How to use bootlace ferrules on stranded cables

Helpful video reference. We use eFIXX's short "Doubling up on the Bootlace Ferrules" as the video reference here. eFIXX covers the UK-specific technique correctly, including the twin-ferrule approach for double-termination that is required when two conductors share a single screw terminal.

1. Understand what ferrules are and when you need them

A bootlace ferrule (also called a wire end sleeve or wire end ferrule) is a small tinned or insulated copper tube that you crimp onto a stripped stranded conductor. When tightened by a screw terminal, the ferrule barrel carries the compression load evenly across all the strands, rather than the screw crushing and breaking individual wires.

You need ferrules whenever you are terminating stranded conductors: flexible cable, multicore cable, or any cable where you can see multiple thin wires bundled together rather than a single solid one. Standard twin-and-earth solid-core cable does not usually need ferrules, but ring main spur cables using flex, equipment leads, or commercial multicore cables do.

2. Choose the right ferrule size for the cable

Ferrule size is measured by conductor cross-section in mm². The ferrule must match the cable conductor exactly, or the connection will be loose (too large) or the conductor will not fit at all (too small).

• 1.0 mm² -- lighting circuits, small appliance spurs
• 1.5 mm² -- standard lighting circuits, 5 A circuits
• 2.5 mm² -- ring final and radial socket circuits, cooker supplies up to 4 kW
• 4 mm² -- 20 A and some 32 A radial circuits
• 6 mm² -- EV charger supplies, large 32 A radials

For double-termination (two conductors sharing one terminal), use a twin-bootlace ferrule rather than two single ferrules. Twin ferrules are available in the same sizes and are colour-coded identically to single ones but have two entry holes.

3. Strip the cable to the correct length

Use wire strippers set to the correct cable size. Strip back just enough insulation to fill the ferrule sleeve. The manufacturer's strip length is usually printed on the ferrule packaging, typically 8 to 10 mm. Check the stripped conductor sits fully inside the sleeve with no bare conductor visible beyond the ferrule end when it is pushed in fully.

Over-stripping is a common mistake: any bare conductor that sticks out beyond the ferrule tip can contact an adjacent terminal, creating a short circuit or shock hazard.

4. Insert the conductor and crimp the ferrule

Push the stripped end of the conductor into the ferrule opening until it bottoms out. Ensure every strand is inside the sleeve. Place the ferrule into the correct jaw of a ratchet crimping tool. These tools are colour-coded to match the ferrule insulation colour, so use the right jaw for the size you are fitting.

Squeeze the handles until the ratchet releases. The ratchet mechanism ensures the crimp is complete before the tool opens: do not force it open partway through. A finished crimp should show a hexagonal or square indentation in the barrel with no cracked insulation at the entry end.

5. Check the joint and insert into the terminal

Grip the ferrule and pull the conductor gently. It should not move. If it pulls out, the crimp has failed, so replace the ferrule and repeat. Visually confirm no strands are splayed around the outside of the sleeve.

Insert the ferrule end into the screw terminal and tighten the terminal screw. The screw should land on the ferrule barrel, not on the plastic insulating collar. The collar is a handling aid, not a structural part of the joint. Apply firm, even pressure and check the conductor does not rotate when you try to move it.

6. Common mistakes to avoid

• Using pliers instead of a crimping tool. Pliers apply uneven force and can crack the ferrule barrel or the insulating sleeve. A ratchet crimping tool is not expensive and does the job correctly every time.
• Fitting the wrong size ferrule. A ferrule that is too large will not grip; one that is too small will not accept all the strands.
• Over-stripping. Bare conductor beyond the ferrule tip is a shock and short-circuit risk.
• Double-terminating with two single ferrules. Two ferrules in one terminal will not seat correctly and may loosen over time. Use a twin ferrule instead.
• Tightening onto the collar rather than the barrel. The screw must contact the metal barrel, not the plastic sleeve, or the connection will be intermittent.

When to call us

Fitting ferrules is a straightforward technique once you have the right tool. Where it gets more complex is inside a consumer unit: live busbars, other circuits still energised, and connections carrying the full household load. If you are not confident working in the board, call Richard instead.