From the foregoing it appears that for a line of given cost, length, and resistance, soft copper has the least cross-section and tensile strength; steel, the greatest cross-section, weight, tensile strength, and lowest permissible price per pound; and aluminum, the least weight and highest price per pound.
Relative Properties of Wires
Having Equal Lengths and Resistances.
| Metal in Wire. | Relative Cross Sections. | Relative Weights. | Relative Tensile Strengths. | Relative Prices per Pound for Same Total Cost. | |||
|---|---|---|---|---|---|---|---|
| Soft Copper | 1.00 | 1 | .00 | 1 | . | 1 | .00 |
| Hard Copper | 1.02 | 1 | .04 | 1 | .34 | .98 | |
| Very Hard Copper | 1.02 | 1 | .02 | 2 | .10 | .98 | |
| No. 1 Silicon-Bronze | 1.02 | 1 | .02 | 1 | .79 | .98 | |
| No. 2 Silicon-Bronze | 2.17 | 2 | .17 | 6 | .38 | .46 | |
| Aluminum | 1.66 | .50 | 1 | .46 | 2 | .00 | |
| Phosphor-Bronze | 3.84 | 3 | .84 | 11 | .29 | .26 | |
| Annealed Iron | 7.14 | 6 | .21 | 11 | .56 | .16 | |
| Mild Steel | 9.09 | 8 | .09 | 26 | .70 | .12 | |
The relative cross sections and weights of both iron and steel wires are so great as to prevent their general use because of the labor and cost of their erection.
So far as the first cost of the wire alone is concerned, iron may be approximately equal to copper in some metal markets. The only practical place for an iron wire, however, is one where copper would be too small or not strong enough. Steel wire finds a place, in spite of its high resistance, in those exceptional cases where a single span of several thousand feet must be made, requiring high tensile strength. In such cases it is usually better to give the steel span a greater resistance than an equal length of the main portion of the line, so as to avoid excessive size and weight of the span. Even when this is done the resistance of the steel span would be very small compared with that of a long transmission line.
Phosphor-bronze finds little use as conductors in transmission systems because of its relatively high electrical resistance. If great tensile strength is wanted, iron or steel will supply it at a fraction of the cost of phosphor-bronze. As a conductor simply, phosphor-bronze is worth only 0.26 as much per pound as soft copper, while its actual market price is greater than that of copper.
Silicon-bronze of relatively high resistance, requiring 2.17 times the section and weight of copper for equal conductivity, is entitled to little or no consideration as a transmission line material. This alloy, in order to give equal conductivity at equal cost with copper, must sell at only 0.46 of the price of copper per pound. But the price of silicon-bronze is equal to, or greater than, the price of copper, so that the cost of the high-resistance silicon-bronze for a line of given resistance will be more than twice that of copper. For this more than double cost the bronze gives 6.38 times the tensile strength of a soft copper line of equal conductivity.
Taking the market price of steel at one-fifth that of copper, which is amply high for the steel, as a rule, a steel wire of equal conductivity with the copper will cost only 1.6 times as much and will have 26.7 times the tensile strength of the copper, or four times the tensile strength of a wire of equal conductivity made from the high-resistance silicon-bronze. From this it is clear that steel offers a cheaper combination of conductivity and strength than does silicon-bronze of high resistance. That grade of silicon-bronze having the lowest resistance may cost 0.98 as much per pound as soft copper, and will have 1.79 times the strength of the copper for equal conductivity. This bronze actually costs more per pound than copper, so that it cannot give equal conductivity at equal cost.
Very hard-drawn copper has a conductivity equal to that of the best silicon-bronze, and the tensile strength of this copper is seventeen per cent greater than that of the bronze. Silicon-bronze costs more per pound than hard copper, but even with equal prices the hard copper gives equal conductivity and higher strength at the same cost. Furthermore, the conductivity of silicon-bronze is much more liable to serious variations than that of hard copper. Between hard-drawn copper and steel there is very little apparent place for any grade of bronze in electric transmission lines.
The hardest copper wire is very stiff, and is more liable to crack when twisted or bent than is wire of only medium hardness. Such medium-hard copper has a tensile strength of thirty-four per cent greater than soft copper of equal conductivity, and is much used on long transmission lines. Aluminum is the only metal which, for given conductivity in a transmission line, combines a smaller weight, a greater tensile strength, and a higher permissible price than soft copper for the same total cost. For equal conductivity an aluminum wire has a greater tensile strength than one of medium-hard copper, and costs less than copper of any grade when the price per pound of the aluminum is less than twice that of copper, which is usually the case.