Which law is used to determine the economic size of a conductor? 

Kelvin’s law

• This law is used to determine the economic size of a conductor.
• The total annual cost of the transmission line can be divided broadly into two parts, the annual charge on capital outlay and the annual cost of energy wasted in the conductor.
• "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."

Gauss’s law

• Gauss Law states that the total electric flux out of a closed surface is equal to the charge enclosed divided by the permittivity.
• The electric flux in an area is defined as the electric field multiplied by the area of the surface projected in a plane and perpendicular to the field.

Ampere’s law

• This relates the integrated magnetic field around a closed loop to the electric current passing through the loop.
• Ampere’s Law can be stated as: “The magnetic field created by an electric current is proportional to the size of that electric current with a constant of proportionality equal to the permeability of free space.”

Newton’s law

Newton's laws of motion are three basic laws of classical mechanics that describe the relationship between the motion of an object and the forces acting on it. These laws can be paraphrased as follows:

• A body remains at rest, or in motion at a constant speed in a straight line, unless acted upon by a force.
• When a body is acted upon by a force, the time rate of change of its momentum equals the force.
• If two bodies exert forces on each other, these forces have the same magnitude but in opposite directions.