Can I connect different wires to each other?

A sharp drop in voltage at the end of the line or local overheating of the insulation at the twisting site often indicates that the cable veins are selected and docked with a violation of the physical laws of current distribution. Direct connection of wires of different cross-sections with each other is technically possible, but only with strict observance of the rules for calculating the throughput and using specialized transition elements, since ignoring the difference in the cross-sectional area leads to catastrophic consequences. Electric current, passing through a section with a smaller cross section, meets increased resistance, which causes heat release according to the Joule-Lentz law, and if this unit is not protected by automation designed for a thin vein, the melting of the insulation begins.

The main problem lies in the current density, which for copper and aluminum conductors is normalized strictly, and when connected heterogeneous The effect of living without an intermediate link is the “bottleneck” effect. The thin section of the circuit becomes a critical point where energy is converted to heat while the thick cable remains cold, creating the illusion of the whole system working properly. Understanding the physics of the process dictates the need for the use of transition terminals, sleeves or tires, which allow you to safely switch veins with different cross-sectional areas, distributing the load and providing reliable electrical contact.

Modern electrical installation standards, including PUE (The rules of the Device of Electrical Installations) regulate the permissible methods of switching, prohibiting simple twisting of heterogeneous wires due to the risk of oxidation and weakening of the contact. The professional approach requires the use of methods that exclude direct contact of heterogeneous metals and compensate for the difference in mechanical rigidity of the veins. Properly executed connection unit ensures the stable operation of electrical appliances and eliminates the risk of fire, turning a potentially dangerous situation into a reliable element of the power system of the house or enterprise.

Physics of the process and the danger of overheating

When the flow of electric current through the conductor is heating, the intensity of which directly depends on the resistance of the material and the area of its cross-section. When you connect thick and thin wires, the current is forced to move from the low-resistance zone to the high-resistance zone, causing a local temperature rise at the transition site. If the cross-section of a thin wire is, for example, 1.5 mm2, and a thick wire is 4 mm2, then with a load permissible for a thick cable, the thin section will instantly overheat, since its throughput is much lower.

A critically important parameter is the current density, which for copper wires in closed wiring should not exceed certain values, usually about 10 Amps per 1 mm2 for short-term loads, but in long-term mode these figures are lower. Overheating in the connection of heterogeneous cross-sectional veins leads to softening of the insulation, its cracking and eventual short circuit. It is especially dangerous if the connection is made by the method of twisting, where the contact area can be unstable, and oxide processes are accelerated under the influence of high temperature.

⚠️ Attention: Never rely on an automatic switch to protect a thin wire if it is matched by a thick cable cross section. The machine will work only with a short circuit current or a significant overload of the line, but the thin section can begin to warm up with currents that are working for a thick line, but for a thin one - already emergency.

The thermal expansion of metals also plays a negative role: during cyclic heating and cooling (on/off powerful devices), metals expand and contract at different intensity, which leads to a weakening of the contact in the twist or terminal. This phenomenon, known as “heat fatigue” of contact, increases the transient resistance, which in turn causes even stronger heating, closing the circle of destruction. That's why. straight-twisting The use of intermediate elements without the use of different diameters is considered a gross violation of electrical safety rules.

ISP regulatory requirements and standards

Regulatory documentation, in particular PUE 2.1.21, states that the connection, branching and termination of wires and cables must be carried out by means of pressing, welding, soldering or compressions (screw, bolt). In the same paragraph it is expressly stated that the connection of wires must be so that the resistance of the contact does not exceed the resistance of a single wire of the same length. When connecting wires of different cross-sections, this requirement becomes difficult to fulfill without the use of special adapters.

The standards also stipulate that the connection of copper and aluminum wires should be carried out only through bimetallic plates or with the use of special lubricants that exclude galvanic pair. If a difference in cross-section is added to this, the requirements for the quality of the contact are tightened. The use of improperly selected terminals, not designed for a specific range of sections, can lead to the fact that the thin vein will not be clamped properly, and the thick one will not enter the seat completely.

