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Correction Factors
Correction factors for ambient temperature, grouping, thermal insulation, and semi-enclosed fuses.
Ambient Temperature (Ca) — Table 4B1
Ambient Temperature Correction Factor (Ca)
| Temp (°C) | PVC (70°C) | XLPE (90°C) |
|---|---|---|
| 25 | 1.03 | 1.02 |
| 30 | 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 |
BS 7671: Table 4B1
Grouping Factor (Cg) — Table 4C1
Grouping Correction Factor (Cg)
| No. of Circuits | Bunched/Enclosed | Single Layer on Wall | Single Layer Ceiling |
|---|---|---|---|
| 1 | 1.00 | 1.00 | 0.95 |
| 2 | 0.80 | 0.85 | 0.81 |
| 3 | 0.70 | 0.79 | 0.72 |
| 4 | 0.65 | 0.75 | 0.68 |
| 5 | 0.60 | 0.73 | 0.66 |
| 6 | 0.57 | 0.72 | 0.64 |
| 7 | 0.54 | 0.72 | 0.63 |
| 8 | 0.52 | 0.71 | 0.62 |
| 9 | 0.50 | 0.70 | 0.61 |
| 12 | 0.45 | 0.70 | 0.61 |
| 16 | 0.41 | 0.70 | 0.61 |
| 20 | 0.38 | 0.70 | 0.61 |
BS 7671: Table 4C1
Thermal Insulation (Ci) — Reg 523.7
Thermal Insulation Correction Factor (Ci)
| Condition | Factor (Ci) |
|---|---|
| Not enclosed in insulation | 1.00 |
| One side touching insulation | 0.75 |
| Totally surrounded 50mm | 0.89 |
| Totally surrounded 100mm | 0.81 |
| Totally surrounded 200mm | 0.68 |
| Totally surrounded 400mm+ | 0.55 |
For cables totally surrounded by thermal insulation for distances > 0.5m. Values from Table 52.2.
BS 7671: Reg 523.7
Semi-enclosed Fuse Factor (Cc)
When BS 3036 semi-enclosed (rewirable) fuses are used, apply a correction factor Cc = 0.725 to the tabulated cable current rating.
BS 7671: Reg 433.1.1
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