Cable Cross-Section per Circuit Breaker

The protective device protects the wiring, not the connected appliance. A breaker that is too large for the wiring can allow the wiring to heat for too long during an overload. But choosing correctly involves more than looking up a size: the load, the cable and the installation conditions must be assessed together. This guide explains what AREI Book 1 V06 Table 4.11 does establish and what a complete design still has to check.

Reading Table 4.11 in the other direction

Table 4.11 in subsection 4.4.1.4 gives, for domestic installations, the maximum rated current of a fuse and a circuit breaker for each conductor cross-section. The inverse overview below is therefore a legal table ceiling for the cross-section, not a universal guarantee that a cable of that size is adequately designed in every situation.

The 2.5 mm² detail matters: Table 4.11 gives 16 A as the maximum rated current for a fuse and 20 A for a circuit breaker. That does not automatically make a 20 A breaker correct on every 2.5 mm² cable. Installation method, load, voltage drop, short-circuit conditions and cable type still need checking. For a socket circuit, the minimum cross-section from 5.2.1.2 still applies even if a lower-rated protective device is selected.

The complete Table 4.11

The two protection columns are not interchangeable. Replacing a 16 A fuse with a 20 A breaker on 2.5 mm² increases the permitted rated current and therefore calls for a new assessment; the table only places both maximum values next to each other.

Why the table is not enough

A correct design starts with the design current Ib, meaning the current expected from the load. The rated current In of the protective device and the permissible current-carrying capacity Iz of the wiring are then selected. A common check is Ib ≤ In ≤ Iz, together with the required protective-device characteristics. Iz is not a fixed value determined by cross-section alone.

The assessment must include, among other things:

• conductor material, insulation and cable construction;
• installation method (in conduit, in thermal insulation, on a wall, in a bundle or on a cable tray);
• the number of simultaneously loaded conductors and grouping factors;
• ambient temperature, heat dissipation and external influences;
• length, load and permissible voltage drop;
• disconnection conditions during a short circuit and fault-loop impedance;
• the protective device’s breaking capacity compared with the prospective short-circuit current.

The protection must therefore be compatible with the cross-section thermally and with the real circuit. A long cable run may need a larger cross-section to control voltage drop. An unfavourable installation method can reduce Iz. A breaker with an acceptable rated current can still be unsuitable if the fault loop does not permit the required automatic disconnection or if its breaking capacity is too low for the prospective fault current.

Table 5.1: limited exceptions below 2.5 mm²

Subsection 5.2.1.2 generally prohibits insulated conductors smaller than 2.5 mm² when they are not an integral part of a machine or appliance. Table 5.1 lists the exceptions:

The 1 mm² row in Table 4.11 is therefore not, by itself, permission to install new fixed 1 mm² wiring in a home. Table 4.11 sets the maximum protective rating when another applicable rule permits that cross-section; Table 5.1 defines the field of use here.

Existing 1 mm² EMCB and EMCVB wiring

Part 8 matters for existing sections of an installation. For installations whose on-site execution began before 17 May 1986, the derogation can allow an EMCB or EMCVB 1 mm² wiring system to remain in service when it forms part of a circuit without socket outlets, subject to the conditions of that derogation. Its protective device must be adapted to the cross-section. This is an allowance for existing wiring, not a rule permitting 1 mm² in a new installation. The actual condition, date and available documentation must be established during an inspection.

Voltage drop: 3% and 5% are not AREI limits

For a preliminary check, designers often use 3% for lighting and 5% for other final circuits. These are useful design or calculation assumptions, but they are not percentages that AREI 5.2.5 sets as general hard limits. That provision refers to the rules of the trade. Present 3% and 5% explicitly as a non-binding preliminary check. A long run, a motor, an EV charging point or sensitive equipment may require a larger cross-section after the actual conditions are calculated.

Replacing an old fuse

On an existing circuit, first identify the wiring actually installed, its installation method, the expected load and any derogations that apply to its construction date. The Table 4.11 columns come afterwards. For 2.5 mm², 16 A for a fuse and 20 A for a breaker are the table ceilings; they are not instructions to increase the protective rating. For 1.5 mm², the breaker ceiling is 16 A, while a socket circuit remains subject to the applicable minimum cross-section.

Design and document

With PlanElec, you can record cross-sections, protective devices, circuits and lengths in the editor, export a PDF and run an orientational self-check. The tool helps organise data and open points. It is not a complete cable calculation, an AREI compliance certificate or a replacement for measurements or an official inspection.

Further reading

• Voltage drop and cable sizing
• Electrical inspection in Belgium
• Create a single-line diagram