Copper sets the standard for conductivity

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Copper is the first choice for electrical conductors as it has the highest electrical and thermal conductivities among commercial metals

In fact, all metals are benchmarked on copper’s electrical conductivity – in 1913 the International Electrotechnical Commission defined the International Annealed Copper Standard (IACS) in terms of copper’s electrical conductivity, setting it at 100% IACS (= 58.00 MS/m). Since 1913, processing technology has improved to the point where commercially produced high conductivity copper (HCC) routinely reaches or exceeds an electrical conductivity of 102 % IACS. Only silver at 106% IACS is more conductive. However, its high cost combined with its low tensile strength limits its use to special applications. By comparison, pure aluminium has an electrical conductivity of only 61% IACS with the alloys of aluminium performing even worse.

The relative performance of the metals is similar when it comes to thermal conductivity - silver is 9% more conductive than copper, while aluminium lies at 63 %.

The benefit of copper’s superior electrical conductivity is that the conductor cross-section needed to carry a given current is at least 40% smaller. For busbars and insulated wires and cables, this additionally means lesser material use in insulation, sheathing and armoring and smaller containment sizes, a great advantage in space constrained constructions. For enameled wires used in electric motors and transformers, it means smaller winding slots and windows resulting in less magnetic material, smaller equipment sizes, smaller civil foundations and less space requirements.

When thermal conductivity is considered as well, the heat generated while carrying an electric current is dissipated much faster by a copper conductor, resulting in more uniform temperatures across its cross-section, thus avoiding hot spots. This is particularly important under  overloading conditions ,such as during the starting of motors direct on line, when currents can shoot up as much as 6 times and a large amount of heat is generated in a short period. The smaller diameter of the conductor also means that the heat has to traverse a shorter path to the surface, making the transfer even quicker. Seen together with the much higher melting point of the metal, conductors made of copper can thus withstand higher  overloading conditions without softening or losing mechanical properties like recrystallisation or creep, which are thermo-activated processes and scale roughly with the absolute melting temperature. These properties are also useful in applications like commutators. Alloying with a small quantity of silver can raise the softening temperature even further.

Another aspect to consider is the vastly different impact on conductivity due to the oxides that inevitably form on the surface of both copper and aluminium when exposed to air at connections and terminations. The oxides of aluminium are hard, tenacious and effective electrical insulants  , whereas the oxides of copper are softer and conductive.  As a result, connections and terminations with copper seldom overheat and do not require surface preparation of the use of oxide-inhibiting compounds.

References

1. High Conductivity Copper for Electrical Engineering, ECI publication no. Cu0232, Feb 2016 
2. White Paper – A Comparative Evaluation of Copper and Aluminium Wires and Cables in Building Installations, ECI publication no. Cu0172, Dec 2015