Skin Depth ($\delta$):

$$ \delta = \sqrt{\frac{\rho}{\pi \cdot f \cdot \mu_0 \cdot \mu_r}} $$

AC Resistance (Round Wire Approximation):

When radius $r \gg \delta$, current is confined to the outer ring of area $A_{ac} \approx 2 \pi r \delta$. Therefore, $R_{ac} = \rho \frac{L}{2 \pi r \delta}$.