Factor for proximity effect of conductors

Proximity effect is the tendency for current to flow along one side of a conductor due to interaction of the magnetic fields of the current in the conductor considered and the currents in adjacent conductors.

$x_p$ should not exceed 2.8 for an accurate estimation of the proximitiy effect factor $y_p$.

Symbol
$x_p$
Formulae
$\sqrt{{10}^{-7}\frac{8\pi f}{R_{cDC}}k_p}$