---
title: High-frequency litz wire selection for custom transformers
description: Engineering framework for litz wire selection. Mathematical analysis of skin depth, strand topologies, and proximity loss mitigation in custom transformers.
image: https://www.mycoil.info/images/og-image.jpg
---

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# High-frequency litz wire selection for custom transformers

mycoil.info Engineering Desk • Jun 4, 2026, 11:21:57 AM

[Litz wire](https://www.mycoil.info/high-frequency-litz-wire-construction-matrix) consists of multiple individually insulated strands woven or braided together in a precise geometric pattern. This construction is engineered to minimize high-frequency AC losses caused by the [skin effect](https://www.mycoil.info/glossary#term-skin-effect) and [proximity effect](https://www.mycoil.info/glossary#term-proximity-effect), making it an essential component in high-frequency custom transformers, resonant inductors, and wireless power transfer coils.

![Cross-sectional diagram of litz wire showing individual insulated strands bundled together](https://www.mycoil.info/image_agent_tag_651583970910277486)

## Understanding the skin effect

The [skin effect](https://www.mycoil.info/glossary#term-skin-effect) is the tendency of high-frequency alternating current ($AC$) to distribute itself unevenly within a conductor, forcing current density to concentrate near the outer surface (the "skin"). This reduces the effective cross-sectional area of the conductor, leading to a significant increase in $AC$ resistance ($R\_{\\text{ac}}$) and power losses.

The depth to which the current penetrates is known as the skin depth ($\\delta$). For copper at 70°C, it can be approximated using the formula:

 $$\\delta = \\frac{66}{\\sqrt{f}}$$

Where $\\delta$ is the skin depth in millimeters ($\\text{mm}$) and $f$ is the operating frequency in Hertz ($\\text{Hz}$).

### Skin depth at typical frequencies

| Frequency ($f$) | Skin depth ($\\delta$) | Max recommended strand gauge |
| --- | --- | --- |
| 10 kHz | 0.660 mm | [AWG 22](https://www.mycoil.info/magnet-wire-awg-to-metric-conversion-reference) (0.644 mm) |
| 50 kHz | 0.295 mm | [AWG 28](https://www.mycoil.info/magnet-wire-awg-to-metric-conversion-reference) (0.321 mm) |
| 100 kHz | 0.209 mm | [AWG 32](https://www.mycoil.info/magnet-wire-awg-to-metric-conversion-reference) (0.202 mm) |
| 250 kHz | 0.132 mm | [AWG 36](https://www.mycoil.info/magnet-wire-awg-to-metric-conversion-reference) (0.127 mm) |
| 500 kHz | 0.093 mm | [AWG 38](https://www.mycoil.info/magnet-wire-awg-to-metric-conversion-reference) (0.101 mm) |
| 1 MHz | 0.066 mm | [AWG 40](https://www.mycoil.info/magnet-wire-awg-to-metric-conversion-reference) (0.079 mm) |

For operating frequencies extending into mega-Hertz ranges, individual strand requirements cross into the ultra-fine domain (down to 0.010 mm); see our detailed breakdown on the [mechanical physics of ultra-fine wire processing](https://www.mycoil.info/physics-ultra-fine-wire-winding-0-010mm).

## The proximity effect in custom transformers

While the skin effect looks at an isolated conductor, the [proximity effect](https://www.mycoil.info/glossary#term-proximity-effect) occurs when multiple current-carrying conductors are packed closely together, such as in transformer windings. The magnetic field generated by adjacent wires distorts the current distribution within each strand, causing severe current crowding and localized overheating.

In multi-layer transformer windings, the proximity effect is often the dominant source of high-frequency loss, causing the $AC$-to-$DC$ resistance ratio ($R\_{\\text{ac}}/R\_{\\text{dc}}$) to spike exponentially if improper wire configurations are chosen. These losses increase thermal dissipation (see [thermal dissipation & power loss calculations](https://www.mycoil.info/thermal-dissipation-power-loss-calculations-fine-wire-coils)) and shift the component's [self-resonant frequency (SRF)](https://www.mycoil.info/parasitic-capacitance-self-resonant-frequency-srf) by altering distributed capacitance and effective coil resistance.

