Cable manufacturers publish current ratings at 30 °C ambient air, single isolated cable, in free air. Real Saudi installations run at 50 °C ambient, bundled in conduit, behind insulation. The derating factors are not optional, they're the difference between a working installation and an MEP-riser fire.
Four ideas to carry into the next specification conversation you have.
IEC 60364 Table B.52.14. At 50 °C ambient, derate to 78% of the 30 °C rating. At 55 °C, 71%. Riyadh summer MEP riser easily hits 55 °C.
Cables in a tray or conduit heat each other. 4 cables together = 80% of single. 9 cables together = 70%. 16 = 60%.
Standard ρ = 1 Km/W. Saudi sandy soil often runs ρ = 2.5 Km/W, derate buried cables 15-20% further than published.
Itable × ambient × grouping × thermal = actual Iz. Design current must be ≤ Iz, not ≤ Itable.
Per IEC 60364-5-52 Annex B. Multiply factors together.
| Condition | Correction factor | Note |
|---|---|---|
| Ambient 30 °C (reference) | 1.00 | Catalogue baseline |
| Ambient 40 °C | 0.87 | Mild Saudi shade |
| Ambient 50 °C | 0.78 | Typical Saudi summer riser |
| Ambient 55 °C | 0.71 | Hot MEP riser, full sun |
| 4 cables grouped | 0.80 | Same tray / conduit |
| 9 cables grouped | 0.70 | Common in risers |
| Soil ρ = 2.5 Km/W (Saudi) | 0.85 | Direct burial |
| Conduit fill > 40% | 0.80 | Per NEC reference |
Five steps that turn IEC tables into a safe install.
Calculate continuous design current. For motors, add 25% margin (IEC 60364). For air conditioners, use the running current at peak summer ambient. Headline current on the nameplate is start, not design.
At 30 °C ambient, single cable, free air. This is the manufacturer's marketing number. We're not done, we're at step 1.
Look up the Saudi ambient at the cable's location. A roof-run cable in July reads 60 °C. A buried cable runs at 30 °C. An indoor AC'd MEP riser still hits 45 °C+ during a building's first summer of operation.
Count the loaded cables in the same conduit, tray, or trench. The correction is across all of them, not just adjacent. Use IEC 60364 Table B.52.20.
Even if Iz passes, the voltage drop at the load might fail. Iz protects against thermal failure. ΔV protects against equipment voltage starvation. Both must pass, use the bigger cable.
Every NLC installation comes with a cable schedule showing Itable, ambient + grouping + thermal corrections, final Iz, and ΔV at full load, not just catalogue numbers.
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