As alternating current passes through a conductor, the highest current density occurs on the surface of the conductor and decreases exponentially toward the center. This phenomenon is called the skin effect. The skin depth is the distance at which the current density drops to 1/e (about 0.368) of the surface density. 86.5% of the heat caused by the passage of current is generated in the skin depth.
Equation for calculating the skin depth:
[m] skin depth
vacuum permeability (4π×10-7 Hm-1)