| Breaker / Fuse Rating | Minimum Earth Size (Copper) | Minimum Earth Size (Aluminum) | | :--- | :--- | :--- | | | 14 AWG | 12 AWG | | 20 Amps | 12 AWG | 10 AWG | | 30 Amps | 10 AWG | 8 AWG | | 40 Amps | 10 AWG | 8 AWG | | 60 Amps | 10 AWG | 8 AWG | | 100 Amps | 8 AWG | 6 AWG | | 200 Amps | 6 AWG | 4 AWG |
When in doubt, While using a slightly thicker wire than necessary only costs a bit more in copper, using a wire that is too thin can lead to catastrophic system failure.
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: earth cable sizes
[ S = \frac\sqrtI^2 \times tk ]
The formula (from BS 7671) is: $$S = \frac\sqrtI^2 \times tk$$ | Breaker / Fuse Rating | Minimum Earth
Where:
= A factor based on the resistivity and temperature of the conductor material. Key Factors Influencing Selection A correctly sized earth cable ensures: [ S
Here is a guide to earth cable sizes based on common international standards (IEE/BS 7671 UK and NEC USA).
Example: 10 mm² phase → 10 mm² earth. 25 mm² phase → 16 mm² earth.
| Line Conductor Size ($S$) | Minimum Earth Conductor Size ($S_pc$) | | :--- | :--- | | | $S$ (Same size as line) | | $16\text mm^2 < S \le 35\text mm^2$ | $16\text mm^2$ | | $S > 35\text mm^2$ | $S / 2$ (Half the line size) |