Effects of Temperature On Cables – Faulty Electrical Wiring

In one project that I got involved with, I came up with several options for cable selection. In all options, the current carrying capacity of the cables after applying derating factors are all acceptable. The voltage drop in all options also satisfy the requirements. Fault-loop impedance as well is not a problem.

The options I came up using a PVC/Cu/PVC 75OC cable are:

  1. Cable - 2 x 300 mm²
    Voltage Drop - 1.61V (0.40%)
    Cable Temperature - 62.0OC
  2. Cable - 2 x 240 mm²
    Voltage Drop - 2.53V (0.63%)
    Cable Temperature - 71.8OC
  3. Cable - 3 x 240 mm²
    Voltage Drop - 1.2V (0.30%)
    Cable Temperature - 45.8OC

With all parameters satisfying the project requirement, all options will be alright. And the most economical will be using a 2 x 240 mmÃ'² cable. But hold-on, cable operating temperature is only 3 degrees below the cable rated temperature, will I be using it? Yes or no. Honestly, I will not! Even if it satisfy my design parameters. Some cable manufacturers allow for overloads on their cables but again, it is I, as the Design Engineer who will be accountable and not the cable manufacturer.

Exposure to high temperatures particularly for cables with thermoplastic insulation will make the insulation to crack and later on, the insulation will become carbonized. A carbonized insulation will become a conductor. That will be when the problem occur. The problem will not pop up during the early years of the project but may take decades. This problem plus poor maintenance will lead to another occurrence of 'faulty electrical wiring'.