Connecting thick wires in a car: reliable methods for power circuits

Thick wires (cross-section from 6 mm² and above) is the basis of power circuits in a car: from the battery to the starter, from the generator to the on-board network, in audio preparation systems or additional equipment. Their incorrect connection leads to voltage sags, overheating, and in the worst case, a fire. Unlike thin wires, twisting or heat-shrinking tubing cannot be used here: special methods, tools and accounting are required current load.

In this article we will analyze 5 professional ways to connect thick cables (up to 70 mm²), which auto electricians use: from classical soldering to bolt clamps and sleeves for crimping. You will find out which method is suitable for DC (battery circuits), and which one is for variable (audio systems), how to avoid oxidation and why aluminum wires require a special approach. At the end there is a table comparing methods in terms of reliability and complexity of execution.

1. Preparation of wires: stripping, tinning and selection of tools

Before connecting thick cables, they must be properly prepared. First step - stripping insulation. For wires with cross section 6–35 mm² use knife with heel (special mounting knife), and for 50–70 mm² — stripper with adjustable blade. The length of the exposed part should be:

• 🔹 For soldering/welding: 20–30 mm (enough to form a reliable connection).
• 🔹 For crimping with sleeves: equal to the length of the sleeve plus 2–3 mm.
• 🔹 For bolted connection: 30–40 mm (to form a loop).

After stripping copper conductors be sure to tin with solder (for example, POS-61 or Sn60Pb40) using rosin-based flux. This will remove the oxide film and improve contact. For aluminum wires tinning is ineffective - they are cleaned alum paste and immediately connect (more details in the section on aluminum).

Tools you will need:

• 🔧 Crimper (crimping pliers) for sleeves - e.g. Knipex 97 53 06 (up to 35 mm²) or HT-750 (up to 70 mm²).
• 🔥 Gas burner or blowtorch for welding/soldering.
• 🔨 Torque wrench (for bolted connections) - the tightening torque should be 8–12 Nm.
• 🛠️ Hot air gun and heat shrink tube with an adhesive layer (for example, 3:1 for sealing).

⚠️ Attention: Do not use side cutters or pliers for stripping - they cut through some of the wires, reducing the cross-section of the wire. This is critical for power circuits, where each reduction in cross-section by 10% reduces the permissible current by 15–20%.

2. Soldering thick wires: when is it justified?

Soldering is one of the most reliable connection methods copper wires with cross section up to 25 mm². It provides minimal contact resistance and corrosion protection. However, for wires 35 mm² and thicker soldering becomes less practical due to:

• 🔥 High heat capacity - requires a powerful soldering iron (150-200 W) or a gas burner.
• 🕒 Long heating time - risk of insulation overheating.
• 💧 The need for sealing - without heat shrinking with glue, the connection will quickly oxidize.

Step by step instructions:

1. Strip and tin the wires (as described above).
2. Twist the wires sequentially (not a “pigtail”, but a tight parallel fit).
3. Apply flux (FKET or LTI-120) and heat the connection with a torch.
4. Enter solder (POS-40 or Sn96Ag4 for high currents) into the connection area, and not onto the soldering iron.
5. After cooling, insulate with heat-shrinkable tubing with an adhesive layer (heat with a hairdryer to 120°C).

Stripping 25–30 mm|Tinning each core|Using active flux|Heating with a torch, not a soldering iron|Sealing with heat shrink 3:1-->

Soldering doesn't fit for:

• 🚗 Aluminum wires - due to the formation of brittle intermetallic compounds.
• ⚡ Circuits with currents >150 A - The solder may overheat and melt.
• 🔄 Movable joints — soldering does not withstand vibrations.

3. Sleeve crimping: professional method for power circuits

Crimping (crimping) with sleeves - the most reliable way to connect thick wires in a car, recommended by standards DIN 46228 and GOST 23586. It provides minimal contact resistance and withstands vibration. The sleeves are:

• 🔘 Tinned copper (for copper wires) - marking GML.
• 🔘 Copper-aluminum (for copper-aluminum transitions) - GAM.
• 🔘 Aluminum (only for aluminum) - GA.

How to choose a sleeve:

Wire cross-section, mm²	Case brand	Insulation color	Max. current, A
6–10	GML-10	Red	80
16–25	GML-25	Blue	120
35–50	GML-50	Yellow	200
70	GML-70	Black	250

Step-by-step crimping instructions:

1. Strip the wires to the length of the sleeve.
2. Insert the cores into the sleeve all the way (for stranded wires use bushing insertso that the cores do not move).
3. Crimp the sleeve two presses (to distribute pressure evenly): first from one edge, then from the other. For sleeves 35–70 mm² use hexagonal matrices.
4. Insulate the connection with heat shrink tubing and glue or electrical tape 3M Scotch 23.

⚠️ Attention: Do not use pliers or a hammer for crimping - this will lead to uneven pressure and an increase in contact resistance. Consequences: heating of the connection at current >100 A and risk of fire.

4. Bolted connection: simple, but not always reliable

A bolted connection (through washers) is the most affordable way to connect thick wires without special tools. It is often used for temporary circuits or in the field. However, the method has disadvantages:

• ⚡ High contact resistance (due to point contact).
• 🔄 Weakening over time (periodic check of tightness is required).
• 🚫 Leakage - oxidizes without additional insulation.

How to make a bolted connection correctly:

1. Strip the wires 30–40 mm and form loops (loop diameter = bolt diameter).
2. Thread onto the bolt in this order:

   Bolt → Flat washer → Wire loop 1 → Lock washer → Wire loop 2 → Flat washer → Nut

3. Tighten the nut with a torque wrench to 8–12 Nm.
4. Insulate the connection with heat shrink or electrical tape in 3 layers.

