How to Repair a Cut Wire: Splice It Properly, Step by Step

A wire cut by a sharp chassis edge, a slipped knife or a stray drill bit is one of the most common faults on any 12V system. The good news is that you can repair a cut wire properly in about five minutes, and a well-made splice is as reliable as the original wire.

The bad news is that a quick twist and a wrap of electrical tape isn't a repair. Tape lifts with heat and water, the bare twist corrodes, and the fault comes back, usually at the worst possible time.

This guide shows how to repair a cut wire step by step with a solder seal connector such as a Solderstick, how to handle wire that's too short or multi-core cable, and when you shouldn't splice at all.

Quick answer

• Isolate the circuit first: disconnect the battery negative.
• Cut both ends back to clean, bright copper and strip about the length of the connector's solder ring.
• Pick a connector sized for the wire, slide it on, overlap or twist the strands, and heat it with a heat gun until the solder flows and the ends seal.
• Let it cool, test the circuit, then secure and protect the wire so it can't be cut again.

Before you start: safety first

Disconnect the battery negative before you touch the wiring. A cut wire may already be shorting to the body, and you don't want to make that worse while you're working. On a caravan or boat, also isolate the house battery, solar input and any shore power charger.

This guide is for low-voltage DC wiring only: 12V and 24V automotive, marine, caravan, solar DC and low-voltage garden lighting. If the cut wire is a 230/240V mains lead or fixed wiring in a house or caravan, don't splice it. In Australia, fixed mains wiring must be repaired by a licensed electrician.

Can you repair a cut wire, or should you replace it?

Most cut wires can be spliced. Replace the wire instead of splicing when:

• The damage runs a long way, such as a wire melted along its length or crushed in several places.
• The copper is black or green well beyond the cut, which means water has travelled under the insulation. Cut back to bright copper, and if you can't find it, replace the run.
• It's a battery cable or anything heavier than 6mm2. Use crimped or soldered lugs or replace the cable rather than splicing.
• It's part of the airbag (SRS) system. Many vehicle makers don't allow SRS wiring to be spliced at all, and those that do specify their own repair kit. SRS wiring is often yellow or in yellow loom, but not always, so check the service information for your vehicle first.

Tools and parts

• Solderstick connectors sized for the wire
• Wire strippers and side cutters
• A heat gun (recommended; a lighter is a last resort and can scorch the tube)
• A multimeter or test light
• Spare wire of the same size or heavier, if the cut wire is too short to rejoin
• Split loom or tape and cable ties to protect the repair

Choosing the connector size

If you don't know the wire size, check the printing on the insulation or compare it with the connector size chart. Auto cable is often sold by its overall size rather than the copper area, so the guide to auto cable sizes is worth a read.

How to repair a cut wire with a Solderstick connector

1. Cut back to clean wire

Trim both ends square with side cutters, removing any frayed, nicked or discoloured strands. You want bright copper on both sides.

2. Strip the ends

Hold the connector against the wire to judge the strip length. The bare strands should sit under the solder ring in the centre, and the insulation should sit under the coloured sealing rings at each end. Don't cut through strands as you strip, as every lost strand reduces the wire's capacity.

3. Slide on the connector and join the strands

Slide the connector onto one wire. Then either overlap the two stripped ends side by side, or twist them together in a straight inline twist, and centre the join under the solder ring.

4. Heat it

Use a heat gun and start in the middle. When the solder ring melts and flows into the strands, move out to each end until the tube shrinks tight and the sealing rings melt. Keep the heat moving so you don't scorch the tube.

5. Check and let it cool

Through the clear tube you should see solder flowed into the strands and a small bead of melted adhesive at each end. Leave the join to cool before you bend or move it.

6. Test the repair

Reconnect the battery and check that the circuit works. For a more thorough check, measure the voltage at the load with the circuit switched on and compare it with the battery voltage. A big difference means there's still a poor connection somewhere.

The full guide to using Solderstick connectors has more detail on technique.

