Selecting cable size based on maximum conductor operating temperatures

We are often required to select a conductor size for a particular job.
How we do this in Australia and New Zealand is covered by the Standard AS/NZS 3008.1.

The current carrying capacity of a cable is a heat loss equation that depends on the electrical resistance of the conductor; the thermal resistance of the layers surrounding the conductor; the environment in which the cable is installed and the available temperature rise between the maximum temperature that the conductor can tolerate and the ambient temperature of the surrounding medium, eg air or soil. 

A cable with a 110 °C maximum conductor temperature rating can carry more current than one of the same size with a maximum operating temperature of 90°C, due to the increased heat rise parameter i.e. 110°C minus 40°C (70°C rise) vs 90°C minus 40°C (50°C rise)*. This often leads designers and installers to select a cable of smaller conductor size when selecting 110°C rated cables and gain an apparent cost saving compared to selecting a larger size conductor.

*Based on the assumption cables are installed in open air with a maximum ambient temperature of 40°C.

But what are the consequences of operating at these tem peratures?
Since conductor resistance increases with increasing temperature, the heat produced by a conductor carrying a specified current increases. If the installer/designer also chooses to install a smaller conductor size with 110°C insulation, the effect is magnified. The increased losses under this scenario can equate to more than 30% for a 110°C rated cable. Even when considering the reduced cost of installing the smaller size cable, the total cost of losses plus cable cost is often lower in the first year of operation. This “payback” for installing the larger conductor size yields a strong argument for its selection as for each year of operation the cost benefit accumulates.
See example below.

**Approximate cost based on 450A, 3 phase load, single core copper cables installed in
air on cable tray over a distance of 75m.

Safety considerations:

• A cable operating at 110°C at the conductor, the outer surface of the cable can be near 100°C. This will be quite unsafe to touch.
• Insulation and sheath of cables operating at 110°C will undergo more rapid ageing of the materials in contact with the conductor.