Why Copper Is Used in Cables?

Copper is an ideal conductor for building wiring, but not because it’s the most conductive metal. Silver is actually the best conductor, with an electrical conductivity 106% of that of annealed copper. However, the high cost of silver combined with its low tensile strength limits its use to special applications, such as joint plating and sliding contact surfaces.

Pound for pound (literally speaking), copper’s not even as good a conductor as aluminum. When you compare the cubic densities of copper (559 lb./cu. ft.) and aluminum (169 lb./cu. ft.), and factor in that aluminum has 56% the conductivity of copper, the result is that one pound of aluminum has the same electrical capability as nearly two pounds of copper.

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At first blush this sounds very promising, but the decreased conductivity of aluminum means that for the same current carrying capacity, an aluminum conductor’s cross section will be ~50% larger than that of copper, making copper better suited for applications where space is a premium. For applications where conductor thickness is an advantage, such as aerial electric power transmission cables, copper is rarely used.

Copper has a number of other benefits:

• Tensile Strength: Copper’s high strength (~2x that of aluminum) resists stretching, neck-down, creep, nicks and breaks, which reduces field failures and service interruptions.

• Ductility: Copper’s ductility makes it easy to draw down to diameters with very close tolerances. This doesn’t conflict with tensile strength, per se–copper is just less susceptible to breaking under tensile load than other conductors.

• Thermal Expansion: Copper has a low coefficient of thermal expansion. By comparison, aluminum expands about 30% more than copper. This higher degree of expansion, along with aluminum’s lower ductility, can cause electrical problems when bolted connections are improperly installed.

• Corrosion Resistance: Copper is fairly resistant to moisture, industrial pollution, and other corrosives. A significant differentiating factor is that oxides or other chemical compounds that do form on copper are conductive. Therefore, copper connections and terminations will not overheat from corrosion due to increased conductor resistance.

• Pliability: Usually the stronger a metal is, the less pliable it is. This is not the case with copper. A unique combination of high strength and high ductility makes copper ideal for wiring systems. At junction boxes and at terminations, for example, copper can be bent, twisted, and pulled without stretching or breaking. As pliable as it is, though, it’s typical for larger cables to use stranded copper (versus solid). Stranding improves cable life by making it more flexible and has nearly the same conductivity as a single-strand (solid) conductor. Solid wire is typically reserved for smaller diameters.

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