How Cable Installation Methods Affect Current-Carrying Capacity
2026-08-26 17:25When designing an electrical installation, selecting the correct cable size is only half the battle. The way the cable is installed has a profound impact on its ability to carry current safely. Two identical cables—same conductor size, same insulation—can have significantly different current ratings depending on whether they are installed in free air, buried directly in the ground, pulled through a conduit, or bundled with other cables. This is because the installation method determines how effectively the cable can dissipate the heat generated by the current flowing through it. Understanding these effects is essential for proper cable sizing, safety, and long-term reliability.
1. The Thermal Balance: Heat Generation vs. Heat Dissipation
A power cable generates heat when current flows due to the resistance of the conductor (I²R losses). The cable's temperature rises until the heat generated equals the heat dissipated to the surrounding environment.
The installation method affects the rate of heat dissipation. If the cable is in a well-ventilated area, heat can be carried away by convection. If it is buried in soil, heat must be conducted through the ground. If it is in a conduit, the conduit traps heat, reducing dissipation. If it is bundled with other cables, they heat each other, further reducing cooling.
The effective current-carrying capacity (ampacity) is therefore determined not just by the cable itself, but by its ability to lose heat in its specific installation environment.
2. The Reference Conditions: Free Air
The base ampacity of a cable is typically specified for free air—the cable is installed in a well-ventilated space with no other cables nearby. This is the reference condition from which all derating factors are applied.
Example: A 16 mm² XLPE copper cable has a base ampacity of approximately 100 A in free air at 30°C ambient. This is the maximum current it can carry continuously without exceeding its temperature rating.
If the cable is installed in a less favourable environment, the ampacity must be reduced.
3. Installation Method 1: In Free Air (Clipped Direct or in Cable Tray)
What it is: The cable is installed on a surface (wall, ceiling) or in a ventilated cable tray, with free air circulation around it.
Key factors:
Good air circulation allows heat to be carried away by convection.
The cable can radiate heat to the surroundings.
No additional thermal barriers.
Ampacity effect: This is the reference condition. The ampacity is the highest for a given cable size.
Derating: None (base ampacity applies, subject to ambient temperature correction).
Best for: Indoor installations, substations, industrial plants with good ventilation.
4. Installation Method 2: In Conduit or Trunking
What it is: The cable is pulled through a rigid or flexible conduit (pipe) or enclosed in a trunking (channel).
Key factors:
Conduit traps heat, limiting air circulation.
The thermal resistance of the conduit reduces heat dissipation.
The conduit acts as an insulating layer, raising the effective ambient temperature.
Ampacity effect: Reduced by approximately 10–20% compared to free air.
| Conduit Type | Reduction in Ampacity |
|---|---|
| Steel conduit (in air) | ~15–20% |
| PVC conduit (in air) | ~10–15% |
| Trunking (with other cables) | ~20–30% |
Derating: Apply grouping and installation factors from standards such as IEC 60364 or NEC.
Best for: Building wiring, where cables need mechanical protection or are installed in walls or floors.
5. Installation Method 3: Buried Directly in the Ground
What it is: The cable is laid directly in a trench and covered with soil.
Key factors:
Heat is dissipated through the soil by conduction.
The soil's thermal resistivity (ability to conduct heat) is critical.
Moisture content of the soil affects thermal resistivity—dry soil is a poor conductor.
Ampacity effect: Generally lower than free air, but can be higher than conduit if the soil has low thermal resistivity and good moisture content.
| Soil Type | Thermal Resistivity | Ampacity Effect |
|---|---|---|
| Wet clay | Low | High (good cooling) |
| Moist loam | Moderate | Moderate |
| Dry sand | High | Significant derating required |
Derating: Apply soil thermal resistivity correction factors (Cs) and depth of burial correction factors (Cd).
Best for: Underground distribution networks, renewable energy farms (wind, solar), and long cable runs where trenching is feasible.
