Breaker rating vs cable size
The breaker rating must match the cable size and the circuit.
Uitleg
AREI Book 1 requires the rated current of the protective device \(In\) to be no greater than the admissible current of the conductor \(Iz\). If In is higher than Iz, the cable can overheat before the protection reacts.
In plain language: the copper conductor determines how much current the cable can safely carry. The breaker protects the cable against overload and must therefore not be rated higher than the cable can safely handle. AREI sets maximum values for this.
What this looks like in practice
For household installations, Table 4.11 gives the following maximum rated currents for circuit breakers:
| Cable cross-section | Maximum breaker rating |
|---|---|
| 1.5 mm² | 16 A |
| 2.5 mm² | 20 A |
| 4 mm² | 25 A |
| 6 mm² | 40 A |
| 10 mm² | 63 A |
These are maximum values, not automatic approval for every situation. The correct rating also depends on the installation method, length, ambient temperature, load and the rest of the installation.
A lower breaker rating is possible if the connected load and the circuit requirements allow it. For example, a 10 A breaker can protect a 2.5 mm² cable. However, for a new circuit with ordinary socket outlets, 2.5 mm² is in principle the minimum cross-section. Using 1.5 mm² with a 10 A breaker is therefore not automatically allowed for that purpose. A 1.5 mm² conductor is, for example, commonly used for a circuit without socket outlets, such as a lighting-only circuit.
The value in the table refers to the cross-section of the copper conductor, not the outside diameter of the complete cable and its insulation. The cable cross-section is often printed on the cable. Otherwise, you must determine the cross-section of a separated, bare conductor; the cable's outside diameter is not a reliable measurement for this.
Typical mistakes:
- A circuit intended for lighting in 1.5 mm² has an ordinary socket outlet added to it and the breaker is increased to 20 A. Both the breaker and the circuit application may then be wrong.
- A 32 A breaker on a long 2.5 mm² cable run, for example to an outbuilding, without considering the installation method and voltage drop.
On the one-line diagram, this appears as a circuit whose breaker rating is clearly higher than what you would normally associate with that cable size.
Useful in Condui
Condui compares the cable cross-section with the breaker rating and reports when they do not match.
Why AREI cares about this
The overcurrent device protects the cable first and foremost, not just the appliances. If a cable has to carry more current than it was designed for:
- its insulation ages faster,
- conductors can overheat inside walls or insulation,
- and in the worst case this creates a fire risk.
Correctly matching the breaker and cable cross-section is one of the most fundamental safety rules in AREI.
Can you use 230 V on a thin cable if the breaker is small?
A low breaker rating does not automatically make a cable suitable for 230 V. A signal cable, such as an SVV cable, is not automatically designed for the voltage, insulation requirements and installation conditions of a 230 V installation.
Do not use such a cable for 230 V without checking this, even with a breaker as low as 1 A. Use a cable rated for 230 V and also check the minimum cross-section and requirements for the circuit concerned.
How to fix it in a real installation
You realistically have two options:
- Choose a breaker whose rated current In is at or below the admissible current for the existing cable cross-section.
- Example: change a 25 A breaker on 1.5 mm² to 16 A, if 1.5 mm² is permitted for that circuit. For a new circuit with ordinary socket outlets, you must also increase the cable cross-section.
- Replace the cable along the entire circuit with a larger cross-section that safely matches the desired breaker rating.
- Example: keep a 25 A breaker, but rewire the circuit in 4 mm².
Mixing cable cross-sections on a single circuit is only acceptable if you treat the smallest cross-section as the reference for protection. In other words: the weakest section determines the maximum breaker rating.
| Solution | When it makes sense | What to check |
|---|---|---|
| Reduce the breaker rating | The existing cable remains in place | The connected load must still fit the lower protection |
| Increase the cable cross-section | You genuinely need the higher power | The entire run must match the new cross-section |
| Split the circuit | Too many or too-heavy loads share one circuit | Update the diagram, distribution board and situation plan |
How to fix it in Condui
- lower the breaker rating to a level that matches the current cross-section, or
- increase the cross-section to a value that matches how you intend to build the installation.
Re-run validation afterwards. The “breaker vs cable size” error should disappear once In and the cable cross-section are consistent with the AREI table.