The conductor inside a cable is described by a class from IEC 60228. The standard sets the maximum d.c. resistance for each conductor size, for plain copper, metal-coated (tinned) copper and aluminium.
The four classes
| Class | Cross-section | Conductor |
|---|---|---|
| Class 1 | Solid. A single wire. Fixed installation |
|
| Class 2 | Stranded. A few thicker wires. Fixed installation |
|
| Class 5 | Flexible. Many fine wires. Cords, portable equipment, machines |
|
| Class 6 | Extra flexible. Even more, finer wires. Constant movement, tight bends |
Drawings show the same nominal size in each class. Wire counts are illustrative; IEC 60228 limits the maximum wire diameter for classes 5 and 6 rather than fixing the number of wires.
Same size, different resistance
Classes 1 and 2 share the same maximum resistance for each size, and so do classes 5 and 6. The flexible classes are allowed a slightly higher resistance, because fine stranding packs less copper into the same nominal size.
| Size (plain copper, 20°C) | Class 1 and 2 | Class 5 and 6 |
|---|---|---|
| 1.5 mm² | 12.1 Ω/km | 13.3 Ω/km |
| 2.5 mm² | 7.41 Ω/km | 7.98 Ω/km |
So two cables sold as 2.5 mm² can differ in resistance depending on their conductor class. Check the class when you size a cable for voltage drop.
Reading the class from a cable code
In harmonised cable codes the letter after the dash gives the conductor type: -U solid (class 1), -R stranded (class 2), -K flexible for fixed installation (class 5), -F flexible (class 5) and -H highly flexible (class 6). H07RN-F, H05VV-F and H07V-K in the Goldberg range all use class 5 conductors, as does NSHTÖU-O/J.
Need a cable for a particular bend radius or amount of movement? Ask our team and we'll recommend the right conductor.
