As current flows through a conductor due to it’s electrical resistance it produces heat. If the current in the conductor increases then the heat also continues to rise and eventually we could reach a temperature which will cause permanent damage to the cable or possibly even a fire within the installation.
In order to prevent this from happening we have a 70 Deg C maximum permitted temperature at the conductor for a standard thermoplastic PVC Twin and Earth cable. (See regulation 523.1 and Table 52.1 BS7671:2018) and we must ensure that our conductor does not go above this temperature.
We must also allow for anything that could restrict a cables ability to dissipate its heat as this too can cause a problem. In a domestic environment although not the biggest problem, this could be by running the cables with other cables or along runs with any central heating pipes etc.
But by far the biggest problem particularly in domestic installations is the amount of thermal insullation present, especially within studded walls and in loft spaces. This thermal insulation is designed to prevent heat loss and as such we should avoid it with our cables whenever possible.
Unfortunantly keeping cables away from thermal insulation is not always possible, so if we are passing through any thermal insulation one method of reducing the amount of heat present in our cable is to reduce the amount of current flowing through it.
Regulation 523.9 and Table 52.2 in BS7671:2018 provides guidance on how much the current should be reduced by dependant on the amount of thermal insulation the cable is passing through.
For example a cable passing through 200 mm of thermal insulation should have its current carrying capacity reduced by multiplying it by 0.63 and a cable passing through 500 mm or more of thermal insulation should be reduced by 0.5 times of its current carrying capacity when clipped direct.
If we can’t reduce the amount of current present to a value below the maximum of the reduced current carrying capacity then we need to increase the conductor size. This larger size will now have a lower resistance and a lower overall conductor temperature with the same amount of current flowing.
Looking at BS7671:2018 Table 4D5 which is specifically for 70 deg C Thermoplastic PVC cable (Twin & Earth) we have the maximum current carring capacities for various conductor sizes depending upon where they are installed and the amount of thermal insulation present, better know as ‘reference methods’.