ECONOMIC size of a CONDUCTOR is determined by Kelvin's law.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:As the annual cost of the conductor is directly proportional to the size of the conductor, it is shown by the straight line C1 in the figure.The annual cost of energy loss is shown by curve C2.The total annual cost curve is OBTAINED by adding the curve C1 and C2.The lowermost point on the total annual cost curve gives the most economical size of the conductor which CORRESPONDS to the intersection point of curves C1 and C2.So, here, the most economical area of cross-section of the conductor is represented by OX and the corresponding minimum cost is represented by XY.