IEC 60228 Explained: Why a 240 mm² Conductor Is Defined by Resistance, Not Area

Almost every cable standard — IEC 60502, IEC 60840, EN 50525, the BS power cable standards and many more — points to the same conductor standard: IEC 60228, Conductors of insulated cables. It rarely gets attention of its own. Yet it quietly decides the most expensive material in a power cable and the diameter every other layer is built on.

This article skips the tables and focuses on the ideas in IEC 60228 that change how cable makers design, price and test.

The four conductor classes in one sentence each

• Class 1 – a single solid wire. Compact and cheap, but stiff.
• Class 2 – stranded conductors for fixed installation: round, compacted or shaped. The workhorse of power cables.
• Class 5 – flexible conductors made of many fine wires, for cables that are moved, vibrated or routed tightly.
• Class 6 – extra-flexible, with even finer wires.

Classes 1 and 2 are for fixed installation; classes 5 and 6 are for flexible cables and cords. The class is usually dictated by the product standard and the application, so the real design freedom lies elsewhere.

Insight 1: the nominal size is a resistance promise

The most important idea in IEC 60228 is easy to miss: the nominal cross-sectional area is a label, not a measured area. A conductor qualifies as "240 mm²" because its DC resistance at 20 °C does not exceed the maximum the standard specifies for that size and material — not because its metal cross-section measures 240 mm².

This changes how you should think about several everyday questions:

• "Is this conductor undersized?" Measuring the geometric area of a compacted conductor and finding it slightly below nominal proves nothing. The question is whether it meets the resistance requirement.
• Where the copper savings are. High-conductivity metal and good compaction let a manufacturer meet the resistance limit with less metal than the label suggests. Across thousands of kilometres, that margin is one of the largest legitimate cost levers in cable making.
• How much margin to keep. Resistance is measured on every production length as a routine test. Designing right at the limit saves metal but leaves no room for variation in wire, stranding or annealing. The right margin is a deliberate engineering decision, not an accident.

Insight 2: the conductor sets the diameter for everything above it

A compacted or shaped conductor is smaller than a loose round one of the same size. Because insulation, screens, sheaths and armour are all applied over it, a smaller conductor saves material in every layer — and can move the design into a different armour or sheath size band.

That is why conductor shape is not just a stranding decision. It is one of the first cost decisions in the whole design. Our layer-by-layer anatomy of a power cable shows how this ripple works.

Insight 3: large conductors behave differently under AC

For very large conductors on high-voltage cables, alternating current crowds towards the surface (skin effect) and is distorted by neighbouring conductors (proximity effect). The DC resistance in IEC 60228 is then only the starting point. Segmental (Milliken) conductors, which split the conductor into insulated segments, exist to fight this — and the conductor construction feeds directly into the AC resistance used in IEC 60287 current rating calculations.

Copper or aluminium: a design question, not just a metal price

For the same resistance, an aluminium conductor needs a noticeably larger cross-section than copper. Aluminium is lighter and usually cheaper per kilometre of conductor, but the larger diameter means more insulation, screen, sheath and armour material, a heavier drum per unit length of copper-equivalent capacity, and larger accessories.

The honest answer to "which is cheaper?" is: compare complete designs, not metals. Two full bills of materials side by side — with current ratings for the real installation — usually settle the argument faster than any rule of thumb.

Flexible classes have practical consequences

Choosing class 5 instead of class 2 is not only about flexibility:

• Fine-wire conductors need suitable terminations (ferrules or lugs designed for flexible conductors).
• Tinned wires, common in PV and marine cables, protect against corrosion but have slightly higher resistance than plain copper.
• Flexible conductors are usually larger in diameter for the same nominal size, which again affects every layer above.

What tender teams should check

When a tender specifies a conductor, a few quick questions avoid expensive surprises:

1. Is the class fixed by the product standard, or is there a choice?
2. Copper or aluminium — and would the customer accept the alternative if it is cheaper overall?
3. Is longitudinal water blocking in the conductor required?
4. Do the customer's accessories constrain the conductor shape or diameter?
5. Is the requested size driven by current, by short-circuit duty or by voltage drop? Each points to a different optimisation.

How CablePLM uses IEC 60228

In CablePLM, IEC 60228 is the conductor basis for every power cable pack in the standards library. When you choose class, material and size, the conductor is carried into the layer-by-layer construction, the bill of materials, the datasheet and the current rating — with every value marked as standard data or estimate.

The Cable Selection & Optimization add-on compares alternatives such as copper versus aluminium or neighbouring sizes, to find the cheapest design your plant can actually make.

Want to see it on your own conductor range? Book a demo.