Because of the helical path of the strand layers there is more length of metal in a given length of stranded conductor than in a solid round conductor of the same AWG size, hence both the weight and dc resistance per unit length are increased. The amount of increase for all-aluminum conductors may be computed according to a method described in ASTM B 23 1, may be used.
The tensile load on a conductor is not always equally divided among the strands. This effect can reduce the total load at which the first strand breaks as compared with that of a solid conductor of equal cross section. However, this effect is more than offset by the fact that the unit tensile strength of commercially cold-drawn wire generally increases as its diameter is reduced, as is evident by the comparison for H19 stranded conductor.
According to ASTM Standards, aluminum conductors that are concentric-lay stranded of 1350 or 6201 alloys in the various tempers have their rated tensile strength (or minimum rated strength) taken as the following percentages of the sum of the minimum average tensile strengths of the component wires, multiplied by rating factors, as below:
7 wires per conductor One layer 96%
19 wires per conductor Two layers 93 %
37 wires per conductor Three layers 91 %
61 wires per conductor Four layers 90%
91 wires per conductor Five layers 89 %
Similarly, the rated strength of ACSR is obtained by applying rating factors of 96, 93, 91, and 90 percent, respectively, to the strengths of the aluminum wires of conductors having one, two, three, or four layers of aluminum wires, and adding 96 percent of the minimum stress in the steel wires at 1 .O percent elongation for cables having one central wire or a single layer of steel wires, and adding 93 percent of the minimum stress at 1 .O percent elongation if there are two layers of steel wires. All strengths are listed in pounds to three significant figures, and these strengths also apply to compact round conductors.
Special Conductor Constructions
Large conductors requiring exceptional flexibility may be of rope-lay construction. Rope-lay stranded cables are concentric-lay stranded, utilizing component members which are themselves either concentric stranded or bunched.
Bunched members are cabled with the individual components bearing no fixed geometric relationship between strands. Rope-lay stranded conductors may be stranded with subsequent layers reversing in direction, or may be unidirectional with all layers stranded in the same direction but with different lay lengths.
Some cables are designed to produce a smooth outer surface and reduced overall diameter for reducing ice loads, and under some conditions wind loading. The stranded cables are smoothed in a compacting operation so that the outer strands loose their circularity; each strand keys against its neighbor and many interstrand voids disappear.
A similar result is commonly obtained by use of trapezoidal strands that intertie with adjacent strands to create a smooth, interlocking surface.
Another cable design, expanded core concentric-lay conductor, uses fibrous or other material to increase the diameter and increase the ratio of surface area to metal cross-section or weight. Designed to minimize corona at voltages above 300 kV, they provide a more economical balance between cable diameter and current carrying capacity.
A “bundled” conductor arrangement with two or more conductors in parallel, spaced a short distance apart, is also frequently used for HV or EHV lines. Although the ratio of radiating area to volume increases as the individual conductor size decreases, the design advantages of bundling are not wholly dependent upon ampacity.
Normal radio interference, etc., and the usual controlling design characteristics are discussed elsewhere, but the current carrying capacity relationship is similar.