For aluminum wires before assembly, apply to contact surfaces alum paste or NOAL (composition for protection against oxidation).

What happens if you don't use a lock washer?

Without a bushing washer (or Grover washer), the connection will quickly weaken due to vibration and thermal expansion. This will lead to sparking, heating and possible burnout of the wire. In power circuits (for example, starter battery), this can cause the engine to fail to start.

5. Wire welding: maximum strength for extreme conditions

Welding is the most durable method of connecting thick wires, withstanding currents up to 300 A and vibrations. It is used in:

• 🚛 Trucks (starter chains, winches).
• 🎵 Car audio systems (amplifiers with power >1000 W).
• ⚡ Electric vehicles (high voltage circuits).

For welding you will need:

• 🔥 Inverter welding machine (for example, Resanta SAI-190) with a current of 60–90 A.
• 🧲 Carbon electrode (a graphite rod from a battery will not work!).
• 🛡️ Flux for copper welding (for example, FSM-4).

Welding technology:

1. Twist the wires tightly, 20–25 mm long.
2. Place the twist in clamp form (for example, from copper foil).
3. Connect the ground of the device to the clamp, and the carbon electrode to the “+”.
4. Briefly (0.5–1 sec) touch the end of the twist with the electrode - a copper ball.
5. Clean the welding area from flux and insulate it.

⚠️ Attention: Welding aluminum wires in a car is prohibited! When heated, aluminum loses strength and the weld area becomes brittle. For aluminum, use only sleeve crimping GAM.

6. Connecting aluminum wires: features and errors

Aluminum wires are still found in older cars (eg. VAZ 2101–2107, GAZ-24) and in charging circuits (from the generator). Their connection requires a special approach due to:

• 🧊 Oxide film - forms in seconds in air and has high resistance.
• 🔄 Aluminum fluidity - weakens contact over time.
• ⚡ Electrochemical corrosion upon contact with copper.

Correct methods for aluminum:

1. Crimping with sleeves GA or GAM:

   - Use alum paste to remove oxide film.

   - Squeeze only hexagonal matrices (not with pincers!).

2. Bolted connection:

   - Apply NOAL or cymol on contact surfaces.

   - Use spring washers to compensate for fluidity.

Prohibited methods:

• ❌ Soldering - fragile connections are formed.
• ❌ Welding - aluminum loses strength.
• ❌ Twisting - quickly oxidizes and heats up.

Why can't you connect aluminum and copper directly?

When in direct contact, copper and aluminum form galvanic couple, which leads to electrochemical corrosion. The aluminum is destroyed and the connection heats up. To go use copper-aluminum sleeves GAM or terminals with brass contacts.

7. Selecting a method based on circuit type and load current

Not all methods are equally good for different circuits. Below are recommendations for choosing a connection method depending on load current and chain type:

Chain type	Current, A	Recommended Method	Alternative
Starter battery	200–400	Welding or crimping with sleeves	Bolted (temporary)
Generator-battery	50–100	Crimping with sleeves	Soldering (if the wires are copper)
Audio system (amplifier)	30–150	Crimping or welding	Bolted (with lock washer)
Additional equipment (winch, refrigerator)	up to 200	Crimping with sleeves	Bolted (with periodic inspection)

For charging circuits (generator-battery) the minimum resistance is critical - here is the best choice: crimping with GML sleeves followed by heat-shrink sealing. B audio systems Vibration protection is important, so it is preferable welding or crimp.

FAQ: Frequently asked questions about connecting thick wires

Is it possible to connect wires of different sections?

Yes, but with reservations:

• 🔹 If the difference in sections is up to 2 times (eg 10 mm² and 25 mm²), use stepped sleeve (for example, GML-10/25).
• 🔹 If the difference is greater, connect via adapter terminal (for example, terminal block WAGO 222 for currents up to 32 A).

In circuits with currents >100 A, avoid direct connection of wires of different sections - this creates a “bottleneck” and an overheating point.

How to check the connection quality?

Use multimeter in millivoltmeter mode:

1. Connect the probes to the wires before and after connections.
2. Pass a current of 10–20 A through the circuit (for example, turn on the headlights).
3. Measure the voltage drop across the connection:
   • 🟢 < 50 mV - excellent connection.
   • 🟡 50–100 mV - acceptable, but requires supervision.
   • 🔴 > 100 mV - needs to be redone.

Also visually inspect the connection after 10–15 minutes of operation under load: it should not heat up (temperatures up to 50°C are acceptable).

How to insulate the connection in the engine compartment?

In conditions of high temperatures and humidity, the following are suitable:

• 🔥 Heat shrink tube with glue (for example, 3:1 adhesive) - withstands up to 125°C.
• 🛡️ Electrical tape 3M Scotch 23 — moisture resistant, temperature up to 105°C.
• 🧊 Liquid insulation Plasti Dip - for complex shapes (for example, bolted connections).

Do not use regular PVC electrical tape - it melts at 80°C and becomes limp with oil.

How to connect aluminum and copper wires in a car?

Direct connection of copper and aluminum is prohibited! Use:

1. Copper-aluminum sleeves GAM - crimp on both sides.
2. Terminal blocks with brass contacts (for example, WAGO 2273 with pasta NOAL).
3. Bolted connection with gasket made of tinned washer.

In any case, apply to the contacts alum paste or NOAL for protection against corrosion.

Do I need to tin the wires before crimping them with sleeves?

No, tinning before crimping not only useless, but also harmful:

• 🔹 Solder reduces the area of contact between the core and the sleeve.
• 🔹 When crimping, the solder can squeeze out, creating voids.
• 🔹 The sleeve itself provides sufficient contact without soldering.

You only need to tinker in front of soldering or welding.