# IEC 60287-3-2:2012 pdf Electric cables – Calculation of the current rating – Part 3-2: Sections on operating conditions – Economic optimization of power cable size IEC 60287-3-2:2012 pdf Electric cables – Calculation of the current rating – Part 3-2: Sections on operating conditions – Economic optimization of power cable size
1 Scope
This part of IEC 60287 sets out a method for the selection of a cable size taking into account
the initial investments and the future costs of energy losses during the anticipated operational
life of the cable.
Matters such as maintenance, energy losses in forced cooling systems and time of day
energy costs have not been included in this standard.
Two examples of the application of the method to hypothetical supply systems are given in Annex A.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60228, Conductors of insulated cables
IEC 60287-1 -1 , Electric cables – Calculation of the current rating – Part 1-1: Current rating
equations (100 % load factor) and calculation of losses – General
IEC 60287-2-1 , Electric cables – Calculation of the current rating – Part 2-1: Thermal
resistance – Calculation of thermal resistance
IEC 60853 (all parts), Calculation of the cyclic and emergency current rating of cables
NOTE The economic conductor size is unlikely to be identical to a standard size and so it is necessary to provide a continuous relationship between resistance and size. This is done by assuming a value of resistivity for each conductor material. The values recommended here for ρ 20 are: 1 8,35 × 1 0 –9 for copper and 30,3 × 1 0 –9 for aluminium. These values are not the actual values for the materials, but are compromise values chosen so that conductor resistances can be calculated directly from nominal conductor sizes, rather than from the actual effective cross-sectional areas.
5.2.3 Effect of charging current and dielectric losses
Dielectric losses and the losses due to charging current are always present in an a.c. system when the cable is energized and therefore operate at 1 00 % load factor. Both types of losses are significant only at high-voltage levels and are dependent on cable capacitance. Evaluation of transmission cable systems often assumes the placement of shunt reactors at the ends of the cable system to supply the reactive VARs required by the cable. The reactors have losses equal to about 0,8 % of power rating. Those losses should be considered in the evaluation of cable system losses and the cost of the reactors added to the cable purchase cost. For a given voltage level and insulation thickness, an increase in conductor diameter results in an increase in cable capacitance and, as a result of this, an increase in voltage dependent losses. Because of this, when dielectric losses are included in the analysis, these losses will tend to decrease the conductor diameter as opposed to the effect of current dependent losses. The dielectric and charging current losses are sometimes referred to as voltage-dependent losses, in contrast to the joule losses which are referred to as current-dependent losses. The cost of these voltage-dependent losses is included in the calculation by the following modification to Formula (1 1 ).
Two example calculations are provided in this annex. The first example relates to a 1 0 kV
cable circuit and the second example concerns a 1 32 kV single circuit.
In the first example, calculations are given for a supply system feeding ten equal loads
uniformly spaced along a route;
a) an application of the first approach (see 5.1 ), the economic current range method, to size