Cable rating tables are published for one specific set of conditions. Your installation is almost never that set of conditions. Derating — applying correction factors to the tabulated current-carrying capacity — is how the two are reconciled.
The DC Cable Sizing Calculator applies only the factors relevant to the installation method you select, and shows each factor separately with its basis.
Reference Conditions
IEC 60364-5-52 rating tables assume:
| Condition | Reference value |
|---|---|
| Ambient air temperature | 30 °C |
| Ground temperature | 20 °C |
| Soil thermal resistivity | 2.5 K·m/W |
| Depth of laying | 0.8 m |
| Grouping | one circuit |
A correction factor applies only where the actual condition differs from the reference — and only where the condition is physically relevant. A cable on a tray has no soil resistivity. A buried cable has no ambient air temperature.
The Derating Chain
$$I_z = I_{table} \times k_{temp} \times k_{group} \times k_{soil} \times k_{depth} \times k_{project}$$
The design current must then satisfy:
$$I_{design} \le I_z$$
Multiplying unrelated factors together — for instance applying both an air-temperature factor and a soil factor to the same cable — is a common and serious error. It either over-derates and wastes copper, or masks the condition that actually governs.
Ambient Air Temperature
For cables in air, above 30 °C:
| Ambient | XLPE/EPR 90 °C factor |
|---|---|
| 25 °C | 1.04 |
| 30 °C | 1.00 |
| 35 °C | 0.96 |
| 40 °C | 0.91 |
| 45 °C | 0.87 |
| 50 °C | 0.82 |
| 55 °C | 0.76 |
| 60 °C | 0.71 |
Note that below 30 °C the factor exceeds 1.0 — a cold environment genuinely allows more current, though most designers do not claim the bonus.
Ambient temperature is not the same as conductor operating temperature. Ambient sets the derating factor; conductor temperature sets the resistance used for voltage drop.
Ground Temperature
For buried cables, corrected against a 20 °C reference:
| Ground temperature | Factor |
|---|---|
| 15 °C | 1.04 |
| 20 °C | 1.00 |
| 25 °C | 0.96 |
| 30 °C | 0.93 |
| 35 °C | 0.89 |
| 40 °C | 0.85 |
| 45 °C | 0.80 |
Soil Thermal Resistivity
Soil thermal resistivity, in K·m/W, measures how poorly the soil conducts heat away from a buried cable. IEC tables adopt 2.5 K·m/W as a precautionary assumption.
| Soil resistivity | Direct buried factor | Buried in duct factor |
|---|---|---|
| 1.0 K·m/W | 1.50 | 1.18 |
| 1.5 K·m/W | 1.28 | 1.10 |
| 2.0 K·m/W | 1.12 | 1.05 |
| 2.5 K·m/W | 1.00 | 1.00 |
| 3.0 K·m/W | 0.90 | 0.96 |
If the project has a soil thermal survey, use its value — the difference between 2.5 and 1.0 K·m/W is worth a full cable size on a large feeder. Where thermal backfill is specified, use the backfill's stated resistivity, not the native soil's.
Never apply a soil factor to a cable in air.
Depth of Laying
Deeper cables shed heat more slowly. Corrected against the 0.8 m reference depth used in the IEC 60502 rating tables:
| Depth | Factor |
|---|---|
| 0.5 m | 1.04 |
| 0.8 m | 1.00 |
| 1.0 m | 0.98 |
| 1.5 m | 0.95 |
| 2.0 m | 0.93 |
| 3.0 m | 0.90 |
The effect is modest compared with soil resistivity and grouping, but it is real, and it applies to buried runs only.
Grouping and Number of Circuits
Cables in a group warm each other, so each one can dissipate less. The factor depends on how many circuits there are and how they are arranged.
| Circuits | Bunched / touching in air | Spaced on tray | In buried ducts | Direct buried, touching |
|---|---|---|---|---|
| 1 | 1.00 | 1.00 | 1.00 | 1.00 |
| 2 | 0.80 | 0.94 | 0.85 | 0.75 |
| 3 | 0.70 | 0.90 | 0.75 | 0.65 |
| 4 | 0.65 | 0.90 | 0.70 | 0.60 |
| 6 | 0.57 | 0.90 | 0.60 | 0.50 |
| 12 | 0.45 | 0.87 | 0.50 | 0.39 |
Two lessons follow. First, spacing is cheap capacity: separating cables by one diameter on a tray keeps the factor near 0.9 instead of dropping to 0.6. Second, there is no universal grouping factor. A design that reports 0.6 for twelve cables in one pipe is reporting that project's arrangement, not a rule you can copy.
Installation Method
Method changes the base ampacity before any factor is applied:
| Method | IEC reference | Character |
|---|---|---|
| Single-core in free air, spaced | F / G | Best cooling, highest rating |
| Perforated tray, touching | E / F | Good cooling |
| In conduit or pipe in air | B2 | Trapped air reduces rating |
| Buried in duct | D1 | Air gap plus soil path |
| Direct buried | D2 | Soil contact, moderate rating |
Buried-cable factors must not be applied to tray cables, and tray factors must not be applied to buried cables. Map the method first, then pick the factor set that belongs to it.
Parallel Cables
Running two cables per polarity is not a derating factor — it changes the circuit. Each cable carries half the current, the loop resistance halves, and the group's own grouping factor must still be applied. Terminations and lengths must be equal so the current genuinely shares.
Worked Example
A 120 A DC feeder, 150 m one-way, buried in duct, 40 °C ground, soil 2.0 K·m/W, 1.2 m deep, 12 circuits in the group.
```text
Base ampacity, 185 mm² copper (method D1) 312 A
Ground temperature factor (40 °C) × 0.85
Soil thermal resistivity factor (2.0) × 1.05
Depth factor (1.2 m) × 0.97
Grouping factor (12 circuits in ducts) × 0.50
Combined factor × 0.43
Derated ampacity 134 A
Design current 120 A -> PASS
```
The same cable at one circuit and reference conditions would be rated 312 A. Grouping alone removed half of it. That is why derating, not the base table, decides most buried feeder sizes.
How Derating Changes the Selection
Derating pushes the ampacity check upward while voltage drop stays where it is. On short, heavily grouped runs, ampacity governs. On long, lightly grouped runs, voltage drop governs. The only way to know which applies is to evaluate both for every candidate size — which is exactly what the DC Cable Sizing Calculator does.