When current flows through a conductor in AC mode, it does not make use of the full cross section of the conductor available. The core of the conductor does not carry any current, rather only the surface and a little portion below it (skin depth) would take part in conduction. Skin depth is the thickness at which the current density is reduced by 63%. Thus a solid conductor in effect acts as a hollow one, increasing the effective resistance offered to the current flow. The amount of conductor cross-section used for current flow is dependent on the frequency. Higher the frequency, lower will be the cross-section used and hence higher resistance for the same conductor. Thus the resistance offered to AC is higher than the DC resistance by some factor. I've seen AC resistance to be taken 1.6 times DC resistance for 50 Hz AC supply (though some sources say it is 1.1 times or higher). Why don't we use hollow conductors when it will save a lot of material in long distance transmission? Saving material means less cost too. Or is it not feasible to make hollow conductors for such long distances?