Helpful video reference. We recommend eFIXX's "SOLID conductors and CRIMP CONNECTORS" as the reference video here. eFIXX is a UK-based electrical training platform used by thousands of electricians for CPD. The video demonstrates crimp technique on solid conductors and explains why the type of tool matters for a reliable long-term connection.
1. Identify the conductor type and cross-section
Before choosing a crimp, work out what you are connecting. Domestic fixed wiring uses solid or stranded copper conductors in standard sizes: 1.0 mm², 1.5 mm², 2.5 mm² and 4 mm². Flexible cable and appliance leads use fine-stranded conductors. Solid conductors are stiffer and need a crimp that can be tightened down on a single strand without cutting into it; stranded conductors can be gathered and inserted straight into the crimp barrel.
Check the current rating for the joint matches the circuit: 1.5 mm² is typical for lighting, 2.5 mm² for ring socket circuits, 4 mm² for high-current appliance circuits.
2. Select the correct crimp type
There are three crimp types commonly used in domestic electrical work:
- Bootlace ferrules (pin terminals) - a copper sleeve that goes over the stripped end of a stranded conductor before it enters a screw terminal. Available in sizes matched to conductor cross-section (0.5 mm² to 10 mm²). Colour-coded: 1.0 mm² is black, 1.5 mm² is red, 2.5 mm² is blue, 4.0 mm² is grey (though colour coding varies by brand).
- Insulated butt-splice connectors - a sleeve that joins two conductors end-to-end. Each half has a barrel; the conductors are inserted from each side and crimped. Suitable for extending a cable run or repairing a break.
- Insulated ring and spade terminals - used for connecting to screw posts and blade connectors in consumer units and appliances.
For most domestic wiring joints, a butt-splice connector is the correct choice. For panel wiring and consumer unit terminations, a bootlace ferrule is used.
3. Match the crimp size to the conductor
Every crimp carries a conductor size range in mm² printed on the body or visible as the colour coding. Using a crimp that is too large gives a loose joint; too small and you cannot insert the conductor fully. If in doubt, strip back a short length and hold the conductor end against the open barrel: the conductor should fill roughly 80–90% of the barrel diameter with no slack.
4. Strip the conductor to the correct length
The barrel of the crimp determines how much insulation to remove. On a butt-splice connector, the strip length should be equal to half the connector's internal length so the conductor ends meet roughly in the middle. On a bootlace ferrule, strip just enough for the conductor to sit fully inside the ferrule barrel without bare copper showing beyond the ferrule collar.
Use wire strippers set to the correct conductor size. Nicking the conductors with a blade weakens them and will cause the crimp to fail under load.
5. Insert and crimp the conductor
Push the stripped conductor firmly into the crimp barrel until the insulation butts up against the barrel entrance. With a ratchet crimp tool, position the barrel in the correct jaw size for that crimp, then squeeze the handles together until the ratchet releases. The tool will not spring open until the compression is complete - this is what prevents under-crimping.
On butt-splice connectors, crimp each side separately. On bootlace ferrules, one crimp per ferrule.
6. Inspect and test the joint
After crimping, grip the conductor at the insulation and try to pull it from the ferrule or butt-splice. A correctly crimped joint will not come free with hand force. Look at the crimp barrel: it should be evenly compressed with no splits or visible cracks. The conductor should not be able to rotate freely inside the crimp.
For an insulated butt-splice on a live circuit that you are about to re-energise, confirm the integrity of the insulation with an insulation resistance test if you have a meter capable of it.
7. Enclose the finished joint
Under BS 7671 Regulation 526.3, all cable joints must be in an accessible enclosure or inside a maintenance-free enclosure (where the connector itself is rated for that installation method). Insulated butt-splice connectors in a wall must be in a back box with a blank plate, positioned in the correct safe zone. In a ceiling void, use a junction box screwed to a joist with a screwed cover.
When to call us
Crimp joints done correctly last decades without maintenance. Done incorrectly - wrong tool, undersized crimp, partial insertion - they overheat over time and become a fire risk. If you have any doubt about the conductor size, the joint type or the enclosure requirements, it is faster and safer to call.
Need a wiring joint made safely in Sandwich?
Richard carries ratchet crimp tools and a full range of connector sizes on the van. Small local jobs at the £10 per 10-minute rate.
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