Extension Cord - Restrictions

Restrictions

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.