The consumer unit is where it shows most whether an installation is properly sized: a wrongly chosen circuit breaker can trip constantly or, worse, fail to protect the circuit when it really matters.
How many modules the unit needs
The number of modules depends on how many circuits the installation has: a standard home usually needs between 12 and 24 modules; larger installations or ones with home automation may need 36-48.
The circuit breaker must match the cable size
This is the rule that can't be skipped: the circuit breaker's amperage (MCB) has to match the cable size of the circuit it protects.
| Amperage | Cable size | Typical use |
|---|---|---|
| 10A | 1.5 mm² | Lighting |
| 16-20A | 2.5 mm² | Sockets |
| 25A | 4 mm² | Kitchen / oven |
| 32-40A | 6 mm² or more | Higher-power circuits |
A circuit breaker with more amperage than the cable can bear doesn't really protect the circuit: it can let through an overload the cable can't withstand before it trips.
The RCD: 30 mA is the norm for protecting people
The residual-current device protects people against earth leakage and electrical contact, cutting the supply on an earth fault. 30 mA is the mandatory sensitivity on domestic circuits. 300 mA is only used for fire protection in certain industrial installations — never as the sole protection of people.
How the ICP is calculated from the contracted power
The ICP (Power Control Switch) limits the power contracted with the supplier, and usually comes sealed by the distributor. It is calculated with the formula:
ICP amperage ≈ Contracted power (W) ÷ 230V
Example: for a contracted power of 5.75 kW: 5,750 ÷ 230 = 25 A. This is the same calculation the PresupuestoYa calculator applies automatically for the Electrician trade. Always verify the contracted power before sizing the rest of the installation — an insufficient ICP will trip constantly.
The surge protector, increasingly demanded
The SPD (surge protective device) protects the home's electronic equipment from transient overvoltages, typically from lightning on the grid. Increasingly demanded by insurers and recommended by the low-voltage regulations (ITC-BT-23), it is installed right after the ICP, before the rest of the protections.
Frequently asked questions
Can I fit a circuit breaker with more amperage than the table indicates? No: if it exceeds what the cable size can bear, the circuit can overload without the circuit breaker tripping in time.
What RCD sensitivity does a home need? 30 mA, the mandatory sensitivity for protecting people on domestic circuits.
How do I know what ICP amperage I need? Divide the contracted power in watts by 230V. For 5.75 kW contracted, the corresponding ICP is 25A.
Conclusion
A properly sized consumer unit isn't just "fitting protections": it's matching the amperage of each circuit breaker to the real size of the cable it protects, and calculating the ICP from the contracted power, not by eye. Automating this calculation avoids both a unit that trips constantly and one that doesn't protect enough.
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