🔌 Cable Size Calculator
Find the minimum cable cross-section (mm²) based on load current, run length, allowable voltage drop, and conductor material. Also shows current-carrying capacity check against standard cable sizes.
What is a Cable Size Calculator?
A cable size calculator determines the minimum cross-sectional area of an electrical conductor (in mm²) required to safely carry a given load current, while keeping the voltage drop within acceptable limits. It uses the conductor's resistivity, the one-way cable length, the load current, and the supply voltage to compute the theoretical minimum size — then rounds up to the nearest standard IEC cross-section. The calculator also checks the selected size against its rated current-carrying capacity (ampacity) to confirm thermal safety.
Correct cable sizing is a legal and safety requirement in all electrical installations. Undersized cables overheat under load, degrading insulation, shortening cable life, and in severe cases causing fire. Oversized cables are safe but wasteful — they cost more and are harder to terminate. The two governing criteria are ampacity (the cable must carry the load current without exceeding its temperature rating) and voltage drop (the voltage at the load must remain within acceptable limits, typically 3–5% for IEC installations per IEC 60364).
The most commonly used conductor materials are copper and aluminium. Copper has lower resistivity (1.72×10⁻⁸ Ω·m vs aluminium's 2.82×10⁻⁸ Ω·m), meaning a smaller copper cable can carry the same current as a larger aluminium one. However, aluminium is significantly cheaper and lighter, making it the standard choice for high-voltage transmission lines and large-scale distribution. For building wiring, copper remains dominant due to its superior conductivity and ease of termination.
Copper Cable Ampacity Reference (IEC, 70°C PVC, clipped direct)
| mm² | Ampacity (A) | Ω/km |
|---|---|---|
| 1.5 | 18 | 12.1 |
| 2.5 | 26 | 7.41 |
| 4 | 35 | 4.61 |
| 6 | 46 | 3.08 |
| 10 | 63 | 1.83 |
| 16 | 85 | 1.15 |
| 25 | 110 | 0.727 |
| 35 | 134 | 0.524 |
| 50 | 159 | 0.387 |
| 70 | 200 | 0.268 |
How the Cable Size Calculator Works
Formula, assumptions, and calculation steps for this engineering tool.
Methodology
Engineering calculators apply standard unit conversions and formula relationships after normalizing measurements to compatible units.
Calculation Steps
- Enter dimensions, loads, rates, or electrical values.
- Convert the inputs into the formula unit system.
- Apply the engineering equation or conversion factor.
- Return the result with units and supporting context.
Assumptions and Limits
- Material behavior is assumed ideal unless fields specify otherwise.
- Code checks, safety factors, and site conditions may require professional review.
- Use a qualified engineer for design-critical decisions.
Frequently Asked Questions
Minimum cross-section (mm²) = (2 × L × I × ρ) / (V_allowable_drop). Where L is one-way length in metres, I is current in amps, ρ is resistivity in Ω·m (1.72×10⁻⁸ for copper), and V_allowable_drop is supply voltage × (% drop / 100). Then round up to the next standard size.
Ampacity is the maximum current a cable can carry continuously without exceeding its temperature rating (typically 70°C for PVC or 90°C for XLPE insulation). Always ensure your load current is below the cable's ampacity for the selected size. Factors like ambient temperature, grouping, and installation method affect the rating.
Always check both. The cable must satisfy BOTH the ampacity requirement (to prevent overheating) AND the voltage drop requirement (to ensure proper equipment operation). For long runs, voltage drop often governs. For short, high-current runs, ampacity is usually the limiting factor. Always use the larger of the two required sizes.
For single-phase: V_drop = 2 × L × I × R_per_metre (factor of 2 for live + neutral). For 3-phase balanced: V_drop = √3 × L × I × R_per_metre ≈ 1.732 × L × I × R_per_metre. This calculator uses the single-phase formula. For 3-phase, the result will be conservative (slightly oversized), which is safe.
Real-World Applications
Common Mistakes
Copper vs Aluminium Conductor Comparison
| Property | Copper | Aluminium |
|---|---|---|
| Resistivity (Ω·m) | 1.72 × 10⁻⁸ | 2.82 × 10⁻⁸ |
| Relative conductivity | 100% | 61% |
| Density | 8,960 kg/m³ | 2,700 kg/m³ |
| Cost (relative) | Higher | Lower (≈40–60% of copper) |
| Equivalent size for same current | Baseline | Approx. 1.5× larger cross-section |
| Common applications | Building wiring, electronics | HV cables, overhead lines, large feeders |
References
- IEC 60364. Low-Voltage Electrical Installations. International Electrotechnical Commission.
- BS 7671. Requirements for Electrical Installations (IET Wiring Regulations, 18th Edition). IET, 2018.
- NFPA 70. National Electrical Code. National Fire Protection Association, 2023.
- Glover, J. D., Sarma, M. S. & Overbye, T. J. Power Systems Analysis and Design, 6th ed. Cengage, 2017.
- Chapman, S. J. Electric Machinery Fundamentals, 5th ed. McGraw-Hill, 2012.
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