In the United States the domestic voltage is 120 V, and the National Electrical Code (NEC) prohibits the use of extension cords in a 20 A circuit unless they are of 16 AWG or larger diameter (for example, 14 AWG or 12 AWG).
As with other flexible cords, the NEC also prohibits their use where attached to building surfaces,concealed inside structures [walls, floors, ceilings] , above suspended ceilings, or run through holes or other openings (windows, doors) in structures.