Cable Ampacity Derating Calculator
Temperature, installation and grouping derating in one step.
Inputs
Result
Formula
| Overall factor | kt × kw × kg |
| Usable capacity | base × factor |
| Three-phase load | √3 × U × capacity × cosφ |
| Single-phase load | U × capacity × cosφ |
| Temperature | 1.00 at 30°C, 0.87 at 40, 0.71 at 50 |
| Grouping | 1.00 for one, 0.85 for three, 0.75 for six |
A 100 A base at 40°C in conduit, single circuit: 0.87 × 0.90 × 1.00 = 0.783, giving 78.3 A usable and 43.8 kW of three-phase load.
FAQ
Why does burying increase capacity?
Because soil conducts heat better than still air, so the cable runs cooler — about 5% more capacity than in air. That assumes good conditions: dry soil, rubble backfill or several cables together destroy it, and codes apply a separate soil thermal resistivity correction. Bury in sand or fine soil and lay a warning tape.
Why so much derating for grouped cables?
Because inner cables are surrounded and cannot shed heat — six side by side can raise the middle pair by over 20°C. Code grouping factors run 0.7-0.9 and apply whenever spacing is under one cable diameter. Spacing them at two diameters or using tiered tray is cheaper than going up a size.
Where do the tabulated ratings come from?
From the permitted conductor temperature of the insulation: 70°C for PVC, 90°C for XLPE, over 250°C for mineral-insulated. XLPE carries 15-25% more than PVC at the same size, so changing insulation and increasing size are equivalent options — the former usually cheaper, provided the equipment and terminals also accept 90°C.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.