In 50 °C air, IEC 60364-5-52:2009 Table B.52.14 leaves one PVC (70 °C) circuit 71% of its 30 °C rating and one XLPE/EPR (90 °C) circuit 82%. Buried cables use other tables, and grouping takes more. This page works one circuit through to Iz and the protective device.
Four ideas to carry into the next specification conversation you have.
IEC 60364-5-52:2009, Table B.52.14, cables in air, one circuit, 50 °C ambient: 0.71 for PVC (70 °C conductor) and 0.82 for XLPE/EPR (90 °C conductor). Each insulation reads its own column. These factors do not apply to buried cables.
Table B.52.17, item 1, cables in air (bunched, on a surface, embedded or enclosed), one insulation type, counts circuits or multicore cables: 0.80 for 2, 0.70 for 3, 0.65 for 4, 0.57 for 6, 0.50 for 9, 0.45 for 12, 0.41 for 16, 0.38 for 20. It multiplies the ambient factor.
Buried cables use Tables B.52.15 and B.52.16, not B.52.14. Table B.52.15, ground temperature, reference 20 °C: in 35 °C ground, 0.84 for PVC and 0.89 for XLPE/EPR; at 50 °C, 0.63 and 0.76 (BS 7671 Table 4B2 has the same ground values). Table B.52.16, cables in buried ducts, reference 2.5 K·m/W: 1.18 at 1.0, 1.10 at 1.5, 1.05 at 2.0, 1.00 at 2.5, 0.96 at 3.0 K·m/W. One buried circuit, no grouping. Measure the soil, or state the value assumed.
IEC 60364-4-43:2023, clause 431.4.2 (433.1 in the 2008 edition): Ib ≤ In ≤ Iz, and I2 ≤ 1.45 × Iz. In the worked example, with the assumed Ib = 35 A, In = 40 A and I2 = 1.45 × In = 58 A, both conditions are met against Iz = 41 A. The device maker supplies the real I2.
IEC 60364-5-52:2009, Table B.52.14, ambient air temperature correction factors for cables in air, one circuit (BS 7671 Table 4B1 gives the same values). The design ambient follows the site conditions, the project requirements and any applicable Saudi code; there is no single Saudi value. 50 °C is the example used here. Buried cables read Tables B.52.15 and B.52.16 instead.
| Condition | Correction factor | Note |
|---|---|---|
| Ambient air 30 °C | PVC 1.00 / XLPE 1.00 | Reference condition |
| Ambient air 40 °C | PVC 0.87 / XLPE 0.91 | Table B.52.14 |
| Ambient air 45 °C | PVC 0.79 / XLPE 0.87 | Table B.52.14 |
| Ambient air 50 °C | PVC 0.71 / XLPE 0.82 | Table B.52.14 · example used here |
| Ambient air 55 °C | PVC 0.61 / XLPE 0.76 | Table B.52.14 |
| Ambient air 60 °C | PVC 0.50 / XLPE 0.71 | Table B.52.14 |
Five points to check before a cable size is final.
NFPA 70 is a separate code with its own rules, for example 125% for continuous loads and for motor circuits. They are not IEC values. Size a circuit to one code, and do not mix the codes in one calculation.
IEC 60364-5-52:2009 uses ten reference methods. A1, A2, B1, B2 and C (enclosed or on a wall) and E, F and G (free air) are referenced to 30 °C air. D1 (in buried ducts) and D2 (buried direct) are referenced to 20 °C ground, and read the ground and soil tables (B.52.15 and B.52.16), not the air table B.52.14.
Cables in air. IEC tabulates the factor; the design ambient follows the site conditions, the project requirements and any applicable Saudi code. Between two rows, the hotter row is the conservative choice: a 52 °C design ambient reads the 55 °C row (0.76 for XLPE). This is design practice, not a clause of the standard.
Cables bunched in air (Table B.52.17, item 1): nine single-core cables in three three-phase circuits read the 3-circuit row (0.70), not the 9-circuit row (0.50).
Example only: one XLPE circuit in air, catalogue rating 100 A at the reference conditions, a 50 °C ambient (Table B.52.14, 0.82) and 9 circuits bunched (Table B.52.17, item 1, 0.50). Apply each factor to the catalogue rating, in turn: Iz = 41 A. Write each factor and its row next to the result, so the next reader can check it.
The installation reference method, the insulation, the ambient row (Table B.52.14 for air, or B.52.15 for ground) and the design ambient behind it, the circuit count as Table B.52.17 defines it, the grouping factor and its row, Iz after every factor, both coordination conditions, and the voltage drop with its method and source stated. A schedule that shows only a final ampere figure cannot be checked.
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