Wire ferrules — crimping flexible conductors for terminal connections

The cable lugs & crimp connections guide covers crimping cable lugs for larger, permanent connections. This article covers a related but different component: the wire ferrule, used to prepare the end of a flexible (multi-wire) conductor for a terminal connection in a switchboard or distribution board.

What a wire ferrule does

A flexible conductor consists of many thin strands rather than a single solid core. Without protection, those strands splay out under the pressure of a screw terminal, allowing loose strands to protrude beyond the terminal — with the risk of a short circuit with a neighbouring terminal or an incomplete, high-resistance contact. A wire ferrule (a metal tube, often with an insulated collar) is slid over the stripped end and crimped around the strands, so the strands enter the terminal as one fixed-shape unit.

Standards

Wire ferrules are standardised under DIN 46228 (parts 1 and 4, for the shape and dimensions of the ferrule), while the requirements for the terminal connection itself fall under IEC 60947-7-1 (low-voltage terminal blocks) and EN 60999-1.

Screw terminal versus spring-clamp terminal — not always the same rule

Screw terminal: for a flexible conductor, a wire ferrule is generally required here — without one, the screw can compress the loose strands unevenly, resulting in protruding strands. For a solid (single-core) conductor, a ferrule is not needed, because the conductor already forms one solid core.
Spring-clamp terminal (push-in connection): this is more nuanced. Some manufacturers of modern spring-clamp terminals explicitly state that a wire ferrule is not needed, or even advised against, for flexible conductors, because the spring clamp itself already applies sufficient, even clamping force to the loose strands, and a ferrule can disrupt the clamping action the manufacturer has tested for.

Note: do not assume by default that "a ferrule can never hurt" in a spring-clamp terminal — check the installation instructions of the specific terminal or device, as this differs by manufacturer and terminal type.

Twin (double) wire ferrules

For bringing two conductors together under one terminal, special twin (double) wire ferrules exist, with a partition that keeps the two bundles of strands separated within one ferrule. This is only permitted when the terminal itself is explicitly rated for two conductors — not every terminal allows this, regardless of whether a (twin) ferrule is used.

The right tool

A wire ferrule must be set with a dedicated crimping tool, matched to the cross-section of the ferrule. An ordinary combination pliers compresses the ferrule unevenly and with too little, non-standardised force, producing a loose or unreliable connection that can still look "crimped" from the outside. With a correctly made connection, at smaller cross-sections a small length of bare strands (on the order of 1 mm) is typically just visible at the end of the ferrule — a ferrule that looks completely "empty" when inserted, or where the strands protrude far beyond it, indicates an incorrect strip length.

Practical relevance

When connecting flexible wiring in a switchboard, consumer unit or machine control cabinet, it must be established per terminal type whether a wire ferrule is required, permitted, or actually undesirable — and the correct crimping tool, specifically intended for that cross-section, must then be used, not a general-purpose pliers that "happens to fit too".

Common mistakes

Using an ordinary (combination) pliers instead of a dedicated crimping tool — this produces an uneven, non-guaranteed crimp, even though the ferrule may look correctly set at first glance.
Fitting a wire ferrule into a spring-clamp terminal by default, without checking whether the manufacturer of that specific terminal actually advises against it.
Using a ferrule that does not match the conductor's cross-section (too small or too large), so the strands are not fully enclosed or the ferrule does not clamp securely.