Copper has a significantly lower electrical volume resistivity: 0.017241 (Ω x mm2)/m for copper, compared to 0.0282 (Ω x mm2)/m for aluminium . This difference is highly relevant for power cables. To attribute to an aluminium conductor the same resistance as a copper conductor, the cross-sectional area of the aluminium conductor must become larger to compensate for aluminium’s higher electrical resistivity. In fact, for the same current-carrying capability, an aluminium conductor needs a 56% larger cross-sectional area than copper. One practical consequence is that less aluminium cable can be stored on a drum, resulting in shorter cable lengths and more joints. Using copper reduces the number of joints thus lowers the risk of system failures.
Copper has a lower coefficient of thermal expansion. This parameter measures the tendency of a material to change in volume as a function of temperature. Copper’s lower value is very important for cables since it reduces the risks of destructive forces in joints.
Corrosion is not an issue for copper. It is resistant to most organic chemicals and can operate indefinitely in most industrial environments. A green patina may form after long exposure to the atmosphere, but this is actually a protective surface film and does not affect performance. The protection of copper is unnecessary, even in salty offshore environments, whereas oxidation is a particular problem with aluminium conductors: the oxide layer should be removed, and an oxide-inhibiting compound applied to reduce oxidation.
Copper shows good resistance to creep; essential to avoid relaxation of contact pressure and for components of complex shape. Aluminium on the other hand exhibits evidence of significant creep at room temperature, whereas a similar rate of creep is only shown by high conductivity copper at 150°C.
Copper is one of the easiest metals to solder and for this reason, combined with its conductivity, finds many applications where good joint integrity is essential.
The significantly higher specific weight of copper (by a factor of three) leads to a more efficient process for laying submarine cables. This is important given the rapidly expanding offshore wind power market. The use of larger turbines and installations further from the shore, in hostile, deep sea environments, creates a multitude of challenges for submarine MV and HV cables. The choice of conductor can have a significant effect on how these challenges are overcome.
Copper does not react with water. This is important because water can find its way into a cable during shipping, handling, outside storage, accidental damage or cable joint or termination failures. In contrast, aluminium reacts with water to produce hydrogen gas. If the pressure of hydrogen inside a cable builds up, it can damage the insulation, resulting in partial discharge activities, failure or even complete destruction.
Stranded copper is available in very low cross-sections, such as 0.5 to 10 mm2, whereas stranded aluminium is only available in nominal cross-sectional areas of 10 mm2 and above. Stranded conductors consist of a number of thinner conductors twisted together into a single cohesive cable. Smaller diameter copper strands result in more flexible cables which are more suitable in applications requiring considerable movement.
Given their smaller cross-sectional areas compared to aluminium, for the same current rating, copper cables are easier to install and to repair. Copper is also less brittle which is important when using 3-core cables which have to be shaped and bent within cable ducts and termination enclosures.
Copper conductors come at a premium price. However, when looking at lifetime costs, there remains no economic advantage to using the initially less expensive aluminium rather than the more technically performant copper.