How To Calculate The Cable Size Jun 2026

Ideally, voltage drop should be kept below 3% for lighting and 5% for power circuits.

| Cable Size (mm²) | Approx. AWG Equivalent | Maximum Current (Amps) | Typical Application | | :--- | :--- | :--- | :--- | | | 18 AWG | 10 – 15 A | Lighting Circuits | | 1.5 mm² | 16 AWG | 15 – 20 A | Lighting, Radial Sockets (Light load) | | 2.5 mm² | 14 AWG | 20 – 25 A | Ring Mains, Sockets, Water Heaters | | 4.0 mm² | 12 AWG | 25 – 32 A | Cookers, Showers, Sub-mains | | 6.0 mm² | 10 AWG | 32 – 40 A | Electric Showers, Sub-mains | | 10.0 mm² | 8 AWG | 40 – 50 A | Main Distribution Boards | | 16.0 mm² | 6 AWG | 50 – 63 A | High Capacity Sub-mains | how to calculate the cable size

Calculating the correct cable size is a multi-step process that ensures electrical safety, efficiency, and longevity of the installation. Using an undersized cable can lead to overheating and fire hazards, while an oversized cable incurs unnecessary costs. Ideally, voltage drop should be kept below 3%

If you know the wattage of the appliance (W) and the voltage (V), you can calculate the amps using Ohm’s Law: $$I = \fracPV$$ Using an undersized cable can lead to overheating

Pro Tip: If math isn't your strong suit, use an online "Voltage Drop Calculator." Input your load, voltage, and length, and it will tell you if your cable size is sufficient.