Cable De-Rating

Cable de-rating is a crucial aspect of electrical wiring and installation. It refers to the process of reducing the ampacity (current carrying capacity) of a cable to account for various factors that can affect its performance and safety. Two key standards that outline the requirements for cable de-rating are AS3000 and AS3008.

AS3000 is the Australian standard for the electrical installation of buildings. It sets out the requirements for the design, installation, and testing of electrical wiring systems. AS3008 is the Australian standard for the installation and testing of cables. It provides guidelines for the installation and testing of electrical cables in various environments, including underground ducts, on the surface, and on trays.

Cable de-rating is necessary because the actual current carrying capacity of a cable is affected by a number of factors, including the method of installation, the number of cables installed together, and the ambient temperature. For example, when cables are installed in underground ducts, they are exposed to higher temperatures than when they are installed on the surface. This means that the ampacity of the cables must be reduced to account for the increased temperature.

To calculate the de-rating of a cable, the first step is to determine the ampacity of the cable at the installation temperature. This can be found in the cable manufacturer's data sheet or by using the tables provided in AS3000 and AS3008. Once the ampacity of the cable is known, the next step is to apply the appropriate de-rating factor based on the method of installation, the number of cables installed together, and the ambient temperature.

For example, let's say we have a cable with an ampacity of 100 amps. We are installing it in an underground duct, and there are four other cables installed in the same duct. The ambient temperature is 40 degrees Celsius. According to the tables in AS3000, the de-rating factor for underground duct installation is 0.5, the de-rating factor for multiple cables installed together is 0.8, and the de-rating factor for an ambient temperature of 40 degrees Celsius is 0.75. To calculate the de-rated ampacity of the cable, we multiply the ampacity by the de-rating factors: 100 amps x 0.5 x 0.8 x 0.75 = 30 amps. This means that the cable can only safely carry 30 amps of current.

It's important to note that cable de-rating is not a one-time calculation. It must be done at the design stage, during installation, and at regular intervals during the life of the cable. This ensures that the cables are operating safely and within the correct parameters.

In conclusion, cable de-rating is a crucial aspect of electrical wiring and installation. It refers to the process of reducing the ampacity of a cable to account for various factors that can affect its performance and safety. Standards such as AS3000 and AS3008 provide guidelines for calculating de-rating based on the method of installation, the number of cables installed together, and the ambient temperature. It's important to regularly check and calculate the de-rating of cables to ensure they are operating safely and within the correct parameters.