It would be difficult to find a household that isn't using at least one extension cord. On the contrary, you'll probably find several in use in any given home. People use them to extend the reach of cords powering lamps and other appliances. Extension cords are only for temporary use, but you'll often see one connected to a permanent appliance, such as an air conditioner or refrigerator.
You can say that an extension cord is basically a set of wires with plugs on either end, but that's an over-simplification. An extension cord consists of wires, wire sheathing, insulation and plugs that include prongs, casing and wire terminals. The characteristics of these parts determine the applications for which the extension cord works best.
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Wires and Wire Gauge
An extension cord can carry electricity, so conducting wires are its most important component. The thickness of the wires, or wire gauge, and their length are the primary factors that determine the applications for which the cord works best. The longer the cord, the less current it can carry, because voltage drop becomes a factor in lengthy cords. Similarly, larger wire gauges are for smaller currents.
Keep in mind that a larger gauge number signifies a smaller-diameter wire. If you're wondering whether a particular cord can handle a particular application, look up the amperage capacity of the wire by noting the wire gauge, which is usually marked on the wire sheathing, and the length of the cord.
Wire Sheathing
The type of sheathing encasing the wires determines whether an extension cord is for indoor or outdoor use. Indoor cords generally have plastic sheathing, which is often colored brown or white, while outdoor cords have waterproof rubber sheathing. Besides being thicker, the sheathing on outdoor cords is usually more flexible than that on indoor cords. Outdoor cords can be orange, white, green or even black, and they usually have the letter W (for waterproof) stamped on the sheathing.
Wire Insulation
The conducting wires inside the sheathing of an extension cord usually have an insulating cover. If you cut an outdoor cord to inspect the wires, you'll find one with black or red, one with white and one with green or no insulation. The black or red wire carries the hot signal while the white wire (return wire) carries the signal back to the source. If the cord has a green or bare wire, that's the ground wire. It's a safety feature found on all outdoor cords. Most cords have standard 300-volt insulation, indicated by the letter "J" on the sheathing. If you don't see this letter, the wires have heavier insulation.
Indoor cords don't always have insulated wires, and if you cut one, you may find two identical stranded wires. If the cord has a polarized plug, you distinguish the hot wire from the return wire by inspecting the wire sheathing. The side with the return wire will have either ribbing or a white line to distinguish it from the other side.
Plugs
Extension cords designed for use with 240-volt power can have one of a variety plug types that usually lock onto the receptacle. Cords designed for use with 120-volt circuits, which are the ones homeowners typically need, usually have one of only two types. All cords have two flat prongs, but outdoor cords and some others have a third, semi-rounded prong for the ground wire. If a plug has only two prongs, one of them is usually larger than the other. This means the cord has polarization, and the return wire connects to the larger prong. You must use this type of cord with a polarized receptacle, and it will only fit into it one way. You might find non-polarized two-prong plugs on older extension cords or on cords purchased in Japan and other countries. The prongs are the same size on these cords. They are safe to use with lamps that have non-polarized plugs but won't work with appliances that have polarized plugs.
A number of plug characteristics vary from cord to cord. For example, plugs on outdoor cords have sheathing to keep water out, but the plugs on some indoor cords are removable. You can separate these plugs by removing two or three screws. In addition, some plugs include a fuse or breaker to prevent power surges from damaging sensitive lights or electronic equipment. Another feature you might find on some plugs is an LED that lights up whenever you plug the cord into a live receptacle.