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IEC 60228: Cable Conductors — The R-Series Behind Every Standard Cross-Section From 1.5 to 2,000 mm²
Why Cable Cross-Sections Follow 1.5, 2.5, 4, 6, 10, 16… The R10 Preferred Number System
IEC 60228:2004 specifies cable conductor nominal cross-sections, constructions, and DC resistance. Every standard cable cross-section follows the R10/R20 preferred number series — ensuring reasonable ampacity gaps (~20–40%) between adjacent sizes.
| Conductor Class | Construction | Max Section | Application |
|---|---|---|---|
| Class 1 | Solid single wire | ≤16 mm² | Fixed wiring (BV-type) |
| Class 2 | Regular stranded | All sizes | General power cables |
| Class 5 | Flexible (fine wires) | All sizes | Portable equipment (RVV) |
| Class 6 | Extra-flexible (extra-fine) | All sizes | Robotics/drag-chain/welding |
Fill factor — the core manufacturing competitive metric: Stranded conductor fill factor = actual Cu area / circumscribed circular area. Class 2: 0.75–0.85; Class 5/6: 0.65–0.78. Higher fill factor means higher ampacity for the same outer diameter — the core competitive metric among cable manufacturers.
Quick DC resistance at 20°C: R₂₀ = ρ₂₀ / A × k
ρ₂₀(Cu)=17.241 Ω·mm²/km, ρ₂₀(Al)=28.264 Ω·mm²/km
k=stranding factor (Class1=1, Class2=1.02–1.04, Class5/6=1.05–1.08)
Example: 2.5 mm² Class2 Cu → R₂₀=17.241/2.5×1.03=7.10 Ω/kmTN Lab — Cable cross-section R-series has been validated by over a century of engineering practice.
