page_title: Overhead power line
section_title: Conductors
hierarchical_section_title: Overhead power line / Conductors
caption_attribution_description: English: Overhead power line conductor comparison: a conventional steel-reinforced round-wire ACSR conductor (left) and a high-capacity, low sag ACCC conductor with light-weight composite core and compact trapezoidal aluminum strands (right)
context_page_description: An overhead power line is a structure used in electric power transmission and distribution to transmit electrical energy across large distances. It consists of one or more uninsulated electrical cables suspended by towers or poles.
  Since most of the insulation is provided by the surrounding air, overhead power lines are generally the least costly method of power transmission for large quantities of electric energy.
context_section_description: The most common conductor in use for transmission today is aluminum conductor steel reinforced (ACSR). Also seeing much use is all-aluminum-alloy conductor (AAAC). Aluminum is used because it has about half the weight and lower cost of a comparable resistance copper cable.  It does, however, require a larger diameter than copper because of lower specific conductivity. 
  Copper was more popular in the past and is still in use, especially at lower voltages and for grounding.
  While larger conductors lose less energy because of their lower electrical resistance, they cost more than smaller conductors.  An optimization rule called Kelvin's Law states that the optimum size of conductor for a line is found when the cost of the energy wasted in a smaller conductor is equal to the annual interest paid on that additional cost of the line construction for a larger conductor. The optimization problem is made more complex by additional factors such as varying annual load, varying cost of installation, and the discrete sizes of cable that are commonly made.
  Since a conductor is a flexible object with uniform weight per unit length, the shape of a conductor hanging between two towers approximates that of a catenary. The sag of the conductor (vertical distance between the highest and lowest point of the curve) varies depending on the temperature and additional load such as ice cover. A minimum overhead clearance must be maintained for safety. Since the length of the conductor increases with increasing heat produced by the current through it, it is sometimes possible to increase the power handling capacity (uprate) by changing the conductors for a type with a lower coefficient of thermal expansion or a higher allowable operating temperature.
  Two such conductors that offer reduced thermal sag are known as composite core conductors (ACCR and ACCC conductor).  In lieu of steel core strands that are often used to increase overall conductor strength, the ACCC conductor uses a carbon and glass fiber core that offers a coefficient of thermal expansion about 1/10 of that of steel.  While the composite core is nonconductive, it is substantially lighter and stronger than steel, which allows the incorporation of 28% more aluminum (using compact trapezoidal-shaped strands) without any diameter or weight penalty.  The added aluminum content helps reduce line losses by 25 to 40% compared to other conductors of the same diameter and weight, depending upon electric current.  The carbon core conductor's reduced thermal sag allows it to carry up to twice the current ("ampacity") compared to all-aluminum conductor (AAC) or ACSR.
  The power lines and their surroundings must be maintained by linemen, sometimes assisted by helicopters with pressure washers or circular saws which may work three times faster. However this work often occurs in the dangerous areas of the helicopter height–velocity diagram, and the pilot must be qualified for this "human external cargo" method.
caption: Conventional ACSR (left) and modern carbon core (right) conductors
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