Electrical resistivity and cross-sectional area of conductor cables: Copper vs. aluminium

One of the major differences between copper and aluminium is that copper has a significantly lower specific electrical resistivity than aluminium. This property quantifies how strongly a given material opposes the flow of electric current. A low value indicates a material that readily allows the movement of electric charge. On a relative scale the differences are significant: copper has a specific electrical resistivity of 100, compared to 160 for aluminium.

This difference is particularly relevant when designing and installing electrical networks and related components. 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, the aluminum conductor will have a cross-sectional area 56% larger than copper for the same current-carrying capability. This leads to a number of serious drawbacks.

More joints means more failures

A larger conductor cross-sectional area means that less cable can be stored on a drum. This results in shorter cable lengths and thus more joints per unit circuit length. Unfortunately, the more joints, the greater the potential for failures due to the following reasons:

  • Joints are man-made and therefore sensitive to jointing errors.
  • Joints are not always effectively tested when they have been installed, so jointing errors and other defects are not always identified during post-installation testing.
  • Joints are a discontinuity in the cable system, which are therefore liable to thermal-mechanical forces due to temperature cycling.

Reduced flexibility

The bending force is proportional to the square of the conductor’s cross-sectional area and thus on the fourth power of the diameter! So the larger the conductor’s cross-sectional area, the less flexible the cable.

Difficult duct installation

For installation in ducts a cable with a larger cross-sectional area and less flexibility will aggravate the installation process.

Copper or aluminium?

Replacing aluminium conductors with copper conductors reduces the number of joints necessary and thus reduces system failures. This leads to benefits in both CAPEX (installation cost) and OPEX (failure costs). Due to its smaller cross-sectional area, the copper cable will be more flexible and easier to install, which is a particular benefit when installing cables in ducts.

References

BICC Electric Cables handbook, third edition

A Comparison of Aluminum vs. Copper as used in Electrical Equipment – Larry Pryor et al, GE Consumer & Industrial.

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