🔌 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.

Load Current (A)

Cable Length — one-way (m)

Supply Voltage (V)

Allowable Voltage Drop (%)

Conductor Material

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

Frequently Asked Questions

How do I calculate the minimum cable size?

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.

What is cable ampacity?

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.

Should I size for voltage drop or ampacity?

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.

What is the difference between single-phase and 3-phase voltage drop?

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.