Crimping Tools: Why You Need One, What to Buy, and How to Crimp Properly

Your electrician has wired the consumer unit. Eight RCBOs, each with a factory-pressed neutral fly lead that's 40mm too long. The leads need shortening, and the cut ends are stranded copper, dozens of fine wires that will splay apart the moment they go into a screw terminal. A loose strand touches the adjacent terminal. That's a short circuit in your consumer unit. The fix is a bootlace ferrule (a small tinned-copper sleeve) crimped onto each cable end, turning floppy strands into a solid pin. The tool that does it is one of the cheapest in your kit. Not having one, or bodging it with pliers, risks a failed inspection or worse.

What it is and when you need one

A crimping tool compresses a metal connector onto the end of an electrical cable, creating a permanent mechanical and electrical joint. Squeeze the handles, the jaws close around the connector and cable together, deforming the metal so it locks tight around the copper strands. Done properly, a crimp is stronger than the cable itself.

For domestic electrical work in the UK, you'll encounter crimping in one specific context: bootlace ferrules on stranded cable ends going into screw terminals. BS 7671 (the IET Wiring Regulations, which underpin Building Regulations Part P) requires that multi-strand conductors use "suitable terminals or suitably treated conductor ends" to prevent strand separation. In practice, that means ferrule crimping. Soldering the cable ends is explicitly prohibited at screw terminals because solder is soft, deforms under the clamping pressure, and develops a high-resistance joint that overheats over time.

The most common scenario where you'll need a crimping tool during an extension project: shortening RCBO neutral and earth fly leads in the consumer unit. RCBOs (the individual circuit breakers in a modern consumer unit) come with pre-made flying leads, but these are often too long for a neat installation. Your electrician trims them. The cut end needs a ferrule.

Ring terminals, spade terminals, and butt connectors are other crimp connector types, but these belong to automotive and industrial applications. For a kitchen extension, bootlace ferrules are what you're buying a crimper for.

Types of crimping tool

Not all crimpers do the same job. The jaws are shaped for specific connector types, and using the wrong tool on the wrong connector gives you a bad crimp that looks fine but pulls apart.

The self-adjusting ratchet ferrule crimper is what you want. Here's why.

A ratchet mechanism physically prevents the tool from opening until the crimp is fully compressed. You can't under-crimp. Squeeze the handles, the ratchet clicks through its stages, and the jaws only release when the connector is properly formed. For a beginner, this removes the guesswork entirely. Basic squeeze crimpers let you release at any point, so whether the crimp is tight enough depends on your hand strength and judgement.

"Self-adjusting" means the jaw automatically adapts to the ferrule diameter. You don't need to select a specific die or slot for each cable size. Push the ferrule into the jaw, squeeze, done. Tools with fixed slots (where you pick slot 1 for 0.5mm, slot 2 for 0.75mm, and so on) work fine but are slower and require you to count slots correctly every time.

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Hex profile vs square profile

Crimping tools compress ferrules into either a hexagonal or square cross-section. This is a topic that fills forum threads with strong opinions. In practice, both work. The ferrule deforms again when you tighten it into a screw terminal anyway.

Hex tends to be slightly better for round terminal holes. Square suits cage-clamp terminals (the spring-loaded type used in Wago connectors) marginally better. If you're buying one tool for domestic work, hex is the safer default because it's more widely available in budget tools. Don't overthink this.

How to crimp properly

Crimping is simple, but doing it badly is also simple. A bad crimp can look identical to a good one from the outside, which is why technique matters.

Strip the cable. Remove insulation from the cable end to expose the copper strands. The strip length must match the ferrule length. Too short and the ferrule won't grip enough copper. Too long and bare wire protrudes beyond the ferrule. Most bootlace ferrules for domestic cable (1.0mm to 6.0mm) need 8-12mm stripped. Use proper wire strippers, not a knife, to avoid nicking the copper strands.

Select the right ferrule size. The ferrule's internal diameter must match your cable's cross-sectional area. A 1.5mm ferrule goes on 1.5mm cable. Forcing a 1.0mm ferrule onto 1.5mm cable (or using a 2.5mm ferrule on 1.5mm cable) creates either a crushed mess or a loose connection.

Insert the cable. Push the stripped end fully into the ferrule until the copper strands are flush with the open end. You should see copper at the tip, not insulation. The insulated collar of the ferrule (the coloured plastic section) should sit snugly against the cable insulation.

Place the ferrule in the jaws. Position it centrally. On a self-adjusting tool, just push it into the open jaw at any point. On a fixed-slot tool, select the slot matching your ferrule size.

Squeeze until the ratchet releases. One firm, steady squeeze. Don't jerk or pulse the handles. The ratchet clicks through its stages and releases when compression is complete. If using a non-ratchet tool, squeeze firmly until you feel the handles bottom out.

