The inferiority of aluminum as an electrical conductor in terms of sectional area is more than offset by its superiority over copper in terms of weight. One pound of aluminum drawn into a wire of any length will have a sectional area 3.33 times as great as one pound of copper in a wire of equal length. This follows from the fact that the weight of copper is 555 pounds while that of aluminum is only 167 pounds per cubic foot, so that for equal weights the bulk of the latter is 3.33 times that of the former metal. As the aluminum wire has equal length with and 3.33 times the sectional area of the copper wire of the same weight, the electrical conductivity of the former is 3.33 ÷ 1.66 = 2 times that of the latter. Hence, for equal resistances, the weight of an aluminum is only one-half as great as that of a copper wire of the same length. From this fact it is evident that when the price per pound of aluminum is anything less than twice the price of copper, the former is the cheaper metal for a transmission line of any required length and electrical resistance.
The tensile strength of both soft copper and of aluminum wire is about 33,000 pounds per square inch of section. For wires of equal length and resistance the aluminum is therefore sixty-six per cent stronger because its area is sixty-six per cent greater than that of a soft copper wire. Medium hard-drawn copper wire such as is most commonly used for transmission lines has a tensile strength of about 45,000 pounds per square inch, but even compared with this grade of copper the aluminum wire of equal length and resistance has the advantage in tensile strength. While the aluminum line is thus stronger than an equivalent one of copper, the weight of the former is only one-half that of the latter, so that the distance between poles may be increased, or the sizes of poles, cross-arms, and pins decreased with aluminum wires. In one respect the strain on poles that carry aluminum may be greater than that on poles with equivalent copper lines, namely, in that of wind pressure. A wind that blows in a direction other than parallel with a transmission line tends to break the poles at the ground and prostrate the line in a direction at right angles to its course. The total wind pressure in any case is obviously proportional to the extent of the surface on which it acts, and this surface is measured by one-half of the external area of all the poles and wires in a given length of line. As the aluminum wire must have a diameter twenty-eight per cent greater than that of copper wire of equal length, one-half of the total wire surface will also be twenty-eight per cent greater for the former metal. This carries with it an increase of twenty-eight per cent in that portion of the wind pressure due to wire surface. In good practice the number of transmission wires per pole line is often only three, and seldom more than six, so that the surface areas of these wires may be no greater than that of the poles. It follows that the increase of twenty-eight per cent in the surface of wires may correspond to a much smaller percentage of increase for the entire area exposed to wind pressure. Such small difference as exists between the total wind pressures on aluminum and copper lines of equal conductivity is of slight importance in view of the general practice by which some straight as well as the curved portions of transmission lines are now secured by guys or struts at right angles to the direction of the wires.