AC and DC voltage wire areas
Skin effect and proximity effects are caused by AC magnetic fields affecting the usable part of a conductor for passing current. As frequency rises skin effect worsens but only as a square root of the frequency increase.

This means that DC resistance is lower than AC resistance (and I'm not talking about inductive reactance here). Skin effect: -



Skin effect is not negligible at 50 Hz for power transmission systems. At 50 Hz copper has a skin depth of about 9 mm making it very problematic for power distribution: -



Proximity effect occurs when AC currents are flowing in side-by-side conductors - the interactions between the magnetic fields produced tend to reduce the usable conductive part of a cable even more: -



The picture above is when the AC currents in each wire are in-phase (as happens in power distribution cable bundles. The picture also shows a level of skin effect.

This is a useful link for understanding both effects.