Why does Kelvin’s Law give different conductor sizes for two identical systems?

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  1. Because the electrical load is different in both systems
  2. Due to variations in interest rates, depreciation, and energy costs
  3. Because one system uses overhead lines and the other uses underground cables
  4. Because the resistance of conductors is always changing

Answer (Detailed Solution Below)

Option 2 : Due to variations in interest rates, depreciation, and energy costs
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Kelvin's Law 

The Kelvin's law states that the most economical size of a conductor is that for which annual interest and depreciation on the capital cost of the conductor is equal to the annual cost of energy loss.

Graphical representation of Kelvin's law:

qImage6812454b39332630ce1f73a8

Limitations of Kelvin's law:

  • Interest and depreciation on the capital cost cannot be determined accurately.
  • It gives different conductor sizes for two identical systems due to variations in interest rates, depreciation, and energy costs.
  • This law does not take into account several factors like safe current carrying capacity, corona loss, etc.
  • The economical size of a conductor may cause the voltage drop beyond the acceptable limits.
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