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IEC 60228 Conductor Classes 1, 2, 5 and 6: Stranding Matters

Quick Answer: IEC 60228 grades conductors by flexibility — solid, stranded, flexible, extra-flexible — and naming the class in the RFQ is the difference between clean terminations and fatigue at the gland. Two cables can carry the same cross-section, the same insulation and the same price — and behave completely differently at the termination, because the copper inside is built differently. A solid rod snaps where a fine-stranded bundle bends; a coarse stranded bundle splays under a screw where a flexible one holds. IEC 60228 exists to make that difference sayable in one number, and this guide makes the numbers chooseable.

Isometric illustration of four conductor class stations from fixed conduit to continuous flex with bend-radius icons

Introduction

Conductor class is the specification detail most often left to chance. Schedules state “4 × 16 mm²” and stop, treating all 16 mm² copper as equal — then the installation reveals the gap: solid conductor fighting every conduit bend, or flexible cord work-hardening in a fixed gland. The class system costs nothing to specify and decides pulling behavior, terminal hardware, vibration tolerance and flex life. It is also a verification point at the factory, because stranding is where marginal production quietly saves money. This article maps the four classes that matter and the discipline of stating them.

The Four Classes, Decoded

IEC 60228 defines conductors by construction, and in practice four classes carry nearly all real work. Class 1 is solid: a single copper rod, cheapest and stiffest, the classic building-wire conductor for fixed conduit runs. Class 2 is stranded: seven or more wires twisted together, still for fixed installation but far easier to pull around bends. Class 5 is flexible: many fine wires in a bundled-rope lay, built for cords, connections to equipment and anything that moves. Class 6 is extra-flexible: the same idea with finer wires and tighter lays yet, for continuous-flex and severe-duty applications. (Classes 3 and 4 exist as intermediate stranded grades but appear rarely in modern schedules.) The same 2.5 mm² exists in all four — the area is identical, the behavior is not.

IEC 60228 Classes Compared
Aspect Class 1 (Solid) Class 2 (Stranded) Class 5 (Flexible) Class 6 (Extra-Flexible)
Construction Single rod 7+ wires, concentric lay Fine wires, rope lay Finer wires, tight rope lay
Typical strands at 2.5 mm² 1 7 50 ~100+
Flexibility None Low High Highest
Bend radius tolerated Largest Large Small Smallest
Termination Direct under screw Direct or ferruled Ferrules below 10 mm² Ferrules always
Motion tolerance None — work-hardens Poor Good Excellent
Cost per meter Lowest Low Higher Highest
Typical duty Fixed conduit wiring Fixed feeders and trays Cords, equipment connections Continuous-flex, robotics

Class 1 vs Class 2: The Fixed-Installation Choice

For anything that never moves again, the choice is solid versus coarse-stranded, and it is a route decision more than an electrical one. Class 1 solid is cheaper, terminates directly under screw terminals without ferrules, and holds its shape permanently — virtues in straight conduit runs and panel risers. Its stiffness fights long pulls with bends, and its single wire work-hardens and fatigues wherever vibration exists, which is why solid copper near vibrating equipment is a recurring entry in the failure catalogs. Class 2 stranded pulls easily through bends, shrugs off minor vibration, and terminates nearly as cleanly; its premium over solid is modest, which is why many feeders and most tray work default to class 2 even where solid would technically serve.

Class 5 vs Class 6: The Flexible Trade

Below 10 mm² the flexible classes live in cords, control pendants and equipment tails; above it they serve mobile equipment, generator sets, crane and reeling duty. Class 5 is the general flexible answer — appliance cords, RVV-family conductors, panel-to-equipment connections. Class 6 buys finer stranding for continuous-flex life: drag chains, robotic arms, reeling drums where the cable bends millions of times. The trade is copper packing and lay complexity, which cost money and slightly increase surface resistance per given area. The fine strands also demand ferrules at every termination — stranded copper splayed under a screw is the classic loose-connection factory, and the hot-spot failures it seeds are catalogued in our cable failure causes guide. For the specialty end of continuous-flex duty, the construction conversation extends beyond class 6 into dedicated high-flex designs, covered in our control and instrumentation cable guide only as far as standard products go.

Choosing the Class by Application
Application Class Why
Building wire in conduit 1 or 2 Fixed forever; solid wins on cost, stranded on pulling
Feeders and tray runs 2 Easy pulls, vibration tolerance, small premium
Panel interior wiring 2 or 5 Bends and terminal access favor stranded
Appliance and equipment cords 5 Motion and handling are the duty
Pendant and portable tools 5 Repeated flex with moderate severity
Drag chains, reeling, robotics 6 (or dedicated high-flex) Millions of bend cycles demand fine stranding

Stranding, Resistance and the Verification Point

Stranding has one electrical cost: fine strands pack less copper into the same area because of the air between wires, so class 5 and 6 conductors carry a slightly higher DC resistance than class 1 at the same nominal size — IEC 60228 publishes a separate, higher resistance limit for the flexible classes, and compliant factories test to it. That is the verification opening: batch resistance reports per kilometer reveal a factory that shorted on copper or strand count long before the cable fails in service. Stranding is also where substitution happens silently — a class 2 drum delivered where class 5 was specified looks similar at the gland and fails in motion. The supplier verification discipline for catching exactly this class of substitution is the one in our power cable manufacturer checklist, and the datasheet fields where class should appear are marked in our equipment datasheet reading guide.

Specifying the Class: RFQ Discipline

The class belongs in the part number’s shadow, not in a footnote: “4 × 16 mm², class 2” is a complete conductor specification; “4 × 16 mm²” is half of one. Where conversion from AWG schedules is involved, the class matters doubly, because American flexible classes and IEC classes do not map one-to-one — state the IEC class explicitly rather than translating from the American construction description. And where terminations are pre-engineered, match the class to the hardware: set-screw terminals want solid or coarse stranded, ferruled or clamp terminals want the flexible classes, and the mismatch discovered on site costs a day of electricians’ time per panel. The sizing arithmetic that runs alongside the class choice is the one in our cable size selection guide.

When Class Selection Is Not the Answer

No class choice rescues a conductor sized wrong: class 6 does not make a too-small cable safe, and class 1 does not make an oversized one cheap in the ways that matter. Nor does class substitute for duty-matched construction at the extremes — continuous-flex applications past standard class 6 need dedicated high-flex designs with different metallurgy and geometry, and reeling duty needs its own engineering. Class selection is a construction decision inside a sizing decision that has already been made correctly; it sharpens specifications, it does not replace engineering.

RFQ Checklist: Ordering by Conductor Class

Make the conductor quotable, so include:

  • Class stated with every cross-section: e.g. 3 × 95 mm², class 2
  • IEC 60228 as the governing standard named explicitly
  • Flex duty described: static, occasional handling, continuous flex cycles
  • Termination hardware matched: screw, ferrule or clamp terminals
  • Resistance limits per class acknowledged in the acceptance criteria
  • Batch resistance reports required, keyed to drum IDs
  • Strand counts spot-checked at delivery against the class
  • Converted sizes double-marked: AWG in parentheses, class in IEC terms
  • Copper price linkage keyed to the class-adjusted construction

Conclusion

Conductor class is the cheapest sentence in a cable specification and the one with the most visible consequences: it decides how the cable pulls, how it terminates, and how long it survives the motion it was bought for. Four classes cover the real world — solid, stranded, flexible, extra-flexible — and a schedule that names the class turns conductor copper from a commodity into a verified component.

Kexingyu Cable Group (KXYE) manufactures across all IEC 60228 classes with strand counts and resistance limits tested per batch, so the class on the schedule is the construction on the drum — verified, documented and export-ready.

Class 1 is a single solid rod; class 2 is the same copper stranded from seven or more wires. Both are for fixed installation. Solid is cheaper and terminates directly under screws but fights bends and fatigues under vibration; class 2 pulls easily, tolerates vibration, and costs only a small premium — which is why most feeders default to it.
Whenever the cable moves or is handled: appliance cords, equipment connections, pendants, portable tools. Class 5's fine-wire rope lay survives repeated bending that would work-harden class 2 copper to failure. The trade is ferrules at terminations below 10 mm² and a modest price premium for the stranding labor.
The harshest flex duty: drag chains, robotic arms, reeling drums — applications with millions of bend cycles. Class 6 uses finer wires and tighter lays than class 5 for longer flex life. Beyond standard class 6, severe continuous-flex duty needs dedicated high-flex constructions with their own engineering.
Slightly, and predictably: fine strands pack less copper into the same nominal area, so class 5 and 6 carry a higher DC resistance limit than class 1 at the same size — IEC 60228 publishes the limit per class. That makes batch resistance reports the quick verification that the factory actually built the class you ordered.
For the flexible classes below 10 mm², effectively yes: fine stranded copper splayed under a set screw is the classic loose-connection factory, running hot and failing intermittently. Class 2's coarse strands terminate acceptably under screw terminals, and class 1 solid needs no ferrule at all. Match the hardware to the class before the panels are built.
Attach it to every cross-section: "3 × 95 mm², class 2, per IEC 60228." Where sizes were converted from AWG, state the IEC class explicitly rather than translating the American construction description, since the class systems do not map one-to-one — and require batch resistance reports keyed to drum IDs as the verification.

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