What derating actually means
Every cable has a rated current-carrying capacity, or ampacity. That rating is measured under a defined set of reference conditions: a particular ambient temperature, a stated installation method and, usually, a cable on its own. Real installations rarely match those conditions, so the rating has to be corrected.
Derating is that correction. You start from the tabulated capacity for the cable type and installation method, multiply it by one or more correction factors, and end up with the reduced capacity the cable can actually carry in place. The corrected value is what has to cover the load and stay above the protective device rating.
The principle is simple, but skipping it is one of the most common ways a cable ends up overloaded. A cable that looks fine in a table can be seriously underrated once it sits in a hot riser alongside several others.
The correction factor formula
The derated capacity is the tabulated capacity multiplied by each applicable factor in turn:
Iz = It × k1 × k2 × k3 …
Here It is the tabulated capacity for the cable and installation method, and the k values are the correction factors for temperature, grouping, soil and any other local condition. When several factors apply, they multiply, and that multiplication adds up quickly.
The corrected capacity must satisfy two relationships: it must be at least the design load current, and it must be at least the rating of the protective device that guards the circuit. If either fails, step up a cable size or improve the installation conditions.
A quick sanity check helps. If the combined factor looks small, the geometry of the installation is working against you, and there is often a cheaper answer than a much larger cable: separate the cables, reduce their number, lower the ambient temperature, or improve airflow around the enclosure.
Ambient temperature correction
Conductors are limited by the temperature their insulation can tolerate. If the surrounding air or soil is warmer than the reference value, the cable starts closer to its limit and can carry less current. Cooler surroundings allow a little more.
The table below shows typical correction factors for PVC-insulated cable in air, with a reference ambient of 30 °C. Values differ for other insulation types, so always confirm against the table for the material you are using.
| Ambient air (°C) | Correction factor |
|---|---|
| 20 | 1.12 |
| 25 | 1.06 |
| 30 | 1.00 |
| 35 | 0.94 |
| 40 | 0.87 |
| 45 | 0.79 |
| 50 | 0.71 |
For cables buried in soil the reference temperature is lower and hot ground matters just as much, so the soil table should be used rather than the air table.
Grouping and conduit effects
Cables grouped together cannot shed heat as easily as an isolated cable, because each one warms the air around its neighbours. The more loaded circuits bunched together, the lower the correction factor. Enclosing cables in conduit or trunking adds to the effect, since the enclosure itself restricts airflow.
| Number of grouped circuits | Correction factor |
|---|---|
| 1 | 1.00 |
| 2 | 0.80 |
| 3 | 0.70 |
| 4 | 0.65 |
| 6 | 0.57 |
| 8 | 0.52 |
These factors assume circuits of similar size and loading. Mixing heavily loaded and lightly loaded cables changes the picture, so a conservative approach is to derate as if all of them are loaded. Spacing cables apart, or using a larger enclosure, restores some of the lost capacity.
Conduit fill and trunking are often checked separately using their own rules, but the thermal effect is the one that changes ampacity. A duct packed to its fill limit both derates the cables inside it and makes them harder to cool, so the two checks belong together.
Soil, burial and other local factors
Buried cables exchange heat with the soil, so soil temperature and thermal resistivity both matter. Dry, sandy or insulated ground holds heat poorly around the cable, and the presence of nearby heat sources or other buried services makes it worse. Reference tables give guiding factors, but the local ground conditions decide the real answer.
- Use the soil-temperature table for buried runs, not the air table.
- Account for thermal resistivity where the ground is unusually dry or insulated.
- Allow for depth of burial and for grouping with other buried cables.
- Avoid known heat sources such as steam mains or other loaded ducts.
Other factors occasionally enter the calculation too, such as exposure to direct sunlight or protection against mechanical damage. The point is not to memorise every table, but to apply a factor wherever the real conditions differ from the reference ones.
Worked mini-example
Take a 4 mm² copper cable with PVC insulation, single-phase, installed in conduit on a wall — a common reference installation method with a tabulated capacity of 32 A.
The cable runs in a hot plant room at 40 °C, and it is grouped with three other loaded circuits in the same enclosure.
- Ambient temperature 40 °C → k = 0.87
- Four grouped circuits → k = 0.65
- Combined factor = 0.87 × 0.65 = 0.566
Derated capacity = 32 A × 0.566 ≈ 18.1 A. If the design load is 20 A, this cable is not acceptable — the corrected capacity is below the load.
Stepping up to a 6 mm² cable with a tabulated capacity of 41 A gives 41 A × 0.566 ≈ 23.2 A, which covers the 20 A load. The same result can also be reached by improving the installation, for example by separating the grouped cables, but choosing the larger conductor is often the simpler fix.
Keeping derating under control
Derating is where careful design pays off. Applying the right factors early avoids the unpleasant discovery, late in a project, that a perfectly good-looking cable schedule cannot be installed as drawn. It also keeps the protective device, the cable and the load in agreement.
GorkyCAD Pro is a browser-based CAD tool for electrical design that carries cable data, installation method and calculated results in one model, alongside IEC 60364 checks, so correction factors stay attached to the circuit they affect. Drawings, panel schedules and the cable schedule all come from that same model, and the app works offline as a PWA for site use.
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FAQ
Do I multiply all correction factors together?
Yes. Each applicable factor is multiplied in turn, and the product is applied to the tabulated capacity to give the real installed capacity.
Is the ambient table the same for all cables?
No. Correction factors depend on the insulation type and the reference temperature, so use the table that matches your cable and installation method.
What if the derated capacity is below the load?
Either step up to a larger conductor or improve the installation, such as separating grouped cables or improving ventilation.
Why does grouping reduce capacity?
Grouped cables warm each other because they cannot shed heat as easily as an isolated cable, so the correction factor falls as circuits are added.