A correctly selected conductor cross-section is the foundation of a safe and durable electrical system. A cable that is too thin overheats under load, damages the insulation and causes short circuits or fire. One that is too thick simply wastes money. In this article, the specialists of TD Odeskabel explain how to calculate cable cross-section by power and current, and provide reference tables for copper and aluminium conductors.
Why cross-section matters
Every conductor has a limit of continuously permissible current. When the actual load exceeds that limit, the conductor overheats. The consequences are accelerated ageing of the insulation, melting, tripping of protection devices or even ignition. That is why the cross-section is always chosen with a margin over the expected load rather than “just enough”.
Two calculation methods
The cross-section is determined in two ways — by current and by power — after which the larger of the two results is chosen. In practice the calculation comes down to two steps: first you work out the operating current of the line, then you select the matching cross-section from a table.
Step 1. Determine the operating current
- Single-phase 220 V: I = P / (U × cosφ)
- Three-phase 380 V: I = P / (√3 × U × cosφ)
Here I is current (A), P is power (W), U is voltage (V), and cosφ is the power factor. For household appliances with a resistive load (heaters, cookers, lighting) cosφ is taken close to 0.95. For motors, pumps and welding machines it is lower — 0.8 or less — so the current is higher. For example, for a 2 kW kettle on a 220 V network: I = 2000 / (220 × 0.95) ≈ 9.6 A.
Step 2. Select the cross-section from a table
Knowing the current, find the nearest larger cross-section in the table. The values below correspond to the Electrical Installations Regulations for open laying; the approximate power is calculated for cosφ = 0.95.
Copper cables
| Cross-section, mm² | Permissible current, A | Power 220 V, kW | Power 380 V, kW |
|---|---|---|---|
| 1.5 | 23 | 4.8 | 14 |
| 2.5 | 30 | 6.3 | 18 |
| 4 | 41 | 8.6 | 25 |
| 6 | 50 | 10.5 | 31 |
| 10 | 80 | 16.7 | 50 |
| 16 | 100 | 20.9 | 62 |
Aluminium cables
| Cross-section, mm² | Permissible current, A | Power 220 V, kW | Power 380 V, kW |
|---|---|---|---|
| 2.5 | 21 | 4.4 | 13 |
| 4 | 28 | 5.9 | 17 |
| 6 | 36 | 7.5 | 22 |
| 10 | 52 | 10.9 | 32 |
| 16 | 64 | 13.4 | 40 |
| 25 | 84 | 17.6 | 52 |
For the same cross-section, a copper conductor carries roughly a third more current than an aluminium one. Roughly speaking, 2.5 mm² copper and 4 mm² aluminium withstand a similar load.
What else affects the choice
- Laying method. An open cable is cooled by air better than one laid in a pipe or wall. For concealed installation the permissible current is reduced by roughly 20–30%.
- Line length. On long runs the voltage drop increases, so the cross-section has to be enlarged to keep the voltage at the load within limits.
- Power margin. Add 15–20% to the calculated load for future needs and inrush currents.
- Number of conductors in a bundle. The more cables laid together, the worse they are cooled — this also lowers the permissible current.
Typical solutions for household wiring
- Lighting — 1.5 mm² copper;
- Socket groups — 2.5 mm² copper (with a 16 A breaker);
- High-power appliances (cooker, boiler, air conditioner) — 4–6 mm² copper on a dedicated line;
- House / apartment input — depending on the allocated power, most often 6–10 mm² copper.
Note: under current standards, indoor wiring in new residential buildings is made with copper cable. Aluminium is used mainly for inputs and external lines with a cross-section from 16 mm².
Conclusion
Choosing the cross-section is a balance between safety, reliability and cost. Calculate the current from the power, select the cross-section from the table taking into account the laying method and line length, and add a margin. If you need to accurately select the cable type and cross-section for your project, contact the specialists of TD Odeskabel — we will help with the calculation and choose products for your specific task.
