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 ($S$).
If an earth wire is too thin, it acts like a fuse. During a high-voltage fault, a thin wire will heat up rapidly, potentially melting its insulation or causing a fire before the circuit breaker has a chance to trip. A correctly sized earth cable ensures:
Note: Standard Twin & Earth cables manufactured in the UK typically have a reduced earth conductor (e.g., 2.5 mm² twin & earth has a 1.5 mm² earth). This is legal because it passes the specific calculation tests for fault current, even though the table above suggests full size.
[ S = \frac\sqrtI^2 \times tk ]
If the live wire is , the earth wire should match its size (unless it's part of a multicore cable where a slightly smaller size is pre-calculated).
