Correction Factors
Derating factors for cable current carrying capacity
BS 7671:2018+A2:2022 Tables 4B1, 4C1
How to Apply Correction Factors
The installed current rating is calculated as: Iz = It × Ca × Cg × Ci
- • It = Tabulated current rating
- • Ca = Ambient temperature correction factor
- • Cg = Grouping correction factor
- • Ci = Thermal insulation correction factor (if applicable)
Ambient Temperature Correction Factor (Ca)
BS 7671 Table 4B1 - For cables not in ground (reference 30°C)
| Ambient Temp (°C) | PVC (70°C) Twin & Earth, SWA PVC | XLPE (90°C) SWA XLPE |
|---|---|---|
| 25 | 1.03 | 1.02 |
| 30(reference) | 1.00 | 1.00 |
| 35 | 0.94 | 0.96 |
| 40 | 0.87 | 0.91 |
| 45 | 0.79 | 0.87 |
| 50 | 0.71 | 0.82 |
| 55 | 0.61 | 0.76 |
| 60 | 0.50 | 0.71 |
| 65 | - | 0.65 |
| 70 | - | 0.58 |
Grouping Correction Factor (Cg)
BS 7671 Table 4C1 - For cables bunched together or in enclosed spaces
| Number of Circuits/Cables | Correction Factor (Cg) | Effective Capacity |
|---|---|---|
| 1(no derating) | 1.00 | 100% |
| 2 | 0.80 | 80% |
| 3 | 0.70 | 70% |
| 4 | 0.65 | 65% |
| 5 | 0.60 | 60% |
| 6 | 0.57 | 57% |
| 7 | 0.54 | 54% |
| 8 | 0.52 | 52% |
| 9 | 0.50 | 50% |
| 12 | 0.45 | 45% |
| 16 | 0.41 | 41% |
| 20 or more | 0.38 | 38% |
Thermal Insulation Correction Factor (Ci)
BS 7671 Table 52.2 - For cables surrounded by thermal insulation
| Length in Insulation | Correction Factor (Ci) |
|---|---|
| 50mm or less | 0.89 |
| 51mm to 100mm | 0.81 |
| 101mm to 200mm | 0.68 |
| 201mm to 400mm | 0.55 |
| Over 400mm | 0.50 |
Ground Temperature Correction (Buried Cables)
BS 7671 Table 4B3 - Reference ground temperature 20°C
| Ground Temp (°C) | PVC (70°C) | XLPE (90°C) |
|---|---|---|
| 10 | 1.10 | 1.07 |
| 15 | 1.05 | 1.04 |
| 20(reference) | 1.00 | 1.00 |
| 25 | 0.95 | 0.96 |
| 30 | 0.89 | 0.93 |
| 35 | 0.84 | 0.89 |
| 40 | 0.77 | 0.85 |
Soil Thermal Resistivity (Buried Cables)
BS 7671 Table 4B4 - Reference resistivity 2.5 K·m/W
| Soil Resistivity (K·m/W) | Soil Type | Correction Factor |
|---|---|---|
| 1.0 | Very wet soil, water-logged | 1.18 |
| 1.5 | Wet soil | 1.10 |
| 2.5 | Damp soil(reference) | 1.00 |
| 3.0 | Dry soil | 0.96 |
Important Notes
- • Multiple Factors: When more than one correction factor applies, multiply them together: Ca × Cg × Ci
- • Significant Derating: If total correction factor is below 0.7, consider revising the installation design.
- • Interpolation: For intermediate values, use the next lower factor (conservative approach).
- • Different Circuits: When grouping circuits with different load types, consider whether they operate simultaneously.
- • Enclosed Trunking: Cables in totally enclosed trunking require additional derating beyond these tables.