Most standard lighting circuits use a 1.0mm² or 1.5mm² twin and earth cable. In these cables, the earth wire is often "reduced"—meaning it might be 1.0mm² even if the live wire is 1.5mm². 2. Socket Outlets and Power Circuits (2.5mm²)

Under the 18th Edition Wiring Regulations, the minimum cross-sectional area (csa) of the protective conductor (CPC) is selected based on the cross-sectional area of the line conductor ($S$).