Inspect the crimp. The ferrule should be evenly compressed all the way around, with no visible gaps or splits in the metal. The cable insulation should butt up against the ferrule collar. Give the cable a firm tug. It shouldn't move. If you can pull the ferrule off, the crimp has failed. Cut it off and redo it with a fresh ferrule.

Ferrule colour codes: the confusion you need to know about

Bootlace ferrules are colour-coded by cable size, but there are three competing colour code systems used across Europe. Ferrules sold in the UK follow either the French (Telemecanique) system or the German (Weidmuller) system, and the colours conflict.

The critical one: a black ferrule means 1.5mm in the French/DIN system but 6.0mm in the German Weidmuller system. Getting that wrong means your ferrule is either too small (crushing the cable) or far too large (loose, no grip).

The two systems agree on 0.75mm (grey), 1.0mm (red), and 1.5mm (black). They diverge on everything else. Always check the packaging or data sheet when you buy a bag of ferrules. The mm size printed on the collar or packet is more reliable than the colour.

Uninsulated ferrules (plain copper with no coloured collar) exist too. These are commonly used inside consumer units where space is tight and the ferrule needs to fit into narrow terminal gaps. They're also standard in 13A plug wiring and RCBO flying leads.

What to buy

For a single extension project, you don't need professional-grade tools. The consistent advice from UK electricians on trade forums is clear: a self-adjusting ratchet ferrule crimper in the £12-£25 range handles domestic work perfectly well.

Budget (basic squeeze crimpers): £6-£12. The Magnusson 9" from Screwfix or the Minotaur 215mm from Toolstation will physically crimp a connector. But without a ratchet, you're relying on feel to judge whether the crimp is complete. For ferrules specifically, these are the wrong shape. They're designed for insulated ring and spade terminals (the red, blue, and yellow pre-insulated connectors used in automotive work). Skip these for domestic electrical.

Mid-range (self-adjusting ratchet): £12-£25. This is the sweet spot. The Essentials Ratchet Crimping Tool from Screwfix (4.7/5 from 115 reviews) or the Minotaur Ratchet from Toolstation both do the job. A Preciva self-adjusting kit from Amazon comes with 1,200 assorted ferrules included, which is enough for several projects and saves buying ferrules separately. Trade electricians on forums report using budget self-adjusting crimpers daily for months without degradation.

Professional (Knipex, CK Tools, Weidmuller). The Knipex PreciForce at Screwfix and Knipex 97 53 08 are factory-calibrated, DIN-compliant, and built to survive 40,000+ crimping cycles. The CK Tools T3680A from RS Online has an auto-stop ratchet that releases at exactly the right pressure. These are superb tools. They're also several times the price you need to pay for a kitchen extension. Buy Knipex if you're rewiring houses for a living.

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Alternatives

There's no real alternative to a crimping tool for ferrule work. You can't squeeze a ferrule with pliers and get a reliable result. Standard pliers don't apply even pressure around the circumference, so the ferrule deforms unevenly and the copper strands don't compress properly. The joint may hold initially but will loosen over time, especially under the vibration and thermal cycling inside a consumer unit.

What you can avoid is buying a crimping tool at all, if your electrician handles all the consumer unit terminations (which is the case on most extension projects). Ask your electrician whether they need you to supply any tools. The answer is almost always no. But if you're doing any first-fix electrical work yourself, or if you're helping with cable preparation, a ratchet ferrule crimper is one of those tools that costs little and prevents expensive mistakes.

How to check your crimps

After crimping, run through these checks before the cable goes into a terminal:

The ferrule should be evenly compressed with a consistent hexagonal (or square) profile along its full length. Lopsided compression means the ferrule wasn't seated properly in the jaw.

No copper strands should be visible outside the ferrule barrel. If strands splay out the back, the cable wasn't pushed far enough in. Cut the ferrule off and redo.

A firm pull test: grip the cable in one hand and the ferrule in the other. Pull hard. A good crimp will not move. If it slides, the ferrule was the wrong size or the crimp was incomplete.

The insulated collar (if present) should sit flush against the cable insulation with no bare copper visible between them. A gap means your strip length was too long.

Where you'll need this

• First fix electrics - crimping ferrules onto cable ends for consumer unit and junction box connections

Safety notes

Crimping itself is low-risk. You're squeezing a hand tool around a small metal sleeve. The hazard isn't the crimping; it's where the crimped cable goes. Consumer units carry lethal voltages. The cable preparation (stripping, ferrule fitting, crimping) is safe to do on your workbench with the cable disconnected. The final termination into the consumer unit is not a DIY job. That's your electrician's work, and it requires isolation of the mains supply and testing afterwards.

Wear safety glasses when cutting cable. Copper strands are stiff enough to flick into your eye when you snip the end of a stripped cable. A trivial precaution, but the kind of thing you don't think about until it happens.