Choosing the right winding wire is one of the most important decisions in magnetic component design. Round magnet wire is simple and cheap, but at high frequency, AC resistance can be several times the DC resistance due to skin and proximity effects. Litz wire solves this problem but adds cost and complexity.
This guide explains when to use litz wire vs round wire in transformers and inductors.
Shaanxi SiRui Technology Co., Ltd. uses both round magnet wire and litz wire in custom magnetics designs. Contact: sales@sirui-tech.com | Web: www.sirui-tech.com
1. The Skin Effect Problem
At high frequency, current concentrates near the surface of a round wire. Skin depth: δ = √((ρ) / (π × f × μ)). For copper at 100 kHz: δ ≈ 0.23 mm. If wire diameter exceeds 2δ, inner copper carries almost no current—AC resistance is much higher than DC resistance.
2. Round Wire
Pros: cheap, readily available, easy to wind, high fill factor, easy to solder. Cons: high AC resistance at high frequency, skin and proximity effect loss. Best for: low frequency (<50 kHz), low current, cost-sensitive applications.
3. Litz Wire
Many thin insulated strands stranded together. Pros: low AC resistance at high frequency, high efficiency, flexible. Cons: 2–5× cost, lower fill factor, harder to terminate. Best for: high frequency (>50 kHz) switching power magnetics, high current inductors, high efficiency applications.
4. When to Use Which
| Frequency | Recommended Wire |
|---|---|
| 50/60 Hz (line freq) | Round magnet wire |
| 20–50 kHz | Round wire (if thin enough) |
| 50–100 kHz | Litz wire for high current |
| 100–300 kHz | Litz wire strongly recommended |
| >300 kHz | Litz wire or planar copper |
Rule of thumb: if wire diameter exceeds 2× skin depth at your switching frequency, use litz wire.
5. Sirui-Tech Winding Capability
- Round magnet wire: all standard AWG sizes
- Litz wire: custom construction per your frequency and current
- Foil winding: for very high current low-voltage applications
- Planar copper: for high-frequency planar transformers
Conclusion
Use round wire for low-frequency, low-current, or cost-sensitive applications. Use litz wire when switching frequency is high enough that skin effect makes round wire inefficient—typically above 50 kHz for high-current magnetics. The cost of litz wire is quickly recovered in efficiency gains and reduced thermal load.
Need help choosing the right winding wire? Contact Sirui-Tech at sales@sirui-tech.com with your frequency and current requirements.