IEC 60287-3-2:2012 pdf free download – Electric cables – Calculation of the current rating
0.1General part
The procedure generally used for the selection of a cable size leads to the minimumadmissible cross-sectional area,which also minimizes the initial investment cost of the cable.lt does not take into account the cost of the losses that will occur during the life of the cable.
The increasing financial and environmental cost of energy, together with the energy losseswhich follow from conductors operating at high temperatures，requires that cable sizeselection be considered on wider grounds.Rather than minimizing the initial cost only，thesum of the initial cost and the cost of the losses over the anticipated operational life of thesystem should be minimized.For this latter condition, a larger size of conductor than would bechosen based on minimum initial cost will lead to a lower power loss for the same current.This, when considered over its anticipated operational life, will reduce the energy losses andthe total cost of the system. Where thermal consideration dictates the use of the largestpractical conductor size，the installation of a second parallel cable circuit can result in areduction in the total cost over the life of the installation.
The formulae and examples given in this standard are arranged to facilitate the calculation ofthe economic conductor size after factors such as system voltage，cable route,cableconfiguration and sheath bonding arrangements have been decided. Although these factorsare not considered in detail, they have an impact on both the installation and operating costsof a cable system. The effect of changing any of the above factors on the total cost over theanticipated operational life of the system can be determined using the principles set out in thisstandard.
Future costs of energy losses during the anticipated operational life of the cable can becalculated by making suitable estimates of load growth and cost of energy. The mosteconomical size of conductor is achieved when the sum of the future costs of energy lossesand the initial cost of purchase and installation are minimized.
The saving in overall cost,when a conductor size larger than that determined by thermalconstraints is chosen， is due to the considerable reduction in the cost of the joule lossescompared with the increase in cost of purchase.For the values of the financial and electricalparameters used in this standard,which are not exceptional, the saving in the combined costof purchase and operation is of the order of 50 % (see A.2.5). Calculations for much shorterfinancial periods can show a similar pattern.
A further important feature,which is demonstrated by examples, is that the savings possibleare not critically dependent on the conductor size when it is in the region of the economicvalue, see Figure A.3. This has two implications:
a) the impact of errors on financial data，particularly those which determine future costs,is small. While it is advantageous to seek data having the best practicable accuracy,considerable savings can be achieved using data based on reasonable estimates;
b) other considerations with regard to the choice of conductor size which feature in the overall economics of an installation， such as fault currents，voltage drop and sizerationalization, can all be given appropriate emphasis，without losing too many of thebenefits arising from the choice of an economic size.
The formulae given in this standard are written for a.c. systems but they are equallyapplicable to d.c.systems.Clearly, for d.c.systems, the d.c.resistance is used in place of thea.c.resistance and the sheath and armour loss factors are set to zero.