| Standard | Method | Key variable | |----------|--------|----------------| | | Adiabatic equation or tabulated minimum csa | Fault current magnitude & duration | | NEC (US) Table 250.122 | Based on overcurrent device rating | Amp rating of upstream breaker/fuse | | AS/NZS 3000 | Tabulated based on active conductor size | Size of phase conductor |
Thus, an must specify which standard it follows. 3. Typical Earth Cable Sizing Chart – IEC / BS 7671 Style Using the adiabatic equation ( S = \frac\sqrtI^2 tk ), or simpler lookup tables derived from it. Below is an abridged table for copper conductors (PVC insulation, ( k = 143 ), fault clearing time ≤ 0.4 s): earth cable sizing chart
| Prospective fault current (kA) | Minimum earth csa (mm²) | |-------------------------------|--------------------------| | 0.5 | 1.0 | | 1.0 | 1.5 | | 2.0 | 2.5 | | 3.5 | 4.0 | | 5.0 | 6.0 | | 7.5 | 10.0 | | 10.0 | 16.0 | | 13.0 | 25.0 | | 17.0 | 35.0 | Many charts simplify by tying earth csa to the phase conductor size. Example (copper, PVC, ≤ 25 mm² phase → earth = same size; phase 35 mm² → earth = 16 mm²; phase 50 mm² → earth = 25 mm², etc.). 4. NEC Table 250.122 – Based on Overcurrent Protection The NEC does not use fault current calculation directly for general wiring; instead it uses the rating of the overcurrent device protecting the circuit. | Standard | Method | Key variable |