Litz wire is a specialized type of multi-strand conductor
(derived from the German word Litzendraht, meaning "braided or stranded wire"). Litz wire is a specialized type of multi-strand conductor engineered to distribute current evenly across all its strands, effectively mitigating high-frequency power losses.
The Physics Behind the Need
To appreciate why Litz wire is constructed the way it is, we have to look at how high-frequency alternating current (AC) behaves in a standard conductor.
When direct current (DC) passes through a wire, the current density is uniform across the entire cross-section. However, when an alternating current passes through a wire, it creates a changing magnetic field. This field induces eddy currents inside the conductor that oppose the original current flow in the center while reinforcing it at the outer edge.
The result is that current crowded toward the outside "skin" of the wire. The depth at which the current density drops to about 37% of its surface value is known as the skin depth (δ). It decreases inversely with the square root of the frequency:
ρ is the conductor resistivity
μ is the magnetic permeability
If a solid wire is significantly thicker than the skin depth at a given frequency, the center of the wire is completely wasted space, driving up the effective AC resistance.
When multiple current-carrying conductors are placed close to one another—such as in the tightly wound turns of a transformer or an inductor—the magnetic field of one wire influences the electron distribution in the neighboring wire. This forces the current to crowd into restricted localized regions of the conductor, compounding the losses already caused by the skin effect.
Construction and Design Philosophy
Litz wire counters these effects through a meticulous, mathematically driven arrangement of individual strands.
Instead of a single large conductor, Litz wire bundles tens, hundreds, or even thousands of microscopically thin, individually insulated magnet wire strands. Simply twisting them together isn't enough, though. If a strand stayed in the center of the bundle the whole time, it would still carry less current than the strands on the outside.
Therefore, true Litz wire uses a specific, braided or transposed twisting pattern. The strands are woven so that every single strand occupies every possible position within the bundle—alternating between the absolute center, the outer edge, and everywhere in between over a specific length of lay.
Because every strand experiences the exact same average electromagnetic environment over the total length of the wire, the total current divides itself perfectly evenly among them. The individual strand diameter is chosen to be equal to or smaller than the skin depth at the target operating frequency, ensuring that each individual strand experiences almost zero skin effect.
Classification of Litz Wire
Litz wire construction is standardly classified into several configurations based on how the bundles are built up:
Type 1: Features a simple twist of individually insulated strands wrapped together, often with an optional textile outer serving (like nylon, silk, or fiberglass) for added mechanical strength.
Type 2: Constructed by twisting bundles of Type 1 Litz wires together. This hierarchical bundling is essential for very high-power applications or lower frequencies where larger aggregate cross-sections are needed.
Type 3: Features bundles of Type 2 Litz wire that are further twisted around a central core (which may be inert or a separate grounding element).
Type 4: Utilizes uninsulated spacers or dummy cores within the twisting matrix to physically position the active strands away from high-intensity magnetic fields at the center.
Type 5: Features Type 2 bundles that are flattened into a rectangular profile to maximize the winding window filling factor in transformers.
Type 6: Flat braided configurations, often chosen for high-frequency grounding straps or ultra-low-profile planar magnetic components.
Key Technical Specifications
When selecting or manufacturing Litz wire, engineers focus on a few critical structural parameters:
Strand Gauge (AWG) vs. Frequency
The operational frequency dictates the maximum allowable diameter of the individual strands.
Recommended Individual Strand Size
30 to 36 AWG (0.25 mm to 0.127 mm)
36 to 38 AWG (0.127 mm to 0.101 mm)
38 to 40 AWG (0.101 mm to 0.079 mm)
40 to 42 AWG (0.079 mm to 0.063 mm)
The individual strands are coated with high-performance film insulations, typically polyurethanes, polyesterimides, or polyimide (Kapton). Polyurethane-based coatings are highly favored because they are solderable—the insulation melts away at standard soldering pot temperatures (380∘C to 480∘C), allowing all strands to be terminated simultaneously without manual stripping.
For the outer overall insulation or jacket, materials like nylon or silk braiding provide excellent abrasion resistance and winding protection, while specialized fluoropolymers (like PTFE or FEP) or silicone are used when high-temperature performance or extreme voltage isolation is required.
Primary Industrial Applications
Litz wire is an essential component across power electronics, telecommunications, and heavy industrial manufacturing:
Switch-Mode Power Supplies (SMPS): Modern high-efficiency power supplies operate at tens or hundreds of kilohertz to minimize size. Litz wire is used in the high-frequency transformers and inductors within these systems to prevent thermal runaway and maintain high efficiency.
