Circuit C01 of the worked example project: a 2 MVA transformer needs 105 A, but 25 kA for 0.47 s needs a 120 mm² conductor. Includes the MV screen check.
| Load | 11 kV feeder to a 2 000 kVA transformer |
| Cable | Single-core Cu/XLPE 6/10 kV, trefoil, buried direct, 180 m, two circuits 0.2 m apart |
| Ground | 40 °C, soil 2.0 K·m/W, depth 0.8 m |
| Fault level | 25 kA three-phase; earth fault 1 kA (low-resistance earthed) |
| Clearing time | Relay 0.40 s + breaker 0.07 s = 0.47 s |
| Screen | 25 mm² copper wire screen |
| Factor | Basis | Value |
|---|---|---|
| Ground temperature | 40 °C, Table B.11 (√((90−40)/(90−20))) | 0.845 |
| Grouping | 2 trefoil circuits, 0.2 m, Table B.19 | 0.83 |
| Soil resistivity | 2.0 K·m/W, Table B.14 (lowest value over sizes) | 0.88 |
| Depth | 0.8 m (reference) | 1.00 |
Table B.2, Cu single-core trefoil buried: 70 mm² 239 A, 95 mm² 285 A, 120 mm² 323 A.
95 mm² fails this check only; it is fine for current and voltage drop.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Current capacity | Iz ≥ 105 A | 199.4 A (120 mm²) | ✓ PASS |
| Conductor short circuit | S ≥ 119.8 mm² | 120 mm² | ✓ PASS |
| Screen short circuit | ≥ 5.3 mm² | 25 mm² | ✓ PASS |
| Voltage drop | ≤ 2 % | 0.07 % | ✓ PASS |
Key insight: On MV feeders close to the source the fault level and the backup clearing time decide the conductor size. Use the actual primary clearing time from the coordination study — the backup time here (0.8 s relay + 0.07 s breaker) would require 163 mm², i.e. 185 mm².
Every input in this example is editable in the live calculator — no signup needed.
Open Cable Engineering Suite (full workbench) calculator →IEC 60502-2 gives soil factors per conductor size. The suite uses the lowest value over all sizes at each resistivity, which is safe side (the spread is under 0.02 at 2.0 K·m/W).
The suite's Design Basis lets you choose primary or backup clearing; the example uses primary (relay + breaker). Many specifications require the backup time for feeders.