What does 1.5 or 2.5 mm² mean?

The number printed on the cable, 1.5, 2.5 or 4, is the area of one conductor, not its diameter. Everything else follows from it.

The usual sizes in a flat

1.5 mm² carries lighting circuits, with a 10 A breaker in front of it.

2.5 mm² carries socket circuits, with 16 A.

4 mm² and above goes to a water heater, a cooker or an air conditioner, with 20 A or 25 A depending on the load.

These are indicative. The real figure depends on how the cable is installed, how long the run is and how warm it gets: a cable inside conduit in an insulated wall behaves quite differently from the same cable in open air.

Why the breaker follows the cable

This is the part people get backwards. The breaker is chosen for the cable, not for the appliance.

The cable is buried in a wall and cannot be inspected. If it carries more than it was sized for, it heats along its whole length, the insulation ages, and eventually it fails somewhere nobody can see. The breaker exists to stop that.

So fitting a larger breaker because one keeps tripping does not solve anything. It removes the only thing standing between an undersized cable and a short circuit.

Aluminium

In flats built before the eighties the wiring is often aluminium rather than copper. Aluminium carries less current for the same cross-section, so a 2.5 mm² aluminium cable is not the equal of a 2.5 mm² copper one.

It also creeps under pressure, so a joint tightened today loosens over the years and then heats up. Aluminium and copper are never joined directly but through a connector rated for it, and getting that wrong is the most common cause of an outlet that gets hot.

Long runs

Voltage drops over distance. For a supply at the bottom of a garden or the end of a long corridor a larger cross-section is sometimes chosen than the current alone would require, not because of heating but so the voltage at the far end is still usable.

All of it is set out on the drawing before anything is chased into a wall, which is part of a new installation.