# Cable Ampacity Derating Calculator

Temperature, installation and grouping derating in one step.

- Page: https://jianzhujisuan.com/en/cable-ampacity
- Category: M&E (https://jianzhujisuan.com/en/c/me)
- Chinese version: https://jianzhujisuan.com/zh/cable-ampacity.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Base rating 100 A, Ambient temperature 40 °C, Installation In conduit (×0.90), Circuits grouped 1 circuits, Voltage 380 V, Power factor 0.85): **Usable capacity 78.3 A**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Base rating | number | 100 | A | Maker data at 30°C in air |
| Ambient temperature | number | 40 | °C |  |
| Installation | select | In conduit (×0.90) | — |  |
| Circuits grouped | number | 1 | circuits |  |
| Voltage | number | 380 | V |  |
| Power factor | number | 0.85 | — |  |

## 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

## Worked example

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.

## Questions

### 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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