Electrical Cable Size Calculator

Why Correct Cable Sizing Matters

Using an undersized cable causes heat buildup, insulation damage, voltage drop, and fire hazard. Oversizing wastes material cost. The two main criteria for cable sizing are: current-carrying capacity (ampacity) and voltage drop.

Cable Sizing Method

1. Calculate the load current: I = P / (V × PF)
2. Apply derating factors for ambient temperature, grouping, and installation method.
3. Find the minimum cable CSA from ampacity tables.
4. Check voltage drop at that CSA.
5. Select the larger of the two requirements.

The Voltage Drop Formula

CSA_min (mm²) = (2 × ρ × L × I) / V_drop_max

Where ρ = 0.0175 for copper, 0.028 for aluminium (Ω·mm²/m)

How to Use This Calculator

Enter load current, cable length, supply voltage, and acceptable voltage drop percentage. Select copper or aluminium. The calculator outputs the minimum CSA and the nearest standard cable size.

Standard Cable Sizes (mm²)

1.0, 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300 mm²

Practical Examples

Example: 32A circuit, 50m run, 230V, 3% max drop, copper. Min CSA = (2 × 0.0175 × 50 × 32) / (0.03 × 230) = 56 / 6.9 = 8.1mm² → use 10mm² cable.