Just a couple of comments about litz wire.
The Wikipedia article is a good one, but one portion of it deserves to be emphasized:
Litz wire uses some different tricks. Instead of using one big conductor, it uses lots of little conductors (strands) in parallel (forming a bundle). Each little conductor is less than a skin-depth, so an individual strand does not suffer an appreciable skin effect loss. However, that is not the complete story. The strands must be insulated from each other -- otherwise all the wires in the bundle would short together, look like a single large wire, and still have skin effect problems. Furthermore, the strands cannot occupy the same radial position in the bundle: the electromagnetic effects that cause the skin effect would still disrupt conduction. The bundle is constructed so the individual strands are on the outside of the bundle (and see low resistance) for a time, but also reside in the interior of the bundle (where the EM field changes are the strongest and the resistance is higher).
Much of the wire offered as "litz wire," including that from Cardas, doesn't meet this criteria, which is really the raison d'etre of litz wire, i.e. to address skin effect.
The Cardas "litz" wire is what's called "Type 1" litz. Which is just individually insulated wires simply twisted together like a typical stranded wire.
Type 1 litz doesn't address skin effect because the individual strands retain the radial position in the bundle from end to end. Current density increases in the outside strands and decreases in the inside strands, no differently than is the case with a solid wire of the same cross-section.
To address skin effect, you need to use a Type 2 construction.
Type 2 litz uses a number of bundles of twisted strands which are then further twisted together.
In this arrangement, each strand more or less occupies every radial position along the length of the wire and current density is more or less equal in every strand, ameliorating losses due to skin effect.
If anyone would like to experiment with Type 2 litz,
Surplus Sales of Nebraska sells an approximately 28 gauge litz (40 strands of 44 gauge wire) for just 30 cents a foot (though they have a $10 minimum order).
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