Effect of Direct Sunlight to Cable Size

The AS/NZS 3008 standard [ref. 1] provides guidelines in the cable selection process while considering the effects of the environment and the specific installation method for working voltages up to and including 0.6/1 kV at 50Hz AC. One of the environmental considerations it covers is the effect of direct sunlight exposure on the current-carrying capacity of the cables. This is an essential consideration because it causes cable derating due to additional heat from the sun.

One standard installation method in AS/NZS 3008 is 'Unenclosed Exposed to Sun'. This method is readily available in Cable Pro Web software and will apply any necessary derating factor to the calculation when selected.

However, there are instances when this does not accurately cover the actual installation method for a design. An example is for cables running through a conduit (enclosed) exposed to the sun. How do we then simulate these installation conditions?

Answer: Apply a 20°C increase in ambient air temperature.

As per Clause 3.5.8 of AS/NZS 3008 (see below), exposure to direct sunlight can be considered by derating the cable based on a 20°C increase in the ambient air temperature (from a standard of 40°C).

As we will show you next, this can be easily done in Cable Pro Web software using the Derating Wizard.

Example - Enclosed (in conduit) exposed to the sun

Consider a cable installation enclosed in a conduit and exposed to the sun. The closest standard installation method to this specific run is the 'Enclosed Wiring enclosure in air'.

This installation method does not yet consider any derating due to sun exposure. To consider direct sunlight exposure, a derating factor due to ambient air temperature must be included in the calculation.

We can use the Derating Wizard in Cable Pro Web software to do this.

In the Derating Wizard, go to the Air Temperature setting and set the Air ambient temperature to 60°C. This is a 20°C increase from a standard ambient air temperature of 40°C.

Assuming that no other derating factors are applied, the overall derating factor due to the simulation of direct sunlight exposure is 0.6. Ensure that 'Use Wiz' is selected (checked) so that the calculated derating factor from the Derating Wizard will be used in the calculation.

Below is a comparison of the cable size with ambient air temperatures of 40°C (standard ambient air temperature) and 60°C (direct sunlight exposure). All other parameters are the same except for direct sunlight exposure.

Cable size at standard ambient temperature (40°C):

Cable size when exposed to direct sunlight (60°C):

As you can see, since a derating factor has been applied due to the simulation of direct sunlight exposure, the recommended cable size is larger.

Reference:

[1] AS/NZS 3008.1.1:2017 Electrical installations—Selection of cables Part 1.1: Cables for alternating voltages up to and including 0.6/1 kV—Typical Australian installation conditions