When calculating the cross-section of the conductors of an electrical installation, the following factors must be considered:

• The current likely to flow through the conductor

In the most usual case, this current is defined by the overload protection located upstream or downstream of the cable.

• Environmental parameters and methods of installation

The circulation of an electric current in a conductor generates heat (Joule effect). In steady state, the environmental parameters (mainly the temperature of the surrounding environment), the method of installation (buried, under conduit, cable tray, etc), the proximity of other cables (grouping factors) and the presence of harmonics allow to determine the ampacity for a given section of the conductor so that its temperature of thermal equilibrium does not exceed its maximum core temperature in service. This temperature is an essential data related to the manufacture of the cable (mainly the nature of the insulation).

• Thermal stress withstood by the conductor in the event of a short circuit

It is also necessary to check that the conductor temperature during a short circuit does not exceed the maximum core temperature on short-circuit condition, which is also a cable manufacturing data. This verification requires knowledge of the different short-circuit currents and the corresponding tripping times of the protections.

• The voltage drop

The conductor cross-section has a direct influence on the voltage drop and can therefore be decisive for the respect of the limits imposed by the different standards or regulations.

• Energy efficiency

IEC 60364-8-1 defines the requirements for energy efficiency. Especially, it specifies that the increase of the cross-section of the cables reduces power losses. The decision must be made considering the savings achieved within a specified period, in the context of the additional costs resulting from oversizing.