Now in recent days electro-technical standards no longer solely deal with issues of safety and functionality (EMC) but also consider the energy efficiency – and hence the life cycle costs – of electrical installations and assets. Thus, 50 m of NYY 5*4 mm² cable costs around EUR 95. With installation method B2 to VDE 0298-4 this cable can carry an operating current of 27 A if three conductors are loaded. The dissipated heat loss is 678 W. At an electricity price of 23 ct/kWh, the loss costs after 611 h have reached the level of the price of the cable. After just one month the cable has therefore been paid for twice, and thirteen times after one year – once to the cable manufacturer and twelve times to the electricity supplier. Using installation method C even increases the factor to seventeen.
However, cables and wires are rarely operated at a constant load, and if so, this load is hardly ever – continually – equal to the maximum permissible load. However, as the bill suggests, even then it is still worth considering to choose a thicker cross-section than required under present standards if the true operating point / operating range is a very long way from this extreme. And yet where is this point, and how can the optimum sizing of the cross-section be found in order to minimise life cycle costs?
While the TBINK-EEE of the DKE, the Technical Advisory Board for International and National Cooperation for Electrical Energy Efficiency, is still working on a roadmap, the committees are already fully involved. This applies, for instance, to committee 544 “Components for Energy-efficient Building Installations”, its working group 544.0.1 “Foundations of Energy-Efficient Appliances, Installations and Systems” and working group 221.5.1 “Energy Efficiency and Smart Grids”. What is there to be done?
In recent times, the trade press has on several occasions discussed load profiles – more precisely: It depends decisively on the load profiles as to whether and to what extent it is worthwhile “over-dimensioning” the conductor cross-sections. Unfortunately, the approaches to date to designing cable and wiring installations for minimum life cycle costs, simply assume the load profiles were known. This is what also IEC 60364-8-1, dealing in particular with “Energy Efficiency”, says in clause 6.2 “Determination of load profile”: “The main load demands within the installation shall be determined. The loads in kVA, together with their durations of operation, and / or an estimate of the annual load consumption (in kWh) should be identified and listed.” Further, clause 6.7.2 says: “Increasing the cross-sectional area of conductors will reduce the power losses. This decision shall be made by assessing the savings within a time scale against the additional cost due to this over-sizing. For cables, the chosen size shall be determined taking into account the cost of losses that will occur during the working life of the cable against the initial cost of the cable. […]
NOTE In some applications (particularly industrial), the most economical cross-sectional area of conductor may be several sizes larger than that required for thermal reasons.” Reference is then made to IEC 60287-3-2 “Economic optimization of power cable size”. This, however, like any other approach in the standards so far, assumes the load profiles to be known. Yet, this is rarely ever the case. So, what about the other situations? Some initial thoughts: