Cable Conductor Classes and Stranding Explained

What does IEC 60228 cover, and what is a conductor class?

IEC 60228 defines conductor requirements for insulated cables. It covers nominal cross-sectional areas, conductor constructions and maximum resistance values. It applies to solid and stranded copper, aluminium and aluminium-alloy conductors for fixed installations, as well as flexible copper conductors.

A conductor class describes how the conductor is built, from a single solid wire to fine strands. Two cables marked with the same nominal cross-section can therefore have different conductor constructions and different flexibility. Nominal cross-section is tied to electrical requirements, not simply to a caliper measurement, so a conductor should not be accepted only because its outside diameter resembles another cable.

The applicable product standard still determines where a conductor construction may be used. State the current edition of the governing standard and of the cable product standard in the purchase specification.

Which conductor class suits which use?

The table compares the four classes most often named in cable specifications. The class describes the conductor only. The insulation, sheath and overall cable construction still have to suit the installation.

Class descriptions follow IEC 60228. The product standard and the manufacturer's datasheet decide which class a cable actually uses, so confirm it on every order.

Class 1 is a solid conductor with a simple construction. It is normally associated with smaller fixed-installation conductors and is not intended for repeated flexing. Check installation handling, terminal compatibility and local code requirements before selecting it.

Class 2 uses multiple strands and is common in fixed power cables. Circular, compacted circular and shaped conductors may be used, depending on size and cable design. Compaction can reduce conductor diameter and overall cable size, but it also changes how the strands look at a cut end. Do not judge a compacted conductor as underfilled from appearance alone. Verify the ordered construction, conductor resistance and relevant dimensional requirements.

When is a flexible conductor class needed?

Class 5 uses finer strands for flexible cables and wires. Class 6 uses still finer construction for applications that need greater flexibility. Flexible products such as the rubber flexible cable range and the welding cable range use these classes for that reason.

The class describes the conductor, not the complete cable's suitability for continuous movement. Insulation, sheath, lay length, reinforcement and bend design also control flex life. The minimum bending radius depends on the cable type and the standard edition, so take it from the manufacturer's datasheet rather than from a general rule.

A highly flexible conductor can be the wrong choice for a fixed power feeder if the terminal is designed for a different conductor type. Flexible strands may require ferrules, lugs or terminals specifically approved for that construction. Before choosing a class, answer these questions:

• Will the cable be fixed, occasionally moved or continuously flexed?
• What terminal, lug or connector will receive the conductor?
• Does the terminal accept compacted, sector-shaped or fine-stranded conductors?
• Is vibration present?
• What minimum bending radius applies to the finished cable?
• Are ferrules, special lugs or crimp tools required?

Is a higher strand count always better?

Buyers sometimes compare cables only by strand count. That can be misleading, because strand diameter, compaction, conductor material and class all matter. Unless a product standard or drawing specifies the strand count, conductor resistance and class are usually more meaningful acceptance controls.

Tinned copper may be specified for corrosion resistance or termination needs, but tinning does not replace a suitable sheath or environmental design. Aluminium conductors need compatible connectors, careful joint preparation and suitable installation practice.

How is conductor resistance measured and corrected for temperature?

DC resistance is commonly measured and corrected to a reference temperature of 20 °C under the applicable standard. The measured sample length, conductor temperature, instrument accuracy and connection method all affect the result. A factory or third-party report should state these conditions rather than show only a final number.

For plain copper Class 1 and Class 2 conductors, the maximum DC resistance at 20 °C under IEC 60228 is 7.41 Ω/km for 2.5 mm², 1.15 Ω/km for 16 mm² and 0.193 Ω/km for 95 mm². Tinned or metal-coated conductors have slightly higher limits. Confirm which edition of the standard applies to your order.

Resistance that is too high may indicate insufficient conductive material, incorrect material, poor joints or a measurement error. Passing resistance does not by itself prove insulation thickness, voltage rating, fire performance or service suitability.

How should the conductor be written in an RFQ, and what should be checked on receipt?

A useful conductor line might read: copper conductor, IEC 60228 Class 2, compacted circular, 4 cores x 95 mm², with conductor resistance compliant with the ordered product standard. For a flexible product, state Class 5 or Class 6 only when the application and termination system require it.

On receipt, check the conductor material and class, the core count and nominal size, and the conductor resistance report. Look at the strand condition at a sample cut, confirm lug and terminal compatibility, and verify the cable marking and drum identity against the order.

The project engineer must confirm conductor class, ampacity, voltage drop, short-circuit duty and termination compatibility together. Flexibility alone is not a complete selection criterion. For the wider selection steps, see the rubber flexible and welding cable guide.

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