Skin Effect

Depends on voltage and frequency; please specify further.

if S= skin effect

d= diameter of the stranded conductor

f= frequency of the system under consideration

then K=f(S,d,f)

Now, i want to know the formula f(S,d,f)

Huh? K=f(S,d,f)?

Save us all some time and look here.

Some equations provided here.

http://en.wikipedia.org/wiki/Skin_effect

I have never used the shortened form, as it's only an approximation. The above link to an excellent article gives much more detail. I used this for calculating loss over "Copperclad" transmission lines, and it was accurate every time. "Copperclad" had steel inside for linear strength, and copper outside for improved conductivity at higher audio frequencies. Litz is great for coil winding where high Q is needed, but only in the MF, HF range. When you get a bit higher, silver plated conductors are necessary.
Examle, the interior of VHF and UHF cavity filters.

"Stranded round conductor" skin effect (skin depth) is basically the same as a solid round conductor UNLESS all individual strands are electrically insulated from each other. This special type of stranded wire is referred to in post #5 and is called Litz wire.