On Friday, January 6, 2017 at 2:32:01 PM UTC-5, John Robertson wrote:

At 60 Hz in copper, the skin depth is about 8.5 mm, slightly thicker
than gauge 0.
You are talking about skin effect which is only starts to show above a
few kilohertz, otherwise the raw diameter of the wire is what carries
the current. Multi-strand wire (such as power cords, or speaker wire) is
convenient as it flexes, but it carries no more current than an
equivalent solid core wire at frequencies under around 10kHz. For
transmission towers where the outer diameter of the wire is much larger
that 8.5mm dia, then uses multi-stranded wire to get around it.

In the words of the prophet: Yabbut!! (Yes, but!)

Most discussions around wire material and types wind up, at least at some point, around speaker wire discussions. Hence the reference to surface area being an advantage. Speaker wires commonly carry in the multi-kHz frequencies.

For normal household use the difference will be negligible, and in some specific conditions actually favor solid conductors. And to be a tiny bit snarky: "There is a formula for that".

My specific choice for speaker wire is #12 19-strand THHN wire chucked and twisted in a drill for easier handling. Quite manageable and extremely tough.

Peter Wieck
Melrose Park, PA