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Nonferrous metals and alloys

Consider why aluminum is used for overhead power lines by comparing the masses of conductors of the same length and the same electrical resistance. Taking the conductivities as 61% IACS for aluminum and 100% IACS for copper, and the densities as 2.70 g/cm³ for aluminum and 8.96 g/cm³ for copper, about how many times the mass of the copper conductor is the mass of the aluminum conductor?

  1. AAbout 0.30 times
  2. BAbout 1.6 times
  3. CAbout 0.49 times
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Answer: C. About 0.49 times

To keep the resistance R = ρL/A the same, aluminum, with 0.61 times the conductivity, needs 1/0.61 ≈ 1.64 times the cross-sectional area. Mass is proportional to cross-sectional area and density, so the ratio is 1.64 × 2.70/8.96 ≈ 0.49, about half that of copper. About 0.30 times compares only the densities and forgets the difference in conductivity, and about 1.6 times mistakes the ratio of areas for the ratio of masses. The wire becomes thicker, but because it is lighter the load on the towers falls and they can be spaced farther apart. For overhead lines, aluminum conductor steel-reinforced cable, with aluminum wires stranded around a steel core to add strength, is used. Where thickness is limited, as in wiring inside equipment, copper, with its higher conductivity per unit volume, has the advantage.

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