A cable's cross-section determines how much current it can carry without dangerous overheating. A wire that's too thin for a heavy-duty circuit is a fire risk; one that's too thick means wasted money and awkward installation. Calculating cross-section correctly isn't a guessing game — it's based on the actual load of a specific circuit.
Below is a practical calculation method, a table of typical values for copper cable, and a look at the mistakes people most often make when sizing wire on their own.
What determines cable cross-section
The main factor is the current the wire has to carry without overheating, which is directly tied to the combined power of the appliances connected to it. Cross-section is also affected by the conductor material (copper carries more current than aluminum at the same cross-section), the installation method (embedded in a wall, in conduit, or exposed), and the circuit length — on longer runs you also need to account for voltage drop, not just heating.
How to calculate the cross-section
First, add up the total wattage of the appliances on the circuit, convert it to current using I = P / U (at 220 V), then pick a copper wire cross-section that can carry that current with a margin. For example, a circuit with a combined load of around 3.5 kW (roughly 16 A) is typically covered by a 2.5 mm² cable. Margin is factored in deliberately — cross-section is chosen not to exactly match today's load but to allow for appliances likely to be added later.
Cross-section table for copper cable
Below are approximate values for copper wire used as a starting point. The final choice should always be checked against the installation method and circuit length.
| Cross-section, mm² | Rated current | Typical load | Use |
|---|---|---|---|
| 1.5 | ~16 A | up to 3 kW | lighting |
| 2.5 | ~21-25 A | up to 4.5 kW | sockets |
| 4 | ~27-30 A | up to 6 kW | kitchen sockets, underfloor heating |
| 6 | ~34-40 A | up to 7-8 kW | electric stove |
| 10 | ~46-50 A | up to 10 kW | main supply line, high-power water heater |
Common mistakes when choosing cross-section
- Eyeballing it based on the thickness of old wire without verifying its actual cross-section.
- Ignoring the installation method — cable in conduit inside an insulated wall runs hotter than exposed wire, and that needs to be reflected in the margin.
- Using one cross-section for every circuit in the apartment instead of sizing each group separately.
- Mismatching wire cross-section and breaker rating — the breaker should protect that specific cable, not just be picked 'with margin' independently of the wire calculation.
When to leave the calculation to an electrician
A DIY calculation is fine for getting a sense of the numbers involved, but for circuits with heavy appliances — an electric stove, water heater, air conditioner, underfloor heating — and for a full apartment rewiring job, it's worth handing the calculation to an electrician. A specialist accounts not just for load but also run length, installation method, ambient temperature at the installation site, and compatibility with the breaker in the panel. ELECTRIC ASTANA provides consultations and wiring installation 24/7, around the clock; wiring installation/replacement starts from 600 KZT per meter, with the exact cost set after an inspection.