STRANDED WIRES AND CABLES
A wire is a single slender rod or filament of drawn metal. This definition restricts the term to what would ordinarily be understood as "solid wire." The word "slender" is used because the length of a wire is usually large when compared to its diameter. If a wire is covered with insulation, it is an insulated wire.
Although the term "wire" properly refers to the metal, it also includes the insulation. A conductor is a wire suitable for carrying an electric current. A stranded conductor is a conductor composed of a group of wires or of any combination of groups of wires.
The wires in a stranded conductor are usually twisted together and not insulated from each other. A cable is either a stranded conductor (single-conductor cable) or a combination of conductors insulated from one another (multiple-conductor cable).
The term "cable" is a general one and usually applies only to the larger sizes of conductors. A small cable is more often called a stranded wire or cord (such as that used for an iron or a lamp cord). Cables may be bare or insulated. Insulated cables may be sheathed (covered) with lead, or protective armor. The illustration below shows different types of wire and cable.
Conductors are stranded mainly to increase their flexibility. The wire strands in cables are arranged in the following order:
The first layer of strands around the center conductor is made up of six conductors.
The second layer is made up of 12 additional conductors.
The third layer is made up of 18 additional conductors, and so on.
Thus, standard cables are composed of 7, 19, and 37 strands, in continuing fixed increments. The overall flexibility can be increased by further stranding of the individual strands. The next illustration below shows a typical cross section of a 37-strand cable. It also shows how the total circular-mil cross-sectional area of a stranded cable is determined.
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