||This article needs additional citations for verification. (July 2010)|
An extension cord, power extender, or extension lead is a length of flexible electrical power cable (flex) with a plug on one end and one or more sockets on the other end (usually of the same type as the plug). The term usually refers to mains (household AC) extensions but is also used to refer to extensions for other types of cabling. If the plug and receptacle are of different types, the term "adapter cord" may be used. Extension cable is also used, but that has a distinct meaning from extension cord for many people.
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 colors, lengths, 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. An extension reel is an extension lead that rolls up, usually into the socket end, which in some cases has more than one socket on it (often 2 or 4). Another type of extension reel hangs near the plug end and permits the user to draw the cord out by grasping the socket end.
Some cords contain multiple female connectors. Others have female connectors spaced along the length of the cord. Cords generally contain either grounded or ungrounded connectors. While a grounded male connector can be forced into an ungrounded female socket, this is unsafe,
A power strip is a block on the end of a power cable with a number of sockets (usually 3 or more), often arranged in a line. This term is also used to refer to the whole unit of a short extension cord terminating in a power strip.
The term "extension cord" has been in use since at least 1946.
To avoid the need to roll-up any excess length, and to avoid the need for the user to cut the cord to size, extension cords are sold in prefabricated lengths 1 to 150 feet (30 cm to 45 m). The longer the cord the larger the diameter of the conductors (wire gauges are larger for smaller diameter wire) should be to minimise voltage drop. Every extra foot of cord increases the electrical resistance, which decreases the power the cord can deliver to connected devices. Because of this, it is best to use a cord that's exactly as long as needed and no more.
In the United States 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.
Within the United States, Underwriters Laboratories certifies extension cords as complying with the NEC. Key standards are UL 817 for the entire extension cord, and UL 62 for the electrical cabling itself. The United States General Services Administration also maintains a standard for extension cords, J-C-1270, that references the UL standards and provides additional critera.
Other countries also regulate the use of extension cables but the specific conditions and the nature of the regulation varies. In Europe and elsewhere where the normal domestic voltage is around 230 V, there is less risk of causing fire through overheating of cables for any given power due to the lower current. However most European extension reel cables now include an automatic current cut-out to avoid misuse of the cable. This requires manual re-setting if excess current is drawn through the cable. (American multi-plug cords also include such a device but single- or triple-outlet cords do not.)
An extension reel can only be used to carry full rated current when fully extended since the portion on the reel constitutes a concentration of the loss power (the result of its series resistance) which is not suitably dissipated unless most of the cable is unreeled to expose it to ambient air.
Signal cable extenders 
Extension cords sometimes refer to cables that add to the length of signal transmission cables or combined signal/electrical cables (for example, USB cables). They are more likely to be called "extenders" or "extender cables" or "cable extenders". Such extenders have one male and one female connector.
Lead content 
Extension cords sold in the United States may contain lead in the PVC insulation sheathing. California Proposition 65 (1986) requires manufacturers to warn consumers when products contain toxic chemicals. Many extension cords carry warning labels that advise consumers to wash their hands after handling them.
The European directive, RoHS, restricts the use of hazardous materials like lead in the manufacture of electrical products such as extension cords. Some manufacturers have moved to RoHS compliance but there is currently no widespread movement in the USA to stop the manufacture of products containing lead.
See also 
- "Extension cords". Retrieved 2012-03-25.
- Coleman Cable Inc.
- UL 817 (2001-03-16). "Standard for Cord Sets and Power-Supply Cords". Retrieved 2011-09-26.
- UL 62 (2010-05-18). "Flexible Cords and Cables". Retrieved 2011-09-26.
- J-C-1270B (1973-10-05). "Cable Assembly, Power, Electrical". Retrieved 2011-09-26.