Why choosing a machine for 4 mm² wire is not just a formality

Choosing a circuit breaker for a cable with a cross-section of 4 mm² is a task that at first glance seems simple: look at the table, select the rating and you're done. But in practice, errors here lead to two critical consequences: overheating of wiring (if the machine is too “powerful”) or false positives (if weak). For example, in a garage with a welding machine or in a house with an electric stove, an incorrectly selected machine can cause a fire or constant power outages.

In this article we will analyze not only standard correspondence tables, but also hidden nuances: how the wire material (copper vs aluminum), line length, load type (inductive or active) and even ambient temperature are affected. You will find out why for same cable 4 mm² in different conditions, automatic machines may be required 25A, 32A or even 40A — and how not to make a mistake with your choice.

Spoiler: for most household networks with 4 mm² copper wire, a 25A (B25) circuit breaker is optimal, but this rule does not always work - read on to understand why.

Correspondence table: machine for 4 mm² wire according to PUE

Let's start with the official data. According to Electrical Installation Rules (PUE), cable cross-section and machine rating must correspond continuous permissible current. For a 4 mm² wire these values are:

Wire material Section, mm² Continuous current, A Recommended automatic, A Machine type
Copper 4 38 25 B or C
Aluminum 4 28 16 B or C
Copper (lining in pipe) 4 32 20 B
Copper (open gasket) 4 46 32 C

Please note: the table gives basic recommendations, but does not take into account:

  • 🔹 Ambient temperature (in a hot garage or bathhouse, the current load is reduced by 10–15%).
  • 🔹 Line length (if the cable is longer than 30 meters, the voltage drop requires increasing the cross-section or reducing the rating of the machine).
  • 🔹 Load type (for example, for engines or welding machines, machines with the characteristic D, not C).

A practical example: in a country house with a copper wire of 4 mm² and a line length of 50 meters to the barn, a 25A automatic circuit breaker can lead to a voltage drop of up to 180V when the pump is turned on. The solution is to either reduce the rating to 20A, or increase the cross-section to 6 mm².

📊 What kind of wire do you have in your house?
Copper 4 mm²
Aluminum 4 mm²
Copper, but I don’t know the cross-section
Another option

Copper vs aluminum: why different machines are needed for the same section

Aluminum wire 4mm² withstand 30% less currentthan copper of the same cross-section. This is due to the physical properties of metals:

  • 🔸 Aluminum resistivity 1.6 times higher than copper - it heats up more.
  • 🔸 Thermal conductivity of aluminum lower, so heat is dissipated less well.
  • 🔸 Oxidation: Aluminum forms a film with high resistance, which further increases heating.

Therefore, for an aluminum wire of 4 mm², the maximum machine is 16A (type B), and for copper - 25A (type C). But there are exceptions:

When can you install a 20A machine for aluminum?

If the wire is laid openly (not in the wall) and the room temperature does not exceed +25°C, a 20A circuit breaker with the characteristic B. However, this option requires regular checking of connections - aluminum “flows” over time, and the contacts weaken.

Important! In old houses (built before 2000), 4 mm² aluminum with 25A or even 32A circuit breakers is often found. This gross violation, leading to melting of the insulation. If this is the case, urgently replace the machine with a 16A one or re-wire the wiring with copper.

Cable length and voltage drop: when 4 mm² requires a 20A circuit breaker instead of 25A

The longer the cable, the greater its resistance and the greater the voltage drop. For a 4 mm² wire, the critical length is - 30–50 meters (depending on load). For example:

  • 📏 10 meters: Voltage drop up to 1% (normal).
  • 📏 30 meters: up to 3–5% (lights flickering noticeably).
  • 📏 50 meters: up to 8–10% (equipment may not start).

If your line is longer than 30 meters, use the formula to calculate the voltage drop:

ΔU = (2  ρ  L * I) / S

where:

  • ΔU — voltage drop, V;
  • ρ — resistivity (0.0175 for copper, 0.028 for aluminum);
  • L — cable length, m;
  • I — load current, A;
  • S — cross-section, mm².

Example: for a 4 mm² copper wire with a length of 40 meters and a load of 25 A, the voltage drop will be ~7V (at 220V this is 3.2%). To meet the standard (maximum 5%), you will have to either reduce the load (set the machine to 20A) or increase the cross-section to 6 mm².

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If you don’t want to count manually, use online voltage drop calculators, for example, on the websites Cable Calculator or Electrician Info.

Type of machine (B, C, D): what to choose for a 4 mm² wire

The characteristics of the machine determine at what excess current it will operate. For household networks, three types are used:

  • 🔌 Type B: triggered when 3–5 times excess current (for example, a 25A circuit breaker will turn off at 75–125A). Suitable for lighting and sockets with active load (kettles, heaters).
  • 🔌 Type C: triggered when 5–10 times exceeding (for 25A this is 125–250A). Optimal for refrigerators, washing machines, air conditioners (inductive load).
  • 🔌 Type D: triggered when 10–20x exceeding Needed for powerful motors, welding machines, pumps.

