Conductor classes IEC 60228 — class 1 through 6, and why the flexibility class determines the termination method

The guide on ferrules covers how a wire-end ferrule protects a stranded conductor from splaying under a screw terminal. This article takes a step back: what actually determines whether a conductor is "solid" or "flexible", and why does that matter for which termination technique is appropriate? The answer lies in the conductor class of IEC 60228.

The classes in brief

IEC 60228 defines conductor classes based on the number of strands, the maximum strand diameter and the maximum resistance per kilometre for a given nominal cross-section:

Classes 1 and 2 are primarily intended for fixed installations; classes 5 and 6 are intended for flexible cables and cords, although in practice they are also used in fixed installations where extra flexibility is wanted for pulling. For classes 1 and 2 the standard mandates the number of strands; for classes 5 and 6 the standard gives the maximum strand diameter, and a lower strand count is permitted as long as the resistance limit is not exceeded.

Why the class determines the termination method

• Class 1 (solid): a solid conductor can generally be terminated directly under a screw or spring terminal; no ferrule is needed because there are no loose strands that can splay.
• Class 2 (stranded, fixed installation): a limited number of thicker strands; here too a ferrule is usually not mandated, although not every terminal type accepts a stranded conductor without a ferrule as reliably as a solid one.
• Class 5/6 (flexible/extra flexible): many thin strands that splay and break under a screw terminal, and that can "creep" (cold flow) under sustained pressure, reducing the clamping force and causing a loose, overheating connection. For these classes a suitable ferrule (see the dedicated guide on that) is, in nearly every case, the correct termination method under a screw terminal.

Practical relevance

When connecting a variable-frequency drive, a moving machine part, or a cable in a cable carrier, a class 5 or class 6 conductor is used in nearly every case because of the required flexibility. Even where the terminal "accepts" the conductor at first glance, a ferrule should be fitted — precisely because the visible, short-term result (a solid-looking connection) can hide a creeping contact problem that only surfaces months later, after temperature cycling and vibration, as a loose, overheating terminal.

Common mistakes

1. Clamping a class 5/6 conductor directly under a screw terminal without a ferrule — the clamping force is then carried by only a few outer strands, while the rest of the bundle remains loose; this reduces the effective contact area and increases the transition resistance.
2. Treating class 1 and 2 as "the same, rigid enough" — some terminal types (in particular spring terminals designed for solid conductors) clamp a stranded class 2 conductor less reliably than a solid class 1 conductor of the same cross-section.
3. Fitting a ferrule of the wrong cross-section — a ferrule that is slightly too loose or too tight for the actual strand count gives an unreliable crimp connection, even though the ferrule initially "fits" over the conductor.
4. Losing sight of the resistance limit for a conductor with fewer strands than the table value — for class 5/6 a lower strand count is permitted as long as the maximum resistance per kilometre is not exceeded; a conductor that meets the strand count but not the resistance requirement is not compliant, even though it looks similar.

Related

• Ferrules — crimping flexible conductors
• Terminals — spring terminal versus screw terminal
• Flexible cables H05VV-F / H07RN-F — application range
• Cable lugs — crimp connections