CalcCable Cross-Section Calculator
Last updated: 2026-08-10
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| Circuit length (m) | Current (A) (A) | Max voltage drop (%) (%) | |
|---|---|---|---|
| Alumbrado (1.5 mm²) | 15 m | 8 A | 3 % |
| Enchufes dormitorio (2.5 mm²) | 20 m | 16 A | 3 % |
| kitchen/horno (4 mm²) | 25 m | 25 A | 5 % |
| Aire acondicionado (6 mm²) | 35 m | 32 A | 5 % |
| Derivación garaje (10 mm²) | 60 m | 40 A | 5 % |
Choosing the right cable size is a critical step for any electrical installation. An undersized wire can overheat, while an oversized one wastes money. Our Cable Cross-Section Calculator helps you instantly determine the minimum copper cable cross-section in mm² based on your circuit's current, cable length, and allowable voltage drop, following the guidelines of IEC 60364. This free tool requires no sign-up and gives you results the moment you enter your numbers, making it essential for electricians, engineers, and DIY enthusiasts working on safe and efficient wiring.
What This Calculator Does and When to Use It
The Cable Cross-Section Calculator is designed to solve a common electrical problem: finding the appropriate wire thickness to prevent excessive voltage loss over a distance. Voltage drop occurs because every cable has some electrical resistance; the longer the cable and the higher the current, the more voltage you lose. If the drop exceeds safe limits (typically 3% to 5% of the supply voltage), connected equipment may malfunction or operate inefficiently.
Use this calculator when planning any copper-wired circuit, including:
- Household lighting and power circuits (e.g., 230 V or 120 V systems).
- Solar panel or battery connections where distance matters.
- Low-voltage landscape lighting and security systems.
- Industrial machinery and control panels requiring exact sizing.
By entering four simple values—current in amperes, cable length in meters, supply voltage in volts, and the maximum allowed voltage drop as a percentage—you receive both the exact calculated cross-section and the nearest standard commercial size (e.g., 1.5, 2.5, 4 mm²). The calculator automatically uses copper's conductivity factor of 56 (S/m at 20°C) and adheres to IEC 60364's typical drop limit recommendations.
Formula Explained: Breaking Down Each Variable
The core of the tool uses a simple but powerful formula derived from Ohm's Law and the resistance of copper conductors:
Calculated cross-section (mm²) = (2 × Length × Current) / (Conductivity × Allowed Voltage Drop)
Let's examine each variable and what you need to enter:
- corriente_a (Current in Amperes): The total load current the cable must carry. For example, a 10 A circuit for several lamps.
- longitud_m (Length in meters): The one-way distance from the power source to the load. The formula multiplies it by 2 because the current must travel both to and from the load (a complete circuit).
- tension_v (Voltage in Volts): The system voltage. The default is 230 V (common in Europe and many other regions), but you can change it to 120 V, 24 V, or any value that matches your system.
- caida_max_pct (Maximum Voltage Drop in %): The acceptable voltage loss at the load. For example, 3% means the voltage at the load must not be less than 97% of the supply voltage. The tool defaults to 3%, which is typical for lighting circuits per IEC 60364.
- Conductivity (sigma = 56): This is a fixed value representing the electrical conductivity of copper in S·m/mm² at 20°C. It is not adjustable—it’s built into the formula for accurate results.
After computing the exact value, the tool rounds up to the next standard cable size (1.5, 2.5, 4, 6, 10, 16, 25, or 35 mm²) because you cannot buy a 3.2 mm² cable; you must choose the next available commercial option that meets or exceeds the calculated requirement.
Two Worked Examples with Concrete Numbers
Example 1: 230 V Household Circuit
You need to run a 230 V circuit for a workshop outlet 30 meters away, carrying 16 A of current. You want a maximum voltage drop of 3%.
Step 1: Calculate the allowed voltage drop in volts: 230 V × 3% = 6.9 V.
Step 2: Apply the formula: S = (2 × 30 m × 16 A) / (56 × 6.9 V).
Step 3: Compute: (960) / (386.4) = 2.4849 mm².
Step 4: The normalised commercial size: Since the calculated value (2.4849) is above 2.5 mm², you must go up to the next standard size: 4 mm² copper cable.
Result: Use a 4 mm² cable to stay within the 3% voltage drop limit. If you used 2.5 mm², the drop would exceed 3%.
Example 2: Low-Voltage 12 V LED Strip
You are installing a 12 V LED strip that draws 8 A, and the cable run from the driver to the strip is 10 meters. Voltage drop is critical at low voltages, so you set the maximum drop to 5%.
Step 1: Allowed drop in volts: 12 V × 5% = 0.6 V.
Step 2: Formula: S = (2 × 10 m × 8 A) / (56 × 0.6 V).
Step 3: Compute: (160) / (33.6) = 4.7619 mm².
Step 4: Normalised size: 4.7619 is above 4 mm², so the next standard is 6 mm² copper cable.
Result: Use a 6 mm² cable to keep the voltage from dropping below 11.4 V at the LED strip. This prevents dimming and overheating of the driver.
Common Mistakes When Sizing Cables
Even with a reliable calculator, errors in input values can lead to wrong results. Watch out for these frequent pitfalls:
- Using the wrong length: Always measure the one-way cable run, not the total loop length. The formula automatically accounts for the return path (hence the factor of 2). If you enter the total distance (there and back), you will double the result incorrectly.
- Confusing voltage and drop percentage: The calculator expects the percentage drop, not the volt drop. For example, if you need a maximum of 5 V drop on a 230 V system, you must convert that to 2.17% (5/230 × 100). Entering "5" in the percentage field would be wrong.
- Ignoring ambient temperature: The conductivity value 56 assumes copper at 20°C. In hot environments (e.g., attics or engine compartments), resistance increases, and voltage drop worsens. For temperatures above 30°C, consider using a derating factor or consult local electrical codes.
- Overlooking the load type: Motors, transformers, and other inductive loads may have a startup current much higher than the running current. This calculator uses steady-state current; for starting surges, you may need a larger cable to handle the momentary drop.
Always double-check your source voltage. Using 230 V when your system is actually 120 V (or vice versa) will produce a cable size that is either dangerously small or unnecessarily large.
Frequently Asked Questions
Does the calculator work for aluminium cables?
No, this version is specifically calibrated for copper conductors, using a conductivity of 56 S·m/mm². Aluminium has a lower conductivity (around 35 to 37), so if you use aluminium wire, the necessary cross-section would be about 1.6 times larger than the copper result provided here. For accurate aluminium sizing, you would need a tool with a different conductivity constant.
What if my calculated cross-section falls exactly on a standard size?
That is perfectly fine. For example, if the formula gives 2.5000 mm², the tool returns the normalised size of 2.5 mm². However, note that if your result is even slightly above a standard size (e.g., 2.5001 mm²), the calculator automatically selects the next larger standard size (4 mm²) to ensure safety and compliance with the voltage drop limit.
Is this calculator suitable for long-distance power transmission lines?
It is primarily designed for low-voltage distribution and final circuits (e.g., building wiring, solar panel connections, and industrial control panels). For very high voltages or extremely long distances (kilometers), other factors like reactance and skin effect become significant, and this simple resistive model may not be accurate. For those applications, consult a power systems engineer or use specialised software.