For 4 mm² wire, in most cases choose type C, as it covers 90% of household appliances. Exceptions:

  • 💡 If the line goes only for lighting - type B.
  • 🔧 If you connect garage equipment (compressor, lathe) - type D.

A mistake made by many electricians: put on a 4 mm² wire automatic type D "just in case." This is dangerous because in the event of a short circuit, such a machine may not operate in time and the wire will overheat.

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For most household networks with a 4 mm² wire, an automatic machine is optimal 25A type C. Exceptions are aluminum wire (16A) or lines >30 meters long (20A).

Top 5 mistakes when choosing a machine for 4 mm² wire

Even experienced electricians sometimes make mistakes. Here are the most common:

⚠️ Attention! If you set the circuit breaker to 32A for a 4mm² copper wire “because it holds 40A”, remember: continuous current for 4 mm² - 38A. With a load of 32A, the wire will heat up and the insulation will age 2-3 times faster.
  1. Ignoring temperature. In a sauna or boiler room, a 25A automatic circuit breaker for 4 mm² must be replaced with a 20A one due to additional heating.
  2. Ignoring starting currents. For example, a refrigerator consumes 5–7 times its nominal value when starting up. Automatic type B may cause false alarms.
  3. Mixing copper and aluminum wires. If part of the line is aluminum and part is copper, the entire calculation is carried out using aluminum (as the weaker link).
  4. Savings on brand. Cheap machines (for example, IEK or EKF) can have a spread of characteristics up to ±20%. For responsible lines take ABB, Schneider Electric or Legrand.
  5. Unverified connections. Poor contact in the terminal block or twist creates additional resistance and heating. For 4 mm² use sleeves for crimping or Wago 222.

An example from practice: in a private house, after replacing the machine with 32A (instead of 25A), a month later the socket caught fire. The reason is a loose contact in the junction box, which began to heat up under increased load.

✅ The wire cross-section was confirmed with a caliper (not according to the markings!).

✅ The length of the line to the load does not exceed 30 m (or the voltage drop is calculated).

✅ The type of machine (B/C/D) corresponds to the nature of the load.

✅ The room temperature does not exceed +30°C (otherwise the nominal value is reduced by 10%).

✅ The connections are made without twists (terminal blocks or sleeves are used).

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Practical examples: which machine for 4 mm² in different situations

Let's look at real cases with solutions:

Situation Wire material/length Load Recommended machine Notes
Kitchen (sockets for kettle, microwave) Copper, 4 mm², 15 m 5 kW (23A) 25A, type C The best option for active loads.
Garage (welding machine 220V) Copper, 4 mm², 25 m 7 kW (32A) 32A, type D Take into account the voltage drop! For lengths >25 m, 6 mm² is better.
Country house (old aluminum wiring) Aluminium, 4 mm², 20 m 3 kW (14A) 16A, type B Replace with copper as soon as possible!
Cottage (heating by electric boiler 6 kW) Copper, 4 mm², 10 m 6 kW (27A) 32A, type C Borderline case - better than 6 mm².

Special Case: 4 mm² wire in automotive electrical (for example, to power a 220V inverter). Here the rules are different:

  • 🚗 Use only flexible copper wire with silicone insulation (for example, PVA 4 mm²).
  • 🚗 Choose an automatic machine at 20–25A, but definitely with thermal protection (for example, ANL fuse).
  • 🚗 Take into account impulse currents (when starting the inverter, they can reach 50–100A).

FAQ: Frequently asked questions about automatic machines for 4 mm² wire

Is it possible to install a 40A circuit breaker for a 4 mm² wire if the load is small?

No! The machine protects wire, not the load. Even if you connect a 100W light bulb, a 40A breaker will not turn off if the wire is overloaded (for example, if there is a short circuit somewhere in the line). For copper 4 mm² maximum - 32A (and then with an open laying and a short line).

Which machine should be installed on the input line if there is a 4 mm² wire after it?

The input machine must be one step higherthan group. For example, if there is a 25A circuit breaker on a line with a 4 mm² wire, then the input can be 32A or 40A (depending on the total load). But be sure to check that input cable cross-section corresponded to the input machine (usually 10–16 mm²).

What happens if you install a 16A circuit breaker for a 4 mm² copper wire?

Nothing dangerous, but inconvenient: the machine will false alarm at a load above 3.5 kW (for example, when turning on a kettle + microwave). For copper 4 mm² 16A is an underload, but if the load is small (lighting, TV), then it is acceptable.

Is it necessary to reduce the rating of the machine if a 4 mm² wire is laid in a corrugated area?

Yes, if corrugated tightly packed with other cables. In this case, heat dissipation deteriorates, and it is better to take the machine to one step down (for example, 20A instead of 25A). If the corrugation is loose or there is only one cable, you can leave the standard rating.

Which machine should I put on a 4 mm² wire for an electric stove?

For an electric stove with a power of up to 7 kW (32A) and a copper wire of 4 mm², you need an automatic 32A type C. But keep in mind:

  • 🔥 If the line length is >10 m, it is better to take a wire 6 mm².
  • 🔥 For stoves with a power >7 kW (e.g. 8–10 kW) it is required 40A and 6 mm².