Litz wire is stranded wire with individually insulated strands,
used to reduce eddy-current losses (skin effect and proximity effect)
in high-frequency (typically 5 kHz to 1 MHz) applications.
Although litz wire manufacturers publish information
on recommended sizes of strands for different frequencies, and
on calculating losses for a given frequency and
construction, the actual mangetic field in a transformer or
inductor is different from what they base the
calculations on. Fortunately, the correct calculations are
usually quite simple. For more information, visit this
list of papers
on this topic and related subjects.