Splicing wires with a stranded core: step-by-step methods and common mistakes

Breakage or need for extension stranded wire in automotive wiring is a common problem encountered when repairing harnesses, installing additional equipment, or repairing a short circuit. If the connection is made incorrectly, the consequences will appear immediately: overheating at the twisting point, oxidation, voltage loss of 0.3–0.7V or complete circuit break after 1–3 months. This is especially critical for power circuits (starter, generator, inverters), where multi-core wires with a cross section of 4–10 mm² require not just mechanical contact, but a full-fledged electrical bridge without transition resistance.

Unlike solid cores, stranded wires (for example, brands PVS, ShVVP or automobile GPT/TXL) cannot be connected by ordinary twisting - individual strands will tear during vibration, and the junction will begin to “breathe” under temperature changes. Even if visually the contact seems strong, micro-tears inside the beam lead to cold soldering effect: current flows only through a few veins, and the rest remain unused, which accelerates corrosion. Proper splicing of such wires requires either mechanical crimping, heat treatment (soldering/welding), or the use of specialized connectors.

1. Why twisting a stranded wire is the worst option

Twisting as a connection method is only permissible for temporary repairs monolithic conductors of small cross-sections (up to 1.5 mm²), but is absolutely not suitable for stranded wires. Reasons:

• 🔹 "Fluffiness" effect: individual wires are not fixed to each other, and during vibration (for example, in a car door harness), they are gradually “pulled out” of the twist, increasing resistance.
• 🔹 Oxidation: microgaps remain between the veins, where moisture penetrates. After just 2–3 weeks, copper is covered with an oxide film, which impairs conductivity by 20–40%.
• 🔹 Overheating: due to point contact of only part of the veins, the cross-section of the “working” area is reduced, which leads to local heating (especially critical for circuits with current >5A).

If you are still forced to use twisting as a temporary solution, be sure to:

1. Divide the wire into two parts and twist them in parallel, not sequentially.
2. Cover the junction conductive paste (for example, Kontakt 60) before isolation.
3. Secure the twist heat shrink tube with adhesive layer, warming it up with a hair dryer until it shrinks completely.

⚠️ Attention: Twisting a stranded wire with a cross-section >2.5 mm² without subsequent soldering or crimping is guaranteed to lead to burnout of the contact within 6–12 months. In automotive wiring, this is a common cause of fires in the engine compartment.

2. Soldering stranded wires: step-by-step instructions

Soldering is the most reliable splicing method if done correctly. She provides molecular bond between the veins, excluding transition resistance. To work you will need:

• 🔧 Soldering iron with a power of 60–100 W (for wires with a cross-section >4 mm² - 150 W).
• 🔧 Solder POS-61 or POS-40 (for automotive wiring, lead-free is better Sn96.5/Ag3.0/Cu0.5).
• 🔧 Flux LTI-120 or FKET (do not use rosin - it does not remove oxides from copper!).
• 🔧 Heat-shrinkable tube with an adhesive layer (shrinkage ratio 2:1).

Soldering algorithm:

1. Strip the wires by 15–20 mm, divide the wires into two groups and twist them flagellum (not a flat braid!).
2. Apply flux to the twist and heat it with a soldering iron for 2-3 seconds to activate the flux.
3. Apply solder from the opposite side of the soldering iron so that it flows into the twist under the action of capillary effect.
4. After cooling, remove any remaining flux isopropyl alcohol (flux is hygroscopic and will conduct current between the veins over time).
5. Put on the heat shrink tube and heat it with a hairdryer until it shrinks completely.

Stripping insulation without cutting the veins|

Checking for wire breaks with a multimeter in continuity mode |

Using flux rather than rosin|

Warming up the twist before applying solder|

Removing flux residues after soldering -->

Typical soldering mistakes:

Error	Consequences	How to avoid
Wire overheating (>250°C)	Melting of insulation, loss of strength of veins	Use a soldering iron with a temperature controller
Insufficient solder	Cold soldered with high resistance	The solder should completely fill the twist.
Using rosin instead of flux	Oxides remain on the veins, contact is unreliable	Use active fluxes (LTI-120, FKET)
No cleaning after soldering	Corrosion due to flux residues	Wash with alcohol or special cleaner

3. Sleeve crimping: professional method for power circuits

Crimping is the preferred method for wires >4 mm², especially in automotive wiring where vibration and temperature changes preclude soldering. For stranded wires use tinned copper sleeves (GML) or combined (aluminum-copper GAM - only for connecting aluminum to copper!).

Step by step process:

1. Select a sleeve according to the cross-section of the wire (for stranded ones, take a size smaller than for monolithic ones). For example, for a 6 mm² wire, a sleeve is suitable GML-4.
2. Strip the wires to the length of the sleeve + 5 mm, twist the wires and insert them into the sleeve all the way.
3. Crimp the sleeve press jaws (not with pliers!) in two places: along the edges and in the center. For sleeves >10 mm², make 3–4 crimps.
4. Check the quality of the crimp: pull the wire along the sleeve. If it slips out, the crimping is unsuccessful.
5. Insulate the connection with heat shrink tubing or electrical tape overlap by 10–15 mm.

