Cable Ampacity Derating Calculator
Temperature, installation and grouping derating in one step.
Quick answer
With the default inputs: Base rating 100 A · Ambient temperature 40 °C · Installation In conduit etc.Usable capacity 78.3 A
Edit the inputs below to recalculate.Inputs
Result
Usable capacity
78.3A
| Temperature factor | 0.87 |
| Installation factor | 0.90 |
| Grouping factor | 1.00 |
| Overall factor | 0.783 |
| Base rating | 100A |
| Capacity lost | 21.7% |
| Three-phase load | 43.8kW |
| Single-phase load | 14.6kW |
| Capacity at 3 grouped | 66.6A |
The three factors multiply rather than add. Three grouped circuits in conduit at 40°C give 0.87 × 0.90 × 0.85 = 0.666, so a 100 A cable is only good for 66.6 A. This is the most commonly missed step: designs account for temperature but forget grouping and conduit, and the cable insulation fails after a year or two in service.
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Enter a target result; the other inputs stay fixed and this one is worked out.
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.
How to work it out
- Look up the base capacity by cable type, core count and cross-section in 19DX101-1 or the manufacturer data. The tabulated conditions are 30°C in free air, single circuit — using the table value directly always overstates reality.
- Temperature factor kt: 1.00 at 30°C, 0.94 at 35, 0.87 at 40, 0.79 at 45, 0.71 at 50. Use the hottest-month mean maximum for the location, not the annual average.
- Installation factor kw: 1.00 in air, 0.90 in conduit, 0.95 on tray, 1.05 direct buried. Buried cables shed heat into soil better than into air, so this one is a credit rather than a penalty.
- Grouping factor kg: 1.00 for one circuit, 0.90 for two, 0.85 for three, 0.80 for four, 0.75 for six. Adjacent circuits heat each other and must be derated.
- Multiply the three factors for the overall K and apply it: actual = base × K. With a 100 A base at 40°C, in conduit, three circuits grouped: 100 × 0.87 × 0.90 × 0.85 = 66.6 A.
- Work back to the load: three-phase P = √3 × U × capacity × cosφ, single-phase P = U × capacity × cosφ. If the load exceeds it, go up a size or improve the installation.
Where it goes wrong
- Reading the catalogue figure and stopping there. Catalogue values assume 30°C, one circuit, free air. Six cables bunched on a tray in summer deliver about 60% of the table value.
- Adding the factors instead of multiplying. 0.87 + 0.90 + 0.85 = 2.62 is absurd; the correct product is 0.665, a factor of four apart.
- Derating a direct-buried cable as if it were in air. Soil has lower thermal resistance, so the same size typically carries 5-15% more buried — provided the depth is adequate and the backfill is not rubble.
- Ignoring solar gain and altitude. A roof-mounted tray in direct sun reaches 70°C at the surface, effectively adding 20°C to ambient; above 2000 m thinner air reduces cooling and requires further derating.
Material and cost
Failing the check means upsizing, which adds 30-60% per size step. Improving heat dissipation is far cheaper: tray dividers, avoiding stacked layers, wider circuit spacing — near zero cost and often worth one or two sizes.
On site
- Stacked tray layers are the most commonly missed item on site. A tray packed solid with cable is a clear violation and the hardest thing to fix once the system is energised.
- Use the hottest-month mean maximum temperature. Correcting against an annual average means no real correction at all in southern summers.
- Fire-rated circuits must be checked for the fire condition (750°C/950°C rated cable); the ordinary temperature correction table does not apply.
- Separate power and control cables with a divider in the same tray, and derate control cables according to the power-cable installation conditions.
Basis
- GB 50217-2018 Standard for design of cables of electric power engineering, cable selection and derating annexes
- GB 50054-2011 Code for design of low voltage distribution, conductor selection
- 19DX101-1 Building electrical data handbook, cable capacity and correction factor tables
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.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Cable Ampacity Derating Calculator. 2026-10-25. https://jianzhujisuan.com/en/cable-ampacity
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.