Bundle conductors are used to increase the amount of current that may be carried in a line. Due to the skin effect, ampacity of conductors is not proportional to cross section, for the larger sizes. Therefore, bundle conductors may carry more current for a given weight.
More important, the bundle conductors result in lower reactance, compared to a single conductor.
As a disadvantage, the bundle conductors have higher wind loading and ice loading.
Previously, bundle conductors were thought to be useful only for very high voltage, such as 500 kV. More recently, the advantage has been proven, and they are more common, for 230 kV and 115 kV. However, in such cases, the increase in cost of construction due to heavy transmission towers and the insulator strings have to be reduced by adopting longer spans.
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