"At high frequencies, current travels near the surface of the wire because of the
skin effect, resulting in increased power loss in the wire. Stranded wire might seem to reduce this effect, since the total surface area of the strands is greater than the surface area of the equivalent solid wire, but ordinary stranded wire does not reduce the skin effect because all the strands are short-circuited together and behave as a single conductor. A stranded wire will have higher resistance than a solid wire of the same diameter because the cross-section of the stranded wire is not all copper; there are unavoidable gaps between the strands (this is the
circle packing problem for
circles within a circle). A stranded wire with the same cross-section of conductor as a solid wire is said to have the same
equivalent gauge and is always a larger diameter.
However, for many high-frequency applications,
proximity effect is more severe than skin effect, and in some limited cases, simple stranded wire can reduce proximity effect. For better performance at high frequencies,
litz wire, which has the individual strands insulated and twisted in special patterns, may be used."
en.wikipedia.org