Conductor Cross Section Calculation in Cables
Electrical energy is transmitted and distributed by cables with various cross-sections and voltage levels from the power plants where it is generated to the end points where it is delivered. When determining the types of these cables, a decision should be made by analysing issues such as in which environments they will be used, what kind of external effects they will be exposed to, and how they will react to possible environmental conditions.
After determining the cable type with possible alternatives, the electrical cross-section (size) of the conductor to be used in these cables should be determined. The cable cross-section calculation should be made at least as detailed and careful as the analyses to be made while determining the cable type. The most important factor in cable cross-section calculation should be safety. In order for the cable to continue its duty without causing any safety problems, the current power and the power analyses that may be added to the system in the future must be done correctly.
The dimensions of the conductor should be determined in such a way that the polymer material wrapping the conductor does not force the limit values of the conductor and does not show a resistance to the current flowing through it above the maximum resistance values determined by international standards. Then, cost factors will come into play. In order to ensure the safety of the system, excessive expenditures should be avoided. The way to do this is to determine the most suitable conductor cross-section by making cost-safety optimisation.
Considering the safety and cost factors, there are three important parameters when calculating the cable cross-section:
Current carrying capacity, voltage drop and short circuit current.
Current Carrying Capacity: It is the maximum current value at which the installed or to be installed cable can be safely continued to be used without deterioration of any of its components, basically on the basis of temperature. Current carrying capacity has a direct effect on the selection of cable cross-section.
Voltage Drop: It is the condition that the voltage decreases in the distance from the source to the load due to the losses caused by the internal resistance of the conductor. As the transport distance increases, this loss increases and the conductor cross-section must be increased.
Short Circuit Current: It is a time-varying parameter in alternating current systems and is the amount of current flowing during the short circuit period in the region where the short circuit occurs.
Using these three parameters, it is possible to obtain three different cable cross-sections.
The process to be performed here is to determine a safe and economical cable conductor size by selecting an upper cross-section equivalent to or closest to the largest of these three cross-sections.
Let's assume that we calculate the conductor cross-section through an example: Let's determine a 3-core cable type and cross-section to supply a pump 200m away, with 235kW power and 1kV voltage level written on the label and allowing a maximum voltage drop of 3%.
Known values;
Power P=235kW
Voltage U=1kV
Power Factor: cosφ=0,8
Distance/Distance : L=200m
Allowable Voltage Drop: %e=3%
In line with these known parameters, it is possible to find a current value with the following power formula;
According to this current value, we can determine the conductor cross-sections of the cables as 50mm2 for PVC insulated cable and 35mm2 for XLPE insulated cable. (See Table: 1)
Finally, we need to decide whether to change the selected cross-section after checking the conformity of these types with the desired maximum percentage voltage drop calculation. If we calculate the voltage drop for cables with two different insulation materials;
Power P=235kW
Voltage U=1kV
Distance/Distance: L=200m
Allowable Voltage Drop: %e=3%
Conductivity Coefficient: k(Cu)=56 m/Ωmm2
Conductor Cross Section: SPVC=50mm2, SXLPE=35mm2
Since the values obtained are less than the permissible voltage drop %e (3%), there is no need to repeat this process; however, if we determine the minimum possible conductor cross-section based on the maximum voltage drop;
According to this calculation, the closest cross-section to the conductor cross-section we obtained is 35mm2.
35mm² cross-section, with XLPE insulation (N2XY), has sufficient current carrying capacity for the calculated current of 169,80A (174A), while with PVC insulation (NYY) it has a lower current carrying capacity (159A). In the calculation made in this way, the need to go to an upper cross-section occurred and was determined as 50mm2.
If a more detailed calculation is desired to be made by adding additional conditions to the example made; For example, in line with the assumption that we will pass the 3-core cable that will feed our system under the ground, the type, temperature and thermal resistivity of the soil will be needed. Assuming that we have an environment with a temperature of 350C, dry sand and a thermal resistivity of 1.5 K.m/W, the correction factors in the standards (IEC 60364-5-52 or DIN VDE 0298-4) under these conditions; fPVC = 0.84 / fXLPE = 0.89 for temperature and 1.1 for thermal resistivity.
In this case, if we rearrange the first determined current value according to these correction factors, we need to find our new current values. Therefore, starting from the upper cross-section of the first predicted cable and the relevant conductor cross-section, the current carrying capacity values must be obtained by multiplying by the above correction factors. If the current value resulting from multiplication by the correction factors is equal to or greater than the first determined current value; it means that the selected cross-section is suitable. Otherwise, it should be continued to be checked by going to the next higher section and multiplying by the correction factor. When we perform this process according to the example we have given, the current carrying values of the cables in the upper cross-section have a value greater than the desired current value when multiplied by the correction factors, so the cross-sections are evaluated as "suitable". New current values of the sections with correction factor applied;
IPVC(70mm2)=232×0,84×1,1= 214,36A
IXLPE(50mm2)= 206×0,87×1,1= 197,14A.
New cables and cross sections to be determined according to these new current values;
NYY - 3x70mm2, In = 232A
N2XY - 3x50mm2, In = 206A
For these additional conditions, under the conditions determined for two types of cables with 50mm2 conductors and PVC insulation, 35mm2 conductors and XLPE insulation, the required voltage drop is below the required level; Since the voltage drop of cables with 70mm2 conductors and PVC insulation, 50mm2 conductors and XLPE insulation will be lower than this level, there is no need to perform a voltage drop calculation again.
