Hot sites: the ambient temperature factor
The reference capacities are for 30 °C air. Above that, IEC 60364-5-52 Table B.52.14 reduces them by Ca = √((θmax − θa) / (θmax − 30)), and the reduction is steeper for PVC because its limit is closer to the ambient. At 40 °C, Ca is 0.87 for PVC and 0.91 for XLPE; the table follows a 16 mm² copper cable, method C, three loaded conductors (76 A PVC, 100 A XLPE at 30 °C).
| Ambient (°C) | Ca PVC | Ca XLPE | 16 mm² PVC (A) | 16 mm² XLPE (A) | XLPE ÷ PVC |
|---|---|---|---|---|---|
| 25 °C | 1.06 | 1.04 | 80.6 | 104 | 1.29 |
| 30 °C | 1.00 | 1.00 | 76 | 100 | 1.32 |
| 35 °C | 0.94 | 0.96 | 71.4 | 96 | 1.34 |
| 40 °C | 0.87 | 0.91 | 66.1 | 91 | 1.38 |
| 45 °C | 0.79 | 0.87 | 60 | 87 | 1.45 |
| 50 °C | 0.71 | 0.82 | 54 | 82 | 1.52 |
| 55 °C | 0.61 | 0.76 | 46.4 | 76 | 1.64 |
| 60 °C | 0.50 | 0.71 | 38 | 71 | 1.87 |
At 30 °C the XLPE cable carries 32 % more; at 50 °C the margin grows to 52 %. In hot climates this is often the reason XLPE allows a smaller size.
Short-circuit withstand
For short faults the conductor heats adiabatically. IEC 60364-4-43 Table 43A gives k = 115 for copper with PVC insulation up to 300 mm² (70 °C initial, 160 °C final) and k = 143 for copper with XLPE (90 °C initial, 250 °C final). The current a conductor can carry for t seconds is I = k × S / √t:
| Size (mm²) | PVC, 1 s (kA) | XLPE, 1 s (kA) | PVC, 0.1 s (kA) | XLPE, 0.1 s (kA) |
|---|---|---|---|---|
| 1.5 | 0.173 | 0.215 | 0.545 | 0.678 |
| 2.5 | 0.288 | 0.358 | 0.909 | 1.13 |
| 4 | 0.46 | 0.572 | 1.45 | 1.81 |
| 6 | 0.69 | 0.858 | 2.18 | 2.71 |
| 10 | 1.15 | 1.43 | 3.64 | 4.52 |
| 16 | 1.84 | 2.29 | 5.82 | 7.24 |
| 25 | 2.88 | 3.58 | 9.09 | 11.3 |
| 35 | 4.03 | 5.01 | 12.7 | 15.8 |
| 50 | 5.75 | 7.15 | 18.2 | 22.6 |
| 70 | 8.05 | 10 | 25.5 | 31.7 |
| 95 | 10.9 | 13.6 | 34.5 | 43 |
| 120 | 13.8 | 17.2 | 43.6 | 54.3 |
| 150 | 17.3 | 21.5 | 54.5 | 67.8 |
| 185 | 21.3 | 26.5 | 67.3 | 83.7 |
| 240 | 27.6 | 34.3 | 87.3 | 109 |
Practical notes
- A cable that runs at 90 °C has a higher resistance than one at 70 °C: for copper the ratio is 1.066, so the voltage drop at full load is about 6.6 % higher. On long runs sized by voltage drop, the XLPE capacity advantage may not reduce the size at all.
- Using the 90 °C rating assumes that the terminals and accessories at both ends are suitable for that conductor temperature. Where the equipment maker limits the conductor temperature, the cable is sized as if it were rated at that lower temperature.
- PVC releases hydrogen chloride and dense smoke when it burns. Where smoke and corrosive gases matter (escape routes, public buildings, data rooms), cables with low-smoke, halogen-free sheaths are specified; XLPE insulation is itself halogen-free, but the sheath of an XLPE cable may still be PVC.
- PVC becomes stiff at low temperature and softens when overheated, which limits it in cold installation conditions and under sustained overload; XLPE is a thermoset and does not melt.