Key points:

• 🔧 For automotive wiring, use sleeves with polymer coating (for example, GML-P), which melts during crimping and seals the connection.
• 🔧 Do not use screw terminals for multi-core wires - they pinch individual wires, reducing the cross-section.
• 🔧 For crimping sleeves with a cross-section >16 mm², use hydraulic press, not hand pliers.

How to choose a sleeve for stranded wire

To calculate the cross-section of the sleeve, add up the cross-sections of all connected wires and round up. For example, two 4 mm² wires require a 10 mm² sleeve. For stranded wires, take a sleeve 10–15% smaller than the calculated cross-section (due to the packing density of the veins).

⚠️ Attention: Crimping sleeves without a special tool (for example, pliers) leads to microcracks in the veins, which will appear in 3–6 months in the form of a break. Press jaws must have trapezoidal jaws, not flat.

4. Welding wires: when use is justified

Welding of stranded wires is used in cases where it is required maximum mechanical strength connections (for example, in circuits starter, winches or battery terminals). For this they use spot welding or gas burner with solder. Advantages of the method:

• 🔥 The connection can withstand current up to 100A without heating.
• 🔥 There is no risk of “pulling out” the veins during vibration.
• 🔥 Minimum transition resistance (less than with soldering).

Welding technology:

1. Twist the wires and crimp them temporary sleeve for fixation.
2. Connect the ground of the welding machine to the twist, and the electrode to a separate piece of solder.
3. Briefly (0.5–1 s) touch the electrode to the twist to melt the solder and strands.
4. After cooling, remove the slag with a wire brush and insulate the connection.

Limitations of the method:

• ❌ Not suitable for wires < 2.5 mm² (risk of burning wires).
• ❌ Requires skills in working with welding equipment.
• ❌ In the car it is used only for stationary harnesses (not for door wires!).

5. Connecting terminals: fast, but not always reliable

Terminal connectors (WAGO, Scotchlok, screw terminals) are convenient for quick installation, but most of them are not intended for stranded wires. Exceptions:

• 🔌 WAGO 222-413 terminals (with lever) - suitable for stranded wires 0.5–4 mm², but require vein tinning before installation.
• 🔌 Scotchlok UY-2 — for wires 0.5–1.5 mm², but only in low-voltage circuits (up to 12V, 5A).
• 🔌 Screw terminals with clamping plate (for example, Weidmüller) - for wires up to 6 mm², but the screw must be tightened with a force of 0.8 Nm.

Rules for using terminals:

1. For wires with a cross-section >2.5 mm² always service the veins before inserting into the terminal.
2. In automotive wiring, avoid terminals in engine compartment - vibrations will weaken the contact.
3. Check the terminals once every 6 months: if the contacts become oxidized, replace them with soldering or crimping.

⚠️ Attention: Terminals WAGO 773 (spring type without a lever) are absolutely not suitable for stranded wires - the spring cuts through individual veins, reducing the cross-section by 30–50%.

6. Common mistakes and how to avoid them

Even experienced electricians make mistakes when splicing stranded wires. Here are the most critical ones:

Error	Reason	How to fix
Using electrical tape instead of heat shrink	The insulating tape comes apart when heated, moisture penetrates into the joint	Use heat shrink tube with adhesive layer
Crimping the sleeve with pliers	Uneven pressure leads to microcracks	Use press jaws with trapezoidal jaws
Soldering without flux	Oxides on the veins prevent solder adhesion	Use active flux (LTI-120, FKET)
Twisting without fixation	Veins “fluff” when vibrated	Fix the twist by soldering, crimping or clamping
Ignoring resistance check	Unnoticeable defects lead to overheating	Test the connection with a multimeter in resistance mode

To check the connection quality, use voltage drop method:

1. Connect the wire to a power source (for example, a 12V battery).
2. Measure voltage up to and after connections under load (for example, at a current of 5A).
3. If the difference is >0.1V, the connection is unreliable and requires rework.

7. Frequently asked questions

Is it possible to connect stranded and solid wire together?

Yes, but only with help crimping sleeve or rations. Twisting in this case is unacceptable - a monolithic wire will “cut” the strands of a stranded wire during vibration. For crimping, use a sleeve the size of a solid wire, and pre-twist and tin the stranded wire.

Which flux is best for soldering automotive wiring?

Flux is optimal for automotive wiring LTI-120 or FKET — they remove oxides from copper and aluminum, and also protect against corrosion. Rosin is not suitable because it does not cope with strong oxides. After soldering, be sure to remove any remaining flux. isopropyl alcohol.

What is the difference between GML and GAM sleeves?

Sleeves GML (tinned copper) are used for connection copper wires, and GAM (aluminum-copper) - for transitions between aluminum and copper. In automotive wiring, only GML is used, since aluminum wires are not used there. GAMs are suitable for stationary wiring (for example, in a garage).

How to protect the connection from moisture in the engine compartment?

To protect against moisture:

1. Use heat shrink tubing with adhesive layer (for example, 3:1 adhesive-lined).
2. Apply to joint silicone grease before shrinking the tube.
3. For critical circuits (such as sensors), additionally wrap the connection self-vulcanizing tape (for example, 3M Scotch 23).

Can WAGO terminals be used for 6 mm² wires?

No, terminals WAGO designed for a maximum of 4 mm². For 6mm² wires use:

• Crimping sleeve GML-6 followed by isolation.
• Welding with copper-phosphorus solder.
• Screw terminals Weidmüller with pressure plate (model ZVH-8).