[Selection Guide] How to choose the most economic size and type of cable?

Significance of selecting the right cable size and type:

Selection criteria of cables:

Derating factors:

  • Temperature derating factor (CT):Temperature derating factor (CT): Cables should be arranged in such a way so they get a minimum space to dissipate heat in their surroundings. This factor is used in calculations of cable sizing so as to consider the arrangement of cable for minimizing heat losses thus improving cable ampacity.
  • Conductor grouping factor (CG): Electromagnetic field around the conductors in a group is created when the current flows which causes the cable ampacity to decrease. For this reason, conductor grouping factor is considered.
  • Thermal resistivity of soil (CR): Standard temperature surrounding cables is 40°C. But if cables are to be buried in soil then temperature surrounding the cables rises up and cable ampacity gets affected. That’s why a factor for thermal resistivity of soil is considered in calculations so as to compensate rise in temperature.
  • Burial depth derating factor (CD): This factor depends on the depth of ground at which conductor is to be buried. Deeper in the ground cable will cause the derating factor to increase.
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Kelvin’s law for economic cable sizing:


  • Accurate interest and depreciation on capital cost can’t be determined.
  • Some factors like cable ampacity, corona effect, etc. are not considered in this law.
  • There may be an excessive voltage drop in the size of the conductor evaluated by Kelvin’s law.

Types of conductor:

  • ACSR (Aluminum Conductor, Steel Reinforced): It consists of steel strands surrounded by aluminum strands. It is the most recommended conductor for transmission lines and used for longer spans.
  • ACAR (Aluminum Conductor, Alloy Reinforced): It consists of aluminum-magnesium silicon alloy surrounded by the aluminum conductor. It has high mechanical strength and conductivity than ACSR, so it can be used for distribution and transmission on a large scale, but it is more expensive.
  • AAC (All Aluminum Conductor): It is also known as ASC (Aluminum Stranded Conductor) and has a conductivity of 61% IACS. Though it has good conductivity, still it is limited in its applications cause of low strength.
  • AAAC (All Aluminum Alloy Conductor): It is made up of aluminum-magnesium-silicon alloy and has a conductivity of 52.5% IACS. Because of better strength, it can be used for distribution but not suggested for transmission. It is suitable to be used in areas with high moisture content.
⁘ IACS (International Annealed Copper Standard) is a standard introduced by US. 
It is a standard against which conductivity of any conductor is compared.
It is value of conductivity of commercially available copper.

Cable shielding and insulation:

  • Insulation: Cable insulation is done by means of any dielectric like PVC so as to prevent leakage of current from a conductor.
  • Sheath: Cable is provided with the sheath for the purpose of the safety of cable from moisture. It should be some non-magnetic material like lead alloy.
  • Bedding: Purpose of bedding is to protect the sheath of cable from any damage caused by armouring.
  • Armouring: Armouring is another layer of galvanized steel over the cable so as to protect it from any kind of mechanical damage.
  • Serving: It adds to the mechanical strength of the cable. It provides overall protection against moisture, dust, etc.


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AllumiaX Engineering

AllumiaX Engineering

Leaders in Industrial & Commercial Power Systems Engineering