At DC and low frequencies, current flows uniformly through a conductor's cross-section. Above a few kHz, it doesn't. The skin effect pushes current toward the outer surface of the wire, and the proximity effect from adjacent conductors distorts the current distribution further. The result: most of a solid wire's copper sits idle while the outer skin overheats. Effective resistance climbs dramatically with frequency.

Litz wire (from German Litzendraht, "braided wire") solves this by replacing one thick conductor with many thin, individually enameled strands, transposed so every strand spends equal time at every radial position in the bundle. Each strand is thin enough that skin effect within it is negligible, and the transposition averages out the proximity effect so all strands share current equally.

Where you'll find it:

• Wireless charging pads (Qi, ~100–200 kHz) — the transmitter and receiver coils are almost always Litz
• Induction cooktops (20–50 kHz) — the work coil under the glass carries tens of amps at high frequency
• High-frequency transformers in switching power supplies (100 kHz–1 MHz)
• Radio transmitter tank circuits and RFID reader antennas
• MRI gradient coils and high-Q resonators

The rule of thumb — skin depth: the depth at which current density drops to 1/e (≈37%) of the surface value. For copper:

δ (mm) ≈ 66 / √f(Hz)

So at 100 kHz, δ ≈ 0.21 mm. Any strand thicker than about 2δ (~0.4 mm, roughly AWG 26) is wasting copper. At 1 MHz, δ ≈ 0.066 mm — you need AWG 40 strands or finer. This is why Litz specs read like "660/44" (660 strands of AWG 44 wire): the strand count sets the current capacity, and the strand gauge is chosen to be thinner than 2× skin depth at the operating frequency.

Design gotchas: Litz only helps if the transposition is real. Simply twisting bundled strands doesn't transpose them — you need a proper served or braided construction where each strand cycles through every position. Poorly-made Litz can be worse than solid wire because the enamel adds insulation losses without averaging the current. Also, Litz loses its advantage above ~2–3 MHz, where the strand-to-strand capacitance starts to dominate and you switch to alternatives like copper foil or tubing.

Termination is a pain: every one of those hundreds of strands must make electrical contact. Standard practice is to strip the ends with a solvent bath (or heated solder pot) to remove the enamel, then tin all strands simultaneously.