What IEC 60228 does and does not define
IEC 60228 is the international standard for conductors of insulated cables. It covers plain and metal-coated (tinned) annealed copper and aluminium conductors, and it sorts them into four classes by construction: Class 1 solid, Class 2 stranded, Class 5 flexible and Class 6 extra flexible. The current edition is IEC 60228 Ed. 4.0 from December 2023, and most national standards in Europe, Asia, Africa and the Middle East refer to it, directly or through a mirror standard such as GB/T 3956 in China.
The most important thing to understand is what the standard uses as its pass or fail criterion. A conductor called 25 mm2 is not required to have a geometric area of exactly 25 square millimetres. It is required to have a maximum DC resistance at 20 degrees C that matches the table value for that nominal size and class. The cross-section is a name; the resistance is the test. For Class 5 and Class 6 the standard also limits the diameter of the individual wires, but it does not fix the number of wires. For Class 2 it sets a minimum number of wires for each size.
This matters commercially. Two plants can make a 25 mm2 Class 2 conductor with slightly different wire counts and diameters and both comply, provided the resistance is within the limit. A plant that saves copper by drawing the conductor thin will fail the resistance test, which is why resistance per drum is the first record the desk asks for. The conductor resistance testing guide goes into the test itself.
The four classes in practice
| Class | Construction | Typical products | Typical designation clue |
|---|---|---|---|
| Class 1 | Single solid wire | Small building wire, some twin and earth, control cable cores | H07V-U, 60227 IEC 01 solid |
| Class 2 | Stranded, round or compacted | LV and MV power cable, larger building wire, armoured cable | H07V-R, Cu/XLPE/PVC to IEC 60502 |
| Class 5 | Many fine wires, flexible | Flexible cords, solar cable, panel wiring | H07V-K, H05VV-F, H1Z2Z2-K |
| Class 6 | Finer wires than Class 5, extra flexible | Welding and highly flexible leads, some robotics and test leads | Stated on the datasheet |
Class 1 is the simplest to make and the easiest to terminate in screw clamps, but it is stiff and only practical in small sizes. Class 2 is the workhorse of fixed power wiring: it bends enough to install and can be compacted to reduce diameter. Class 5 is chosen when the cable moves, vibrates or has to be pulled around tight bends, as in cords and solar strings. Class 6 is for repeated flexing and is less common in general trade.
Class 3 and Class 4 do not appear because they are not used in the current scheme; the numbering jumps from 2 to 5. Aluminium conductors are covered in Class 1 and Class 2 only, which is one reason aluminium is a fixed-installation material and not a cord material. If a quote offers a flexible aluminium conductor, ask which standard it follows before going further, because it will not be an IEC 60228 Class 5 product.
Reading the resistance table
The resistance limits differ by class and by metal. Finer stranding adds a little resistance because of the lay length and the gaps between wires, and a tin coating adds a little more. The figures below are the 20 degrees C maximum DC resistance values in ohm per kilometre for common sizes.
| Nominal size | Copper Class 1/2 | Copper Class 5, plain | Copper Class 5, tinned | Aluminium Class 2 |
|---|---|---|---|---|
| 1.5 mm2 | 12.1 | 13.3 | - | - |
| 2.5 mm2 | 7.41 | 7.98 | - | - |
| 4 mm2 | 4.61 | 4.95 | 5.09 | - |
| 6 mm2 | 3.08 | 3.30 | 3.39 | - |
| 10 mm2 | 1.83 | - | - | 3.08 |
| 25 mm2 | 0.727 | - | - | 1.20 |
| 95 mm2 | 0.193 | - | - | 0.320 |
Two practical readings follow. First, the reference 0.6/1 kV 4x25 mm2 Cu/XLPE/PVC cable on the LV power cables page quotes 0.727 ohm/km or less, which is exactly the Class 2 copper limit; a datasheet that quotes a higher value for a 25 mm2 copper conductor is not a Class 2 conductor of that size. Second, the reference H1Z2Z2-K 1x4 mm2 solar cable quotes 5.09 ohm/km, the tinned Class 5 value, because EN 50618 calls for tinned flexible copper.
Choosing a class for your project
The class is usually fixed by the product standard rather than chosen freely. IEC 60502-1 power cables use Class 1 or Class 2 conductors; flexible cords to IEC 60227 or IEC 60245 use Class 5; EN 50618 solar cable uses tinned Class 5. Where you do have a choice, as with building wire offered as H07V-U, H07V-R or H07V-K, weigh four points:
- Installation: conduit pulls with several bends favour Class 2 or Class 5; short straight runs into screw terminals suit Class 1.
- Termination: fine-stranded Class 5 conductors in screw terminals often need ferrules; check the terminal maker's instructions.
- Movement: anything that moves in service needs Class 5 or Class 6.
- Market habit: some markets and wiring rules expect a particular class for a given size, so confirm with the project engineer or the local standard.
The stranded vs solid conductors article covers the trade-offs in more detail, and the conductor materials page explains copper, aluminium and tinned options.
What to write on the spec sheet
A conductor line that leaves nothing open reads like this: "annealed copper conductor, Class 2 to IEC 60228, 25 mm2, maximum DC resistance 0.727 ohm/km at 20 degrees C, verified per drum by test report". It names the metal, the class, the nominal size, the limit and the evidence. If the market uses AWG, give the AWG size and the standard as well, because AWG sizes and IEC metric sizes are not interchangeable; see AWG vs mm2 conversion.
When the desk prepares a specification it carries this line into the purchase order, the inspection plan and the drum label, so the same requirement is checked at the plant, at pre-shipment inspection and on arrival. If you are preparing an enquiry now, the quote form asks for conductor material, class and size first, and MOQ and lead time explains how many metres per size partner plants usually accept.