International standard IEC 60287-3-2 provides a methodology for calculating the most economic cable cross section. It enters into a lot of detail and requires a reasonably accurate prediction of what is called the loss load factor, without giving any advice on how this should be achieved. It claims to be a standard for all cables, but the examples in the annexes are limited to utility cables. These have a less erratic load profile, making it easier to estimate the loss load factor. This is not the case for cables behind the meter, making it a highly complex task to rigorously follow this standard. The standard’s adoption rate in buildings is most probably very low.
The ECI Whitepaper Economic Conductor Size Optimisation in Buildings proposes a simplified formula, derived from IEC 60287-3-2, but abstracting some minor factors. For the loss load factor, the formula makes use of the typical values per sector and per type of circuit set out in the Ecodesign Economic Cable Sizing Preparatory Study.
International standard IEC 62125-7: Environmental considerations specific to electrical power and control cables – Environmental and energy cost based conductor size optimization develops similar calculations to IEC 60287-3-2, but brings the cost of CO2eq emissions into the equation. The calculations are based on the LCA methodology.