Repairing a cut wire that's too short

If a section of wire is missing, or the ends won't reach without pulling tight, splice in a new piece. Use wire of the same size or heavier, never lighter, and join it at both ends with two connectors. Leave a little slack so the repair isn't under tension. A splice that is stretched tight will pull apart or fatigue with vibration.

Use the same colour wire where you can, or mark the insert with coloured tape, so the next person tracing the circuit isn't confused.

Repairing cut multi-core cable

Trailer plugs, reversing cameras and caravan cables often run several cores inside one outer sheath. To repair a cut in multi-core cable:

1. Slide a length of adhesive-lined dual wall heat shrink over the outer sheath first, big enough to pass over the finished joins.
2. Strip back the outer sheath on both sides.
3. Cut each core to a different length so the joins are staggered. This stops the repair bulging in one spot.
4. Join each core colour to colour with its own connector.
5. Slide the outer tube over the joins and shrink it so it seals onto the sheath at each end.

Staggering the joins makes the repair slimmer and more flexible. The outer sleeve seals the sheath, so water can't get in between the cores. See How to Wire Trailer Lights So They Keep Working for more on trailer cable.

Ways to repair a cut wire compared

For a deeper comparison, read Solderstick vs Crimp vs Solder.

Which should you use?

• If you're repairing a cut wire under a vehicle, trailer or caravan, use a Solderstick connector sized to the wire.
• If the repair will be hosed, submerged or rubbed, add a dual wall heat shrink sleeve over the connector.
• If the wire is too short, splice in a new piece of the same size or heavier with two connectors.
• If it's multi-core cable, stagger the joins and seal the sheath with a larger dual wall tube.
• If it's a battery cable, anything heavier than 6mm2, SRS wiring or 230/240V mains, don't splice it: use lugs, follow the manufacturer's procedure, or call a licensed electrician.

Find out why the wire was cut

A wire rarely cuts itself. Before you put everything back, find the cause, or you'll be making the same repair again.

• Sharp edges: fit a rubber grommet where wire passes through metal, and pad any edge it rests on.
• Movement: secure the wire with cable ties or P-clips every so often so it can't swing and rub.
• Heat: keep wiring away from exhausts and turbos.
• Pinch points: check tailgates, doors, tilt trays and the gap between a car and trailer, where wire can be crushed.

Cover the repaired section with split loom where it runs near hazards. On a fuse-protected circuit, a cut wire that shorted to the body may also have blown the fuse, so check it and replace it with one of the same rating.

Frequently asked questions

Can you repair a cut wire with electrical tape?

Only as a get-you-home fix. Tape lifts with heat and water, and the bare twist underneath corrodes. Replace it with a sealed join as soon as you can.

Is it OK to splice a car wire?

Yes, for most circuits, if you use the right size connector and seal the join. The exceptions are airbag (SRS) wiring, which many manufacturers don't allow to be spliced, and battery cables, which need lugs.

How do you repair a cut wire without a soldering iron?

Use a Solderstick connector. It has its own solder ring inside, so you only need wire strippers and a heat gun, with no iron or crimp tool.

What size wire should I use to extend a cut wire?

Use the same size as the original or heavier, never lighter. A thinner insert can overheat and causes extra voltage drop.

Why does my repaired wire keep failing?

Usually the cause was never fixed, such as a sharp edge or a wire that moves, or the join was made on corroded copper. Cut back to bright copper, seal the join and secure the wire away from the hazard.

Can I repair a cut 240V cord myself?

No. Solderstick connectors are for low-voltage DC wiring only. Damaged mains leads should be replaced, and fixed mains wiring in Australia must be repaired by a licensed electrician.

What to use

For a single repair job, the 50PCS kit (10 white, 22 red, 13 blue, 5 yellow) covers wire from 0.25mm2 to 6.0mm2 and is the compact option to keep in the vehicle. For a workshop drawer, the 100PCS kit gives you more of each size. Add adhesive-lined dual wall tube from the heat shrink range for multi-core cable and exposed repairs.

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