It is also important to consider the temperature of operation. The regulations require that at the maximum design load the temperature of the vein does not exceed the permissible for this type of insulation (usually 70 ° C for PVC). When connecting heterogeneous sections, the calculated load should be determined by lower-section in chains. Ignoring this rule when designing a shield or wiring around an apartment is a common mistake leading to fire-hazardous situations.

Methods of secure connection of wires

For reliable connection of wires of different cross-sections, electricians use several proven methods, each of which has its own advantages and scope. The most common and affordable way is to use screw terminals that allow you to pinch two veins of different diameters in separate sockets, providing their electrical contact through a metal plate. However, when choosing such a method, it is important to ensure that the thin wire is not squeezed to damage to the life, and the thick wire is reliably fixed.

A more professional approach is considered to be the use of connecting sleeves under pressurization. In this case, both wires are inserted into the sleeve, and a high-density inseparable connection is created with the help of press-ticks. For wires of different cross-sections, there are special sleeves with internal partitions or a stepped profile that prevent mixing of the veins and ensure a uniform distribution of pressure during pressing. This method is considered one of the most reliable for hidden wiring.

• 🔌 The terminal pads: Allows you to connect the veins of different diameters due to individual downturn screws, but require regular check of tightening.
• 🛠️ Cartridge-pressing: Creates a monolithic compound resistant to vibrations and thermal expansion, ideal for hidden installation.
• 🔩 Bolt connections: They are used for large cross-sections where wires are separated by washers, providing contact across a large surface area.
• 🧪 Ride with a transition: It is rarely used because of the labor-intensive nature, but allows you to create a connection with minimal transient resistance with the correct flux.

Welding wires, especially using a graphite electrode, also gives excellent results, creating a monolithic droplet at the end of the twist. However, when connecting wires of different sections, it is necessary to carefully select welding modes so as not to burn out the thin vein until the thick one has not yet warmed up. This method requires a high qualification of the performer and special equipment, so it is more often used in industrial installation.

Use of terminals and shells

Climbs are the most versatile solution for household electrical installation. Modern models such as Wago Series 221 or 2273, equipped with lever clips or spring mechanisms that adapt to the diameter of the inserted vein. This allows wires to be connected in a cross section from 0.5 to 4 mm2 in one case without loss of contact quality. It is important to choose terminals with a current load corresponding to the smallest conductor in the pair.

Cartridges for pressing require more accurate selection. If the wire cross-sections are very different, for example 2.5 mm2 and 6 mm2, a standard sleeve may not provide uniform compression. In such cases, composite sleeves are used or voids are filled in the sleeve with additional segments of the vein (tinting), so that the thin wire does not deform when pressed and the thick one is tightly pressed. The tool for pressing must have the appropriate matrices for each type of sleeves.

⚠️ Attention: When using screw terminals, be sure to use end tips (NSHVI) for multi-core wires. Direct clamping of the multicore vein with a screw leads to flattening and damage of individual wires, which reduces the cross-section of the conductor and worsens contact.

Particular attention should be paid to the material of contacts. To connect copper and aluminum wires, use terminals with brass contacts or a special coating that excludes electrochemical corrosion. The shells should also be selected according to the material: copper for copper, aluminum for aluminum, and bimetallic for docking dissimilar metals. Failure to comply with this rule will lead to rapid oxidation and failure of the node.

Folding and soldering: myths and reality

Screwing as a method of connection is still controversial, but in the context of wires of different sections, it is categorically not recommended without subsequent soldering or welding. A simple twist of a thick and thin wire cannot physically be dense: thin veins cannot evenly wrap a thick monolithic vein, there are voids where sparking and oxidation occur. Even if the twisting appears to be tight visually, thermal expansion will quickly weaken the contact.

Salting eliminates many of the drawbacks of twisting, filling the voids between the veins with solder and creating a monolithic structure. However, when soldering wires of different sections, there is a risk of overheating the thin wire with flux or solder, which can damage its insulation or change the properties of the metal. It is necessary to use low-temperature solders and active, but not aggressive fluxes, carefully controlling the temperature of the soldering iron stinger.

