrical Code (NEC) prohibits the use of extension cords in a 20 A circuit unless they are of 16 AWG or larger (for example, 14 AWG or 12 AWG). As with other flexible cords, the NEC also prohibits their use where attached to building surfaces, or concealed inside walls, floors, or ceilings, above suspended ceilings, or where run through holes or other openings (windows, doors) in structures (with limited exceptions). Cords run across the floor should be covered with a suitable device to protect them from physical damage.