Earth Cable Sizing Chart Work -
| 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 |
| Rating of OCPD (A) | Minimum copper earth csa (AWG / kcmil) | mm² approx. | |--------------------|------------------------------------------|--------------| | 15 | 14 AWG | 2.1 | | 20 | 12 AWG | 3.3 | | 30 | 10 AWG | 5.3 | | 40-60 | 10 AWG | 5.3 | | 100 | 8 AWG | 8.4 | | 200 | 6 AWG | 13.3 | | 400 | 3 AWG | 26.7 | | 600 | 1 AWG | 42.4 | | 800 | 1/0 AWG | 53.5 | | 1200 | 3/0 AWG | 85.0 | | 1600 | 250 kcmil | 126.7 | | 2000 | 350 kcmil | 177.3 | Earth conductor size may need to be increased if the circuit length is long (to reduce impedance for fault clearing), but Table 250.122 provides the minimum. 5. AS/NZS 3000 Approach (Australian/New Zealand) Similar to IEC, but with local tables. Abbreviated Table (copper, V-90 insulated): Below is an abridged table for copper conductors
The earth cable (protective conductor, grounding conductor, or earth continuity conductor) provides a low-impedance path for fault currents to return to the source (transformer or generator). This allows overcurrent protective devices (breakers/fuses) to trip quickly.