Electric Vehicle (EV) Powertrains: EV traction motors and onboard charging systems operate under severe space and weight constraints. Utilizing Litz wire in stator windings reduces AC losses under high RPMs, directly extending driving range.
Induction Heating: Industrial induction furnaces and residential induction cooktops rely on generating intense, high-frequency alternating magnetic fields. Litz wire coils carry these massive high-frequency currents without overheating.
Wireless Power Transfer: From small Qi smartphone charging pads to high-power megawatt wireless charging infrastructure for electric buses, Litz wire maximizes the quality factor (Q-factor) of resonant transmission coils, ensuring optimal energy transfer efficiency over air gaps.
Renewable Energy Inverters: Solar inverters and wind turbine power converters use Litz wire in their high-power filter inductors to handle complex harmonic frequencies cleanly.
Advantages and Engineering Trade-offs
While Litz wire delivers unparalleled performance at high frequencies, it does introduce specific manufacturing and economic challenges.
Dramatic Loss Reduction: Minimizes RAC, allowing high-frequency magnetic components to run cool and maintain high efficiency.
Thermal Mitigation: Eliminates localized hot spots within copper windings, extending insulation life and system reliability.
Reduced Footprint: Lower losses allow engineers to design smaller, lighter, and more power-dense components.
Poor Packing Factor: Because every strand is round and individually insulated, there is a significant amount of air and insulation space inside the overall bundle. This results in a lower copper packing factor compared to a solid copper bar or square wire.
Manufacturing Complexity: The multi-step twisting, braiding, and serving processes make Litz wire significantly more expensive to produce than standard magnet wire.
Termination Vulnerability: If even a few microscopic strands fail to properly wet with solder during termination, the resistance profile changes, and performance drops.
To organize the various configurations of Litz wire, the industry classifies them into six primary standard types (defined by NEMA standards) based on how the strands are bundled, twisted, and structurally supported.
Choosing the right type depends entirely on your target operating frequency, power requirements, and the shape of the winding window in your magnetic components.
The 6 Standard Types of Litz Wire
A single bundle of individually insulated strands twisted together. It may be bare or covered with a single outer textile serving (like nylon or silk).
Compact, low-to-medium power inductors and high-frequency transformers.
Multiple Type 1 bundles twisted together. This creates a hierarchical, multi-stage grouping (bundles within bundles).
Higher power applications requiring larger overall cross-sections without losing flexibility.
Multiple Type 2 bundles twisted around a central, non-conductive core (such as a textile string) to provide structural shape.
Large-diameter, heavy-duty industrial cables where physical roundness and stability are critical.
Multiple Type 2 bundles twisted around an uninsulated central spacer or a hollow core.
Applications requiring active cooling or where strands must be physically kept out of high-intensity magnetic centers.
Multiple Type 2 bundles that are mechanically compressed and flattened into a rectangular or square profile.
High-power transformers where you need to maximize the copper filling factor in tight winding windows.
Articulated, flat-braided configurations rather than twisted bundles.
Low-profile planar magnetics, high-frequency grounding straps, and highly flexible connections.
Specialty & High-Performance Variances
Beyond the standard structural classifications, manufacturers offer specialized variants tailored for extreme environments:
1. Extruded Outer Jackets
While standard Litz wire uses textile braiding (serving) to hold the bundles together, high-voltage or rugged applications replace or supplement the textile with an extruded outer layer. Materials include Silicone for ultimate flexibility and temperature resistance, or fluoropolymers like PTFE, FEP, and PFA for chemical inertness and extreme dielectric breakdown strength.
2. Formable / Profiled Litz
Engineers frequently struggle with the "packing factor" (the amount of actual copper vs. empty air space inside the bundle). Special profiled Litz wires are compacted into precise square, rectangular, or trapezoidal shapes during manufacturing. This eliminates wasted air space and allows for ultra-tight, neat windings in high-efficiency electric vehicle (EV) motor stators.
3. UST (Ultra-Fine / High-Voltage) Litz
For specialized medical equipment (like pulse oximetry or SpO2 sensor components) and ultra-high frequency sensors, sub-miniature Litz wire utilizes individual strands as thin as 48 AWG to 52 AWG. These require high-precision handling and micro-soldering termination.
Would you like to see a breakdown of how to calculate the correct strand gauge for your frequency?
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