## Litz wire construction styles

Litz wire is classified into different types based on how the strands are bundled, twisted, and insulated. Detailed bundle parameters are documented in the [**High-Frequency Litz Wire Construction Matrix**](https://www.mycoil.info/high-frequency-litz-wire-construction-matrix):

| Type | Construction | Best suited for |
| --- | --- | --- |
| Type 1 | Single bunching of insulated strands | Low to medium frequencies, simple inductors |
| Type 2 | Bundles of Type 1 twisted together | Medium power transformers, high frequency |
| Type 3 | Braided or woven configuration | High-power, high-frequency switch-mode transformers |
| Type 4 | Bundles served with nylon or silk textile yarn | Applications requiring extra mechanical protection |

## Engineering framework for litz wire selection

To choose the right litz wire for custom transformer configurations, implement this systematic approach:

1. **Determine operating frequency ($f$):** Identify the fundamental switching frequency and major harmonics of your design.
2. **Calculate maximum strand diameter ($d\_{\\text{max}}$):** Ensure that the individual strand diameter is less than or equal to the skin depth ($d \\le \\delta$). For high efficiency, select a diameter matching $d \\approx 0.5\\delta \\dots 0.8\\delta$ using standard [metric wire gauges](https://www.mycoil.info/magnet-wire-awg-to-metric-conversion-reference).
3. **Calculate required total copper area ($A\_{\\text{total}}$):** Determine the overall conductor cross-section based on your continuous current ($I\_{\\text{rms}}$) and target current density (typically $3\\text{--}5\\,\\text{A/mm}^2$). High peak currents must also be evaluated to [prevent core saturation in high-current inductors](https://www.mycoil.info/preventing-core-saturation-high-current-inductor-design).
4. **Calculate required number of strands ($N$):** Divide total copper area by individual strand area: $$N = \\frac{A\_{\\text{total}}}{A\_{\\text{strand}}}$$
5. **Evaluate winding window constraints:** Factor in the litz packing density and outer diameter expansion due to individual strand insulation coatings and outer serving jackets. (For techniques on optimizing spatial fill within narrow envelopes, see [maximizing copper fill factor in miniature coils](https://www.mycoil.info/maximizing-copper-fill-factor-miniature-actuator-coils)).

## Manufacturing & processing considerations

- **Termination and soldering:** Polyurethane-insulated strands ([IEC 60317-11 / Grade 1–2](https://www.mycoil.info/iec-60317-magnet-wire-insulation-grades-dimensions)) can be directly soldered using a high-temperature solder bath ($380\\text{--}430^\\circ\\text{C}$), which melts away the enamel layer cleanly. For higher thermal classes (e.g., polyimide coatings), mechanical or chemical stripping is required before crimping or welding.
- **Bending limits:** Avoid tight bends that can crush individual internal strands, causing voltage breakdowns and unequal current distribution across bundles.
- **Fill factor limits:** Litz wire has a lower [copper fill factor](https://www.mycoil.info/glossary#term-copper-fill-factor) ($0.50\\text{--}0.65$) than solid magnet wire due to air gaps between bundles. Account for this reduced volume allocation during early bobbin and winding window geometries (compare with [orthocyclic vs. wild winding density limits](https://www.mycoil.info/orthocyclic-vs-wild-random-winding-fill-factor-comparison)).
- **Layer lead-out routing:** High-frequency litz coils often benefit from an [alpha winding structure](https://www.mycoil.info/alpha-winding-method-eliminate-lead-out) to ensure symmetrical lead exit points on the outer diameter without internal crossover strands.
- **Self-bonding variants:** For air-core coils and [bobbinless coil architectures](https://www.mycoil.info/bobbinless-coil-engineering-weight-reduction-thermal-optimization), litz wire with an outer [backlack adhesive layer](https://www.mycoil.info/self-bonding-wire-backlack-bonding-methods-reference) can be thermally or chemically bonded (see [self-bonding activation methods](https://www.mycoil.info/guide-self-bonding-wires-backlack-thermal-solvent-activation)).

For detailed strand tables, outer diameter multipliers, core loss dynamics, and serving options, refer to the [**High-Frequency Litz Wire Construction Matrix**](https://www.mycoil.info/high-frequency-litz-wire-construction-matrix) and the [**Core Physics & Permeability Matrix**](https://www.mycoil.info/core-physics-permeability-matrix-core-loss).

Technical classification parameters:

[Material Science](https://www.mycoil.info/tag/material-science)

**mycoil.info**

 Engineering data, empirical winding specifications, and parametric models for fine-wire electromagnetic components. Provided by KUK Coils.

**Technical references**

- [Electromagnetic coil glossary](https://www.mycoil.info/glossary)
- [Core physics & core loss reference sheet](https://www.mycoil.info/core-physics-permeability-matrix-core-loss)
- [Litz wire construction & skin depth matrix](https://www.mycoil.info/high-frequency-litz-wire-construction-matrix)
- [AWG to metric wire conversion table](https://www.mycoil.info/magnet-wire-awg-to-metric-conversion-reference)
- [IEC 60317 insulation grades & dimensions matrix](https://www.mycoil.info/iec-60317-magnet-wire-insulation-grades-dimensions)
- [Self-bonding wire & backlack matrix](https://www.mycoil.info/self-bonding-wire-backlack-bonding-methods-reference)

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