Hi
If you consider: R=pL/A which is valid for both ac and DC.
I think the only difference between rac and RDC is skin effect. In higher frequencies, area where current can pass through is decreased (because of the skin effect), so rac is increased. Now, I'm looking for the relation between them.
skin effect & proximity effect both cause the current to flow in the outer surfaces of the wire at higher frequencies, skin depth (at which the current density, J, falls to 37%) is approx 0.066/sqrt(MHz) so at 60Hz it is about 8.5mm, 100kHz = 0.2mm, so round wires thicker than about 0.1mm at 100kHz (and not in a proper woven litz structure), will get hotter than their RDC suggests...
skin effect & proximity effect both cause the current to flow in the outer surfaces of the wire at higher frequencies, skin depth (at which the current density, J, falls to 37%) is approx 0.066/sqrt(MHz) so at 60Hz it is about 8.5mm, 100kHz = 0.2mm, so round wires thicker than about 0.1mm at 100kHz (and not in a proper woven litz structure), will get hotter than their RDC suggests...