Litz wire

Litz wire is a type of cable used in electronics to carry alternating current. The wire is designed to reduce the skin effect and proximity effect losses in conductors used at frequencies up to about 1 MHz. It consists of many thin wire strands, individually insulated and twisted or woven together, following one of several carefully prescribed patterns often involving several levels (groups of twisted wires are twisted together, etc.). The result of these winding patterns is to equalize the proportion of the overall length over which each strand is at the outside of the conductor, an effect not achieved with simple twisted-strand hookup wire.

The term litz wire originates from Litzendraht (coll. Litze), German for braided/stranded wire or woven wire.

Litz wire reduces the degree of the skin effect and the proximity effect.

The resistance of a conductor at DC (0 Hz) depends on its cross-sectional area. A conductor with a larger area has a lower resistance. The resistance also depends on frequency because the effective cross-sectional area changes with frequency. For alternating currents (AC), the skin effect causes the resistance to increase with increasing frequency.

For low frequencies, the effect is negligible. For AC at frequencies high enough that the skin depth is small compared to the conductor size, the skin effect causes most of the current to flow near the conductor's surface. At high enough frequencies, the interior of a large conductor does not carry much current.

So round conductors such as wire or cables larger than a few skin depths do not conduct much current near their axis, so the central material is not used effectively.

One technique to improve the efficiency is to reduce amount of material that does not carry current by making the conductor of hollow tubing. The large surface area of the tube conducts the current with much less resistance than a solid wire with the same cross-sectional area would. The tank coils of high power radio transmitters are often made of copper tubing, silver plated on the outside, to reduce resistance.

Litz wire is another method, which employs a stranded wire with individually insulated conductors (forming a bundle). Each thin conductor is less than a skin-depth, so an individual strand does not suffer an appreciable skin effect loss. The strands must be insulated from each other—otherwise all the wires in the bundle would short together, behave like a single large wire, and still have skin effect problems. Furthermore, the strands cannot occupy the same radial position in the bundle over long distances: the electromagnetic effects that cause the skin effect would still disrupt conduction. The weaving or twisting pattern of the wires in the bundle is designed so that the individual strands are on the outside of the bundle for a distance (where the EM field changes are smaller and the strand sees low resistance), and are inside for a distance (where the EM field changes are the strongest and the resistance is higher). If each strand has a comparable impedance, current is distributed equally among every strand within the cable. This allows the interior of the litz wire to contribute to the overall conductivity of the bundle.

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