Free Online Calculators
Cable size from current, length & voltage drop – 230/400 V and DC
A cable must pass two checks: it carries the breaker current (IEC 60364-5-52), and the voltage drop over its length stays within the limit, typically 3%. The calculator runs both for copper at 230/400 V or 12/24/48 V DC and converts conductor diameter to mm².
For 16 A over 20 m at 230 V you need 2.5 mm²: 1.5 mm² carries the current, but the voltage drop would be 3.3% instead of the usual 3% maximum. The table covers 10 to 63 A and 10 to 100 m; the calculator also handles three-phase, 12/24/48 V DC and the installation method.
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| Breaker | 10 m | 20 m | 30 m | 50 m | 100 m |
|---|---|---|---|---|---|
| 10 A | 1.5 mm² | 1.5 mm² | 2.5 mm² | 4 mm² | 6 mm² |
| 13 A | 1.5 mm² | 1.5 mm² | 2.5 mm² | 4 mm² | 10 mm² |
| 16 A | 1.5 mm² | 2.5 mm² | 2.5 mm² | 6 mm² | 10 mm² |
| 20 A | 2.5 mm² | 2.5 mm² | 4 mm² | 6 mm² | 16 mm² |
| 25 A | 4 mm² | 4 mm² | 4 mm² | 10 mm² | 16 mm² |
| 32 A | 6 mm² | 6 mm² | 6 mm² | 10 mm² | 25 mm² |
| 40 A | 10 mm² | 10 mm² | 10 mm² | 16 mm² | 25 mm² |
| 50 A | 10 mm² | 10 mm² | 10 mm² | 16 mm² | 35 mm² |
| 63 A | 16 mm² | 16 mm² | 16 mm² | 25 mm² | 35 mm² |
6 mm² at 230 V, for example for a 7.4 kW EV charger: 4 mm² is rated only 30 A in conduit, and on 6 mm² the voltage drop is 2.5%. Beyond about 36 m you need 10 mm².
At 16 A and 230 V, 11.4 V or 5.0% — above the usual 3% limit. 4 mm² gives 3.1%, 6 mm² 2.1%.
Single-phase at 230 V: 43.5 A, so a 50 A breaker and 10 mm². Three-phase at 400 V: 14.4 A per phase, 16 A and 2.5 mm².
Two checks: the cable's current rating must exceed the breaker rating, and the voltage drop ΔU = 2 × L × I / (56 × A) must stay within 3%. Use whichever cross-section is larger.
4 mm²: at 12 V a 3% drop is only 0.36 V, so length, not current rating, sets the size.
A solid 2.5 mm² conductor is 1.78 mm across; 1.5 mm² is 1.38 mm (d = √(4 × A / π)).
Choosing the right cable cross-section is one of the most critical decisions in any electrical installation. Whether you are wiring a new home, installing a solar power system, or upgrading your workshop, an undersized cable can lead to dangerous overheating, excessive voltage drop, and even fire. Our free cable cross-section calculator helps you determine the correct cable size in mm² in seconds – no engineering degree required.
Every electrical conductor has resistance. The thinner the cable, the higher the resistance – and the more energy is lost as heat. This energy loss is expressed as voltage drop, which reduces the performance of connected devices, while a cable carrying more current than it is rated for overheats and becomes a serious fire hazard. International standards such as IEC 60364 and national regulations such as BS 7671 set limits for voltage drop in a circuit (typically 3–5%).
Conversely, using an excessively thick cable is wasteful and expensive. The goal is to find the smallest cross-section that safely carries the required current with acceptable voltage drop – and that is exactly what this calculator does.
The tool checks two conditions. First, current rating: the rating Iz of the cable from IEC 60364-5-52 for your installation method must be at least the breaker rating (Ib ≤ In ≤ Iz). Second, voltage drop: A = (2 × L × I × cos φ) / (σ × ΔU), where A is the cross-section in mm², L is the one-way cable length in metres, I is the current in amperes, σ (sigma) = 56 is the conductivity of copper, and ΔU is the permissible voltage drop in volts (√3 instead of 2 for three-phase). The recommended size is the larger of the two, chosen from the standard sizes 1.5 to 50 mm².
The calculator shows the current and the breaker rating, the size by current rating and the size by voltage drop, and recommends the larger of the two as a metric size (e.g., 1.5 mm², 2.5 mm², 4 mm², 6 mm², 10 mm², 16 mm² – not AWG). It also gives the actual voltage drop in % and volts, the maximum length for that size, and whether current rating or length was decisive.
A radial socket circuit in a living room is protected at 16 A and runs 25 metres at 230 V AC. With installation method B2 and a 3% voltage drop limit, 1.5 mm² would carry the current but the drop would reach 4.1%, so the calculator recommends 2.5 mm² (2.5% drop) – here the length decides, not the current.
A home EV charger drawing 32 A (7,360 W, single phase, 230 V) installed 30 metres from the consumer unit: with a 3% voltage drop limit and method B2, the result is 6 mm² with a 2.5% drop – 4 mm² would already reach 3.7%. Much larger than many homeowners might assume.
Low-voltage DC systems are extremely sensitive to voltage drop. A 12 V refrigerator drawing 10 A over a 4-metre run needs at least a 4 mm² copper cable to stay within 3% drop. At 12 V, even a small resistance causes a disproportionately large voltage loss.
Even experienced installers sometimes make calculation errors. Watch out for these common pitfalls:
For a 20 A circuit at 230 V with a cable run of up to 20 metres, a 2.5 mm² copper cable is typically sufficient (method B2, 3% limit, good for about 24 m). At 30 m the calculator recommends 4 mm², and in a thermally insulated wall (method A2) 2.5 mm² is not enough for 20 A. For other runs or loads, recalculate with the actual parameters.
IEC 60364-5-52 recommends a maximum of 3% for lighting circuits and 5% for other loads in installations supplied from the public low-voltage network; BS 7671 uses the same values. Some countries or applications may have stricter requirements. The calculator starts at 3%, and you can change the limit.
Copper is the standard choice for residential wiring due to its superior conductivity, flexibility, and corrosion resistance. Aluminium is used in large-diameter supply cables because it is lighter and less expensive, but requires special connectors and care. This calculator covers copper conductors only.
For 12 V, 24 V or 48 V DC systems, yes: select the DC voltage, enter the maximum current and the cable length, and the calculator gives the cross-section for your battery or off-grid wiring. Higher PV string voltages are not among the supply options, so size string cables from the manufacturer's data.
Yes. It checks both the current rating to IEC 60364-5-52 (copper, PVC insulation, 30 °C, not grouped) for your installation method and the voltage drop, and recommends the larger size. Grouping and other ambient temperatures are not included, so apply those correction factors yourself where needed.