This article gives a practical description of crimping for joining electrical cables in fixed wiring installations. The techniques shown are generally applicable to other fields such as automotive electrics. This is not, however, a recommended technique for extending flexible mains cables connected to appliances.
It is recommended that you practice this procedure a few times before deploying it on a real installation, to make sure the joints you create are mechanically strong. It is also a good idea to get your practice in when you are not stuck upside down in some uncomfortable corner of a wiring cupboard or other "real life" situation.
Acceptable jointing methods
There are various situations where it may become necessary to join an electrical cable, such as when effecting repairs and upgrades to installations. The wiring regulations basically allow several methods for making cable joints, including:
- Screw terminal connections
- Joints produced by a compression tool (i.e. crimped)
- Soldered Joints
- Welded Joints
1) is most commonly used in electrical accessories such as switches and sockets. However this method is only acceptable where the joint will remain accessible for future inspection and maintenance (BS7671 (17th edition) section 526.03), since screwed terminal connections can become loose over time.
4) Is not an option readily available in many domestic situations. Hence if one needs to make a joint that is robust, and can be safely entombed into a building we are left with 2) & 3) as both of these are covered by the exemption detailed in part (iv) of the above wiring regs section.
Crimping is a quick and reliable method of jointing that is easy to deploy in the field since unlike soldering it does not require time to be spent waiting for a soldering iron to heat up, and poses no burn risk to the operator when working in confined spaces.
Required Tools and Materials
For making reliable crimped joints the correct crimping tool must be used. Adequate crimping tools are of a substantial contruction and include a ratchet tightening mechanism that ensure adequate pressure is applied to the crimp before it is released. Like this:
A suitable tool is available from most good tool shops and electrical retailers such as this[].
Crimp terminals are available in a number of sizes and two basic types: [Insulated] and [Uninsulated]. Both types can be used, but the procedure is different, and uninsulated crimps require the use of a different crimp tool to insulated ones. For the purposes of this article we will use insulated crimps since these are the more commonly available, and require a little less work to use.
In addition to a crimp tool you will need a few basic tools like wire cutters, and strippers. You will also need some heatshrink tubing of a size appropriate for the cable to be joined
The wrong tool for the job
Note that the cheap non ratchet crimp tools like this are not suitable:
If you have one of these, save it for stripping wire and cropping small bolts.
The crimping procedure
Let's assume we need to join a section of cable.
Prepare the wire
The first job is to cut out any damaged sections or wire to leave sound wire to work with:
Strip the cable back a couple of inches (about 50mm) on each side so that there is room to work.
To Reduce the size of the overall crimp, and to make for a neater joint, the lengths of wires can be staggered so that the crimp terminals will not form a bunch:
(It is better to stagger the joints even more than as shown in the photograph - such that there is no overlap in the crimp terminal positions at all being ideal)
Before going any further fit the insulation!
Cut some heatshrink to a length enough to not only cover the joint but also extend an inch of two down the cable. This will make the final joint more moisture resistant. (If waterproofing is important then an adhesive lined heatshrink can be used):
For a 2.5mm² T&E cable use 12.7mm (0.5 inch) diameter heatshrink, for smaller cables 10mm diameter will probably be a better fit.
Also remember to fit sleeving to the earth wires. Slide the heatshrink it up one of the wires until it is out of the way:
Select suitable crimps
For joining cables like this, the type of crimp required is known as a "butt" crimp or splice.
The crimp selected needs to not only match the wire size in question, but also have enough current carrying capacity for the circuit. The colour of a crimp only indicates its size, they are not designed to match any particular colour of wire insulation.
The three most popular sizes / colours being:
1mm² and 1.5mm² T&E
4.00mm² and 6.00mm² T&E
Position the crimp in the jaws of the crimp tool (in the recess marked with the same colour as the crimp), and squeeze gently so as to grip the crimp without squashing it:
You should position the crimp so that the active part of the jaws are directly over the metal part of the crimp, leaving the empty part of the plastic insulation sticking out
You want the length of exposed conductor to be roughly equal to half the length of the inner metal part of the crimp terminal (5 - 8mm is usually about right), such that each wire fills half of the terminal. Note that butt splice crimp terminals like those shown in the picture do not always have a central stop to prevent the wire passing right through the terminal. With thinner wires it does not matter if the wires cross over, however with thicker wires there may not be enough room in the terminal for this to happen - so if you insert a wire too far from one side, you will not be able to get the other wire far enough inserted on the other to make a reliable crimp.
Insert the wire such that the insulation is tucked inside the shrouded part of the crimp:
Once the crimp is made, inspect the crimp to ensure it looks correctly compressed and the wire is still in the correct place with no uninsulated section showing. Attempt to pull the wire out of the crimped connection to verify that it is held properly:
Repeat for the other wires in the cable:
Finally complete the crimp to the other cable:
Once the cable is joined, you can test your work. As described above the first test is mechanical: that the wire should not pull free of the crimp when applying reasonable force. A visible inspection should also be made to ensure that none of the conductor is exposed at the ends of the insulation on the crimp connector.
To test electrically, find where the cable you have repaired connects to the existing circuit at both ends. Disconnect both ends. At one end twist the phase, neutral and earth wires together. Now use a multimeter on a low ohms range to measure the resistance between phase and neutral, phase and earth, and neutral and earth at the untwisted end of the cable. The two readings taken with the earth wire ought to be equal, but larger than the phase/neutral one since the earth wire is typically thinner than the phase/neutral wires.
Compare your results with the anticipated resistance values shown in the table here in the Electrical Circuit Faults article. If all is well untwist the wires and reconnect both ends of the cable to the circuit ensuring the screw connections are well made and tight.
Last job is to slide the heatshrink over the joint and shrink it:
Heatshrink is best shrunk with a small purpose made hot air gun. If this is not available a gas flame can be used, but it must be played over the joint very quickly and sparingly so as not to scorch it.