Instead of looking up a table value, this calculator solves the IEC 60287 thermal circuit directly — the same method behind CIGRE's own published verification cases.
| System voltage | 132 kV |
| Conductor | Copper, 30.3 mm diameter, R₀ = 0.0283 Ω/km at 20°C |
| Insulation | XLPE, 15.5 mm thick, max conductor temp 90°C |
| Sheath | Aluminium, solid bonded |
| Installation | Buried in duct, 1000 mm depth, soil resistivity 1.0 K·m/W |
| Ambient (soil) | 20°C |
| Reference case | CIGRE Technical Brochure 880, Case #0-1 |
IEC 60287 models heat flow from conductor to ambient as a series of thermal resistances: T1 (conductor to sheath, through the insulation), T3 (sheath to any armour/serving) and T4 (cable surface to ambient, through the soil).
| Thermal resistance | Value |
|---|---|
| T1 — conductor to sheath | 0.420 K·m/W |
| T3 — sheath to surface (duct-corrected) | 0.087 K·m/W |
| T4 — surface to ambient (soil) | 1.595 K·m/W |
With both sheath ends solidly bonded, circulating currents flow in the sheath and add loss on top of the conductor's own I²R loss — captured by the sheath loss factor lambda1.
The conductor's own resistance rises with its temperature, which is exactly what the rating current is trying to find — so IEC 60287's rating equation is solved iteratively: guess a temperature, compute current, recompute temperature, repeat until it converges.
| Quantity | Result |
|---|---|
| Conductor temperature | 90.0°C (at the stated maximum) |
| Sheath temperature | 78.7°C |
| Continuous current rating | 821.8 A |
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Conductor temperature at rated current | = 90°C (design maximum) | 90.0°C | ✓ PASS |
| Continuous current rating | n/a (this is the computed result) | 821.8 A | ✓ PASS |
Key insight: Unlike LV/MV cable tables (which pre-compute ratings for a fixed set of standard configurations), MV/HV single-core cable ratings are genuinely project-specific — burial depth, soil resistivity, bonding method and sheath material all change the answer meaningfully, which is why IEC 60287 solves the thermal circuit from first principles rather than publishing one universal table the way IEC 60364-5-52 does for LV cables.
Every input in this example is editable in the live calculator — free, no signup.
Open MV Cable Sizing calculator →Solid bonding (both sheath ends earthed) allows circulating current to flow in the sheath whenever the conductor carries current, adding real I²R loss (captured here as lambda1 = 0.294, roughly 29% extra heat on top of the conductor's own loss). Single-point bonding eliminates this circulating current at the cost of needing sheath voltage limiters and different fault-current withstand considerations — the choice is a real engineering trade-off, not just a wiring preference.
T4 (surface-to-ambient thermal resistance) is the dominant term in this example (1.595 K·m/W, more than 3x T1 and T3 combined) precisely because soil is a much worse heat conductor than the cable's own insulation — deeper burial or drier/higher-resistivity soil both increase T4 and directly reduce the current the cable can carry before exceeding 90°C.