The technology of soldering heterogeneous compounds

For high-quality soldering, first scavenge both wires separately, then twist them, warm them up and add solder. It is important not to overheat the thin wire. Use soldering acid only for ferrous metals, for copper and aluminum you need special fluxes.

There is a myth that a high-quality twist of “a century ago” works still, so you can connect as you like. This is a misconception: old twists worked in conditions of less current loads and often had a margin of safety, which is now exhausted. Modern electrical appliances create pulsating and high currents that quickly detect any defects in the connections, especially in the cross-sectional areas.

Table of correspondence of cross-sections and current loads

When planning connections, it is important to focus on the permissible current loads for each section. Below is a table that helps determine the maximum current that can be passed through the connection, based on the parameters of the smallest wire in the circuit. Remember that the chain is limited by its weakest link.

Wire cross-section (mm2)	Materials	Max. open-current (A)	Max. pipe-current	Recommended machine
1.5	Copper.	19	15	10 A
2.5	Copper.	27	21	16 A
4.0	Copper.	38	29	25 A
6.0	Copper.	46	36	32 A
10.0	Copper.	70	50	40-50 A

From the table it can be seen that if you connect a wire of 1.5 mm2 with a wire of 4.0 mm2, then you need to protect this line with a 10 Amp automatic, despite the fact that a thick wire can withstand much more. Installation of the machine on 25 Amps (over a thick wire) will lead to the fact that the thin section will burn long before the protection is triggered. Calculation of protection It is always done at the minimum section in the chain.

Frequent errors and ways to fix them

One of the most common mistakes is to try to twist wires of different diameters “in one pile” without aligning the ends. This leads to the fact that a thin wire simply presses against the surface of the thick, not providing full contact throughout the area. This can only be eliminated by reworking the node using a sleeve or terminal, where each wire is clamped individually.

Another mistake is the use of insulation to isolate the connection points of dissimilar wires without prior sealing. The insulation dries up over time, and in the cross-sectional transition areas due to different thermal expansion, microslits can form, where moisture and air enter, causing corrosion. It is recommended to use shrink tubes with adhesive layer, which when heated tightly compress the connection of any shape.

• ❌ Ignoring the sweep: Poorly cleaned oxidized wires create high transient resistance, which is critical for heterogeneous compounds.
• ❌ Wrong choice of machine: The protection is picked up over a thick wire, leaving the thin one unattended.
• ❌ A copper aluminium screw: Direct contact of these metals without an intermediate layer causes rapid electrochemical corrosion.

Error elimination requires dismantling the faulty node and re-performing the work in compliance with all technological maps. Don’t skimp on materials: a quality terminal or sleeve costs a penny compared to the consequences of a fire or failure of expensive household appliances. Regular audits of the power grid, especially in older homes, help identify such problem areas before an emergency occurs.

Can I twist copper and aluminum wires of different sections?

Direct twisting of copper and aluminum is prohibited due to galvanic corrosion. To connect heterogeneous metals and different sections, use special transition terminals with paste or bimetallic sleeves. This is the only safe way to avoid oxidation and heating.

What machine to put on the connection of 1.5 mm2 and 4 mm2?

The circuit breaker shall be designed for the minimum cross-section in the circuit. For 1.5 mm2 of copper wire, the maximum nominal value of the machine is 10 Amps. The use of a 16A or 25A machine (over a wire of 4 mm2) is unacceptable, since a thin wire will burn up when overloaded.

Do I need to snooze the wires before connecting in the terminal?

In modern screw and spring terminals, tinting of multi-core wires is not recommended, since solder can "float" under the screw pressure, and the contact will weaken. It is better to use NSHVI tips. There is no need for a monoveal.

What to do if the connection is warm?

If you notice heat, immediately de-energize the line. Most likely, the contact has weakened or the wire cross-section does not correspond to the load. Disassemble the connection, clean the veins, replace the terminal or sleeve with more suitable ones and check the compliance of the machine with the section of the smallest wire.

Can I increase the wire by a section less than the main one?

To increase the wire with a smaller cross section is possible only if the load on this section is guaranteed not to exceed the permissible for a thin wire. For example, to connect a low-power light. If the line is supposed to include powerful devices, the build-up of a smaller cross section is prohibited.