Conductor Classes 1, 2, 5 and 6 Explained
IEC 60228 Classes 1 (solid), 2 (stranded), 5 and 6 (flexible) — what the class number actually means, when to use each one, the minimum wire counts, and how stranded resistance limits vary with class.
Metric conductors are divided into four classes. The class is not a quality grade — it describes the construction, and it changes the diameter, the flexibility, and the resistance the conductor has to meet.
The four classes
| Class | Construction | Intended for |
|---|---|---|
| 1 | Solid | Fixed installations |
| 2 | Stranded | Fixed installations |
| 5 | Flexible | Flexible cables and cords |
| 6 | More flexible than class 5 | Flexible cables and cords |
There is no class 3 or 4. They existed in earlier editions and were withdrawn.
Classes 5 and 6 may also be used in fixed installations. The reverse is not true — class 1 or 2 in an application that flexes will eventually fail.
Class 1 — solid
One single conductor. Cheapest, smallest diameter for a given size, and the easiest to terminate in screw and push-in terminals.
Copper solid conductors must be circular. Above 25 mm², solid copper is intended for particular cable types such as mineral insulated, not general purpose — worth knowing before you specify a solid 35 mm² conductor and find nobody stocks it.
Solid aluminium is available from 10 mm² up, circular through 35 mm², and either circular or shaped above that in multi-core cables.
Class 2 — stranded
Several wires laid up together. The standard sets a minimum number of wires per size, not a fixed count:
| Size | Min wires (circular copper) |
|---|---|
| 0.5 to 35 mm² | 7 |
| 50 to 95 mm² | 19 |
| 120 to 240 mm² | 37 |
| 300 to 500 mm² | 61 |
| 630 to 1600 mm² | 91 |
Class 2 comes in three forms: circular non-compacted, circular compacted, and shaped. Compacting squeezes the strands together, which reduces the diameter for the same cross-section — useful when conduit fill is tight. Compacted conductors need fewer wires, since the minimum for compacted sizes runs lower than for non-compacted.
For sizes of 1800 mm² and above, the wire count is not specified at all. Those conductors may be built from four, five or six equal segments, which is the Milliken construction used to cut skin effect on large AC conductors.
Classes 5 and 6 — flexible
Both are made from plain or metal-coated annealed copper only. No aluminium.
Here is the thing that catches people out: at the same nominal size, class 5 and class 6 have the same maximum overall diameter and the same maximum resistance. On a datasheet they look identical.
The only difference in the standard is the maximum diameter of the individual strands:
| Size | Class 5 max strand | Class 6 max strand |
|---|---|---|
| 0.5 to 1 mm² | 0.21 mm | 0.16 mm |
| 1.5 to 2.5 mm² | 0.26 mm | 0.16 mm |
| 4 mm² | 0.31 mm | 0.16 mm |
| 6 mm² | 0.31 mm | 0.21 mm |
| 10 to 35 mm² | 0.41 mm | 0.21 mm |
| 50 mm² | 0.41 mm | 0.31 mm |
| 70 to 150 mm² | 0.51 mm | 0.31 mm |
| 185 to 300 mm² | 0.51 mm | 0.41 mm |
Finer strands bend more times before they work-harden. That is the entire difference, and it decides whether the cable survives.
Use class 6 for anything that moves in service: drag chains, robot dress packs, cabinet door looms, machine tool wiring, portable equipment.
Class 5 is enough for cable that is flexed during installation and then left alone.
Class 5 runs from 0.5 to 630 mm². Class 6 stops at 300 mm².
Flexible conductors are allowed more resistance
Stranding makes the copper path longer than the cable, because the strands spiral. So a flexible conductor of the same nominal size has slightly higher resistance, and the standard allows for it:
| Size | Class 1 / 2 limit | Class 5 limit |
|---|---|---|
| 2.5 mm² | 7.41 Ω/km | 7.98 Ω/km |
| 4 mm² | 4.61 Ω/km | 4.95 Ω/km |
| 25 mm² | 0.727 Ω/km | 0.780 Ω/km |
| 50 mm² | 0.387 Ω/km | 0.386 Ω/km |
| 70 mm² | 0.268 Ω/km | 0.272 Ω/km |
Look at 50 mm². It is the only size in the entire standard where the flexible limit is lower than the solid one — 0.386 against 0.387. Everywhere else flexible is higher. It is a single-digit artifact of how the two tables were rounded, but if you are checking a conductor against the wrong column at that size, you will get the wrong answer.
Choosing in practice
Ask three questions:
- Does it move after installation? Yes → class 6. Occasionally → class 5. No → class 1 or 2.
- How tight is the space? Compacted class 2 gives the smallest diameter for a stranded conductor.
- What terminates it? Solid seats better in push-in terminals. Fine-stranded usually needs a ferrule.
How each class affects the overall diameter, cross-section by cross-section, is shown in the AWG to mm² calculator.
Zakaria El Intissar
Electrical Engineer
Electrical engineer with 12+ years of experience in power systems. I create practical electrical calculators and standards-based technical guides that engineers can apply immediately in their daily work.
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