[CU0268] Economic Conductor Size Optimisation in Buildings

Author: Bruno De Wachter

Published: December 2020

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Increasing the conductor cross section of a cable reduces the energy losses. The most economic cable cross section is where the investment cost is equal to the lifetime cost of the energy losses. The basic principle of economic cable sizing is straightforward, and yet application of this principle is far from common practice.

Cable sizing is subject to regulation through national building codes, but these only take safety and aspects of functionality into account, not energy efficiency. The most economic cable cross section is often more than double the mandatory minimum.

Particularly in non-residential buildings, there is substantial potential for energy savings from upsizing cables to their economic optimum. In the context of the far-reaching decarbonisation ambition set out in the EU Green Deal, it is a regulatory opportunity that must be considered. Even more so since it comes at no cost, creates local jobs and is economically advantageous for society as a whole.

A savings opportunity of 30 TWh/year

Approximately 8% of electrical energy generated in the EU gets lost in the network between generation and end-use, while 1.5% can be traced back to cable losses in non-residential buildings representing about 50 TWh per year (see Annex 1). By applying the principle of economic cable sizing, 60% of this, or 30 TWh/year, could be saved [2].

The opportunity to make this saving was identified in the Ecodesign Working Plan 2012-2014 and further analysed in the corresponding Preparatory Study (2013-2015) [1]. Such a savings potential is in the same order of magnitude as that of many products already subject to Ecodesign regulations. For many reasons, however, it was decided that Ecodesign was not an adequate regulatory tool to deal with the issue. As a result, the opportunity is still untapped.

This is even more regrettable given the long lifetime of electrical installations in buildings, as a result of which the inability to realise this energy savings potential will be locked-in for a long time where sub-optimal cables continue to be installed.

Calculating the economic cross section

The international standard IEC 60287-3-2 provides a methodology for calculating the most economic cable size. It enters into a lot of detail and requires a reasonably accurate prediction of what is called the loss load factor, which may not always be immediately available (see chapter ‘Calculating the optimised power cable size’). The standard gives no advice on how to estimate the loss load factor. Rigorously following this standard, consequently, makes calculations highly complex.

This white paper proposes a simplified formula, derived from IEC 60287-3-2, but abstracts some minor factors. For the loss load factor, the formula makes use of the typical values per sector and per type of circuit provided by the Ecodesign Economic Cable Sizing Preparatory Study. We also present two additional calculation methods for making a more precise and case-specific estimation of the loss load factor.

Making buildings future proof

Since power cables are installed to be operational for a long time, it is wise to think ahead and actively predict evolutions that might have an impact on the electrical installation. This exercise will be different for each sector. EV charging stations, PV rooftop installations, and the electrification of heating systems are among the factors to take into account in making buildings and their electrical installations future proof, even if it is not yet part of current building plans.