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Read the text. Line Conductors. Line conductors may vary in size according to the rated voltage
Line Conductors. Line conductors may vary in size according to the rated voltage. The number of conductors strung on a pole depends on the type of circuits that are used. Because they strike a happy combination of conductivity and economy, copper, aluminum, and steel are the most commonly used conductor materials. Silver is a better conductor than copper; but its mechanical weakness and high cost eliminate it as a practical conductor.
On the other hand, there are cheaper metals than copper and aluminum; but they would be hopelessly poor conductors. Copper is the touchstone of conductors. Other conducting materials are compared to copper to determine their economic value as electrical conductors. Aluminum-steel or copper-steel combinations have become popular for conductors in particular circumstances. Aluminum alloys are also used as conductors.
Copper Conductors. Copper is used in three forms: hard drawn, medium-hard drawn, and soft drawn (annealed). Hard-drawn copper wire has the greatest strength of the three and is, therefore, mainly used for transmission circuits of long spans (200 ft or more). However, its springiness and inflexibility make it hard to work with. Soft-drawn wire is the weakest of the three. Its use is limited to short spans and for tying conductors to pin-type insulators. Since it bends easily and is easy to work with, soft-drawn wire is used widely for services to buildings and some distribution circuits. Practice, however, has been toward longer distribution circuit spans and use of medium-hard-drawn copper wire.
Aluminumand ACSR Conductors. Aluminum is used because of its light weight, which is less than one third that of copper. It is only 60 to 80 percent as good a conductor as copper and only half as strong as copper. For these reasons it is hardly ever used alone, except for short distribution spans. Usually the aluminum wires are stranded on a core of steel wire. Such steel reinforced aluminum wire has great strength for the weight of the conductor and is especially suitable for long spans. Transmission lines often consist of aluminum conductors steel reinforced (ACSR).
Steel Conductors. Steel wire is rarely used alone. However, where very cheap construction is needed, steel offers an economic advantage. Because steel wire is three to five times as strong as copper, it permits longer spans and requires fewer supports. However, steel is only about one tenth as good a conductor as copper and it rusts rapidly. This rusting tendency can be counteracted (so that steel wire will last longer) by galvanizing, that is, by the application of a coat of zinc to the surface.
Copperweld or Alumoweld Conductors. The main disadvantages of steel are a lack of durability and conductivity. On the other hand, steel is cheap, strong, and available. These advantages made the development of copper-clad or aluminum-clad steel wire most attractive to the utility companies. To give steel wire the conductivity and durability it needs, a coating of copper is securely applied to its outside. The conductivity of this clad steel wire can be increased by increasing the coating of copper or aluminum. This type of wire, known as copperweld or alumoweld is used for guying purposes and as a conductor on rural lines, where lines are long and currents are small.
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