The cable cross-section determines the safety, function and efficiency of any electrical installation. If it is too small, the conductor overheats or a noticeable voltage drop occurs. In this guide you will learn how to determine the right cross-section. For the actual sizing, use our Cable Cross-Section Calculator, which takes all relevant factors into account.
The three deciding factors
Three factors essentially work together when choosing a cross-section:
- Current and fuse rating: How much current flows and which circuit breaker protects the cable?
- Installation method and grouping: Is the cable run free, in conduit, in thermal insulation or bundled with others?
- Cable length: Over long runs the voltage drop becomes the limiting factor, not heating.
Current-carrying capacity and protection
Each cross-section can carry only a certain current continuously without overheating. The protection must match the cross-section: the circuit breaker protects the cable by tripping on overload. The table below shows rough reference values for copper, installation method B2 and 230 V - for orientation only. Real capacity depends on installation method, grouping and ambient temperature (IEC/VDE 0100-520 / VDE 0298-4).
| Cross-section | typ. fuse rating | typical application |
|---|---|---|
| 1.5 mm² | up to ~16 A | lighting, sockets |
| 2.5 mm² | ~20-25 A | socket circuits |
| 4 mm² | ~25-32 A | heavier loads |
| 6 mm² | ~40 A | cooker, instant water heater |
| 10 mm² | ~50 A | large loads, supply lines |
Check the permissible capacity for your case with the Cable Ampacity Calculator (with Grouping Factors) and the matching fuse with the Circuit breaker calculator.
Do not underestimate installation method and grouping
Current-carrying capacity drops significantly when heat is not dissipated well. This happens with grouping of several cables, at high ambient temperature and with unfavourable installation methods, such as inside thermal insulation. In these cases derating factors must be applied, and often a larger cross-section is needed than the reference values suggest.
Common mistakes
- Several cables bundled together without applying a derating factor.
- Installation in insulated walls combined with high load.
- Fuse rated higher than permitted for the cross-section.
Cable length and voltage drop
Over long runs it is not heating but the voltage drop that matters. In final circuits it should not exceed 3 %. An excessive voltage drop causes power loss and can disturb sensitive equipment. Rule of thumb: the longer the run and the higher the current, the larger the cross-section must be. Calculate the value with the Voltage drop / cable length.
Summary and procedure
First determine current and fuse rating, then check installation method and grouping, and finally verify the voltage drop on long runs. For complete sizing, use the Cable dimensioning. When in doubt: work on fixed installations belongs in the hands of a qualified electrician.
