Equations & interpretation

δ = √[1/(π f μ0 μr σ)]; Rdc = length / (σ π radius²), in SI units, μ0 = 4π × 10⁻⁷ H/m. Conductivity is an input, not a temperature-corrected material lookup.

The classical good-conductor skin-depth expression describes exponential decay into a conductor. If the penetration depth is comparable to or larger than the wire radius, it does not justify replacing the wire cross-section with a thin annulus. Proximity effect, plating, roughness, magnetic dispersion and return-path geometry are excluded. No AC resistance, wire current rating or thermal result is inferred.

Equation background: MIT circuit notes, skin depth.