Wire gauge selection is one of the most important decisions in magnetic component design. Choose wire too thin and DCR loss makes the component inefficient and hot. Choose wire too thick and it won’t fit in the winding window. At high frequency, skin effect and proximity effect add another layer of complexity.
This guide covers how to select the right wire gauge for transformers and inductors, from low-frequency line transformers to high-frequency SMPS magnetics.
Shaanxi SiRui Technology Co., Ltd. engineers optimize winding wire selection for every custom magnetic component. Contact: sales@sirui-tech.com | Web: www.sirui-tech.com
1. DC Current Density Basics
| Application | Current Density |
|---|---|
| Dry, natural convection | 2–3 A/mm² |
| Forced air cooling | 4–6 A/mm² |
| Immersion / oil cooled | 6–10 A/mm² |
2. Skin Effect at High Frequency
At switching frequencies above 20 kHz, current flows only near the wire surface. Skin depth: 0.46 mm at 20 kHz, 0.21 mm at 100 kHz, 0.095 mm at 500 kHz. Single wire diameter should be ≤ 2× skin depth. Above that, use litz wire.
3. Wire Types
- Round enameled copper wire: standard for most windings, low to medium frequency
- Litz wire: many thin strands, solves skin/proximity effect above 50 kHz
- Foil winding: flat copper, excellent fill factor, high current low frequency
- Flat wire / ribbon: rectangular, better fill, planar transformers
4. AWG Reference
| AWG | Diameter (mm) | Area (mm²) | Resistance (Ω/m) |
|---|---|---|---|
| 18 | 1.02 | 0.82 | 0.021 |
| 20 | 0.81 | 0.52 | 0.033 |
| 22 | 0.64 | 0.33 | 0.053 |
| 24 | 0.51 | 0.20 | 0.084 |
| 26 | 0.40 | 0.13 | 0.134 |
| 28 | 0.32 | 0.080 | 0.213 |
| 30 | 0.25 | 0.051 | 0.339 |
5. Winding Window Fill Factor
- Round wire on bobbin: 40–50%
- Litz wire: 35–45%
- Foil: 70–85%
6. Common Mistakes
- Using single wire above skin depth: massive AC loss
- Ignoring proximity effect: layers increase AC resistance far beyond skin effect
- Too high current density: overheating
- Too low current density: oversized, poor cost
7. Sirui-Tech Winding Capability
- Magnet wire winding: 0.1–2.0 mm diameter
- Litz wire winding: strand sizes 0.05–0.3 mm
- Foil winding: high-current applications
Conclusion
Wire gauge selection balances DC current density, skin depth, proximity effect and winding window space. For low-frequency, use simple current density rules. For high-frequency SMPS, switch to litz wire matched to skin depth.
Need optimized winding magnetics? Contact Sirui-Tech at sales@sirui-tech.com with your current and frequency.