Cable Derating Factors for Underground and Ambient Temperature
Cable current carrying capacity reduces when installed in underground ducts or at high ambient temperatures. This reduction is applied using cable derating factors to avoid overheating and failure.
Ambient Temperature Derating
| Ambient Temp (°C) | Derating Factor |
|---|---|
| 30 | 1.00 |
| 40 | 0.91 |
| 50 | 0.82 |
| 60 | 0.71 |
Soil Thermal Resistivity (Underground Cables)
- Normal soil (1.2 K.m/W) → derating factor = 1.0
- Dry soil (2.5 K.m/W) → derating factor ≈ 0.8
Example – Cable Derating
A 300A cable installed underground at 40°C: Rated current × 0.91 × 0.8 = 300 × 0.91 × 0.8 ≈ 218A capacity.
FAQ – Cable Derating
Q1: Why is derating necessary?
To prevent overheating and insulation damage.
Q2: Which standards define derating?
IEC 60287 and IS 3961 standards.
Q3: Do cables in groups require derating?
Yes, grouping reduces heat dissipation, so derating must be applied.
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