The conductor resistance increases due to several factors that include conductor skin effect,
conductor proximity effect, shield eddy currents, shield circulating currents, and steel conduit losses. The ac resistance is determined from the following Equation.

Rac = Rdc (1 + Ycs + Ycp + Yse + Ysc + Yp)

where
Rdc is the dc resistivity at reference temperature μohm/m (μohm/ft)
Ycs is the conductor skin effect
Ycp is the conductor proximity effect
Yse is the shield eddy current
Ysc is the shield circulating current
Yp is the steel conduit losses

Note the factors used to calculate Rac are based on a per-unit resistance measured in micro-ohms/meter
(micro-ohms/foot).

Conductor skin effect—Ycs

The skin effect is caused by the varying current intensity that results in varying inductance through aconductor's cross section. The inductance is maximum at the center of the conductor and minimum on the surface. Skin effect varies with temperature, frequency, stranding and coating, and can typically be ignoredfor cables 350 kcmil and smaller (less than 1% impact). The skin effect factor is approximated using Equation (C.7a) for Rdc in μohm/m, and Equation (C.7b) for Rdc in μohm/ft.



Conductor proximity effect—Ycp

This effect is due to the force developed by currents flowing in the same direction in adjacent
conductors, which concentrates electrons in the remote portions of a conductor. Ycp increases as spacing
between conductors is decreased. The factor is calculated using Equation (C., Equation (C.9a), and
Equation













source: IEEE Std 525-2007