6. Installation Method 4: In Underground Ducts or Tunnels
What it is: The cable is installed in a protective duct (pipe) or tunnel underground.
Key factors:
Duct traps heat, reducing heat dissipation.
The duct material adds thermal resistance.
Heat must be conducted from the duct to the surrounding soil.
Ampacity effect: Lower than direct burial because of the additional thermal insulation of the duct.
| Duct Type | Ampacity Effect |
|---|---|
| PVC duct | Significant reduction (~15–25% compared to direct burial) |
| Steel duct | Moderate reduction (~10–15%) |
| Concrete tunnel | Moderate reduction, depends on size and ventilation |
Derating: Apply duct correction factors and soil thermal resistivity factors.
Best for: City distribution networks, where cables must be protected from mechanical damage and future excavation.
7. Installation Method 5: In Cable Trays with Other Cables (Bundling)
What it is: Multiple cables are installed side-by-side in a cable tray or ladder rack.
Key factors:
Cables heat each other (mutual heating).
The closer the cables, the greater the heating effect.
The number of circuits and their arrangement (single layer, multiple layers) affects the derating.
Ampacity effect: Significant reduction, especially for tightly packed cables.
| Number of Cables in Group | Derating Factor (Example) |
|---|---|
| 1 | 1.00 |
| 2 | 0.91 |
| 3 | 0.87 |
| 4 | 0.82 |
| 5 | 0.79 |
| 6 | 0.76 |
Derating: Apply grouping correction factors (Cg) from standards.
Best for: Industrial plants, substations, and other installations with many cables in a common route.
8. Installation Method 6: In Thermal Insulation
What it is: The cable is run through thermal insulation, such as in a wall cavity, loft, or under floor insulation.
Key factors:
Thermal insulation traps heat, preventing dissipation.
The cable may be in contact with insulation, which has very low thermal conductivity.
This is one of the most severe installation conditions.
Ampacity effect: Very significant reduction—up to 50% or more.
Derating: Apply thermal insulation correction factors (Ci) from standards. The derating can be as low as 0.5 for cables in thermal insulation.
Best for: Avoid running cables through thermal insulation if possible. If unavoidable, use larger cables or install them in a protective conduit with adequate spacing.
9. Summary of Derating Factors
| Installation Method | Derating Factor Range | Comments |
|---|---|---|
| Free air | 1.00 | Reference condition |
| Clipped to surface | ~0.90–0.95 | Slight reduction due to reduced convection |
| Conduit in air | ~0.80–0.85 | Conduit traps heat |
| Direct buried | ~0.80–0.95 | Depends on soil thermal resistivity |
| Underground duct | ~0.70–0.80 | Duct adds thermal resistance |
| Cable tray (single layer) | ~0.80–0.90 | Depends on spacing |
| Cable tray (multiple layers) | ~0.60–0.75 | Significant mutual heating |
| In thermal insulation | ~0.50–0.70 | Very poor heat dissipation |
10. How to Correctly Apply Derating Factors
The process for calculating the effective ampacity is:
Determine the base ampacity (from standards, for the reference condition—free air).
Apply ambient temperature correction (Ca) if the ambient temperature differs from the reference.
Apply grouping correction (Cg) if cables are installed together.
Apply soil thermal resistivity correction (Cs) for buried cables.
Apply depth of burial correction (Cd) for buried cables.
Apply thermal insulation correction (Ci) if applicable.
The effective ampacity = Base ampacity × Ca × Cg × Cs × Cd × Ci (etc.).
Example:
A 25 mm² XLPE copper cable in free air has a base ampacity of 120 A. It is installed in a group of 4 cables (Cg = 0.82) in an ambient temperature of 40°C (Ca = 0.91 for XLPE). The effective ampacity is:
Iz = 120 × 0.82 × 0.91 = 120 × 0.746 = 89.5 A.
This means the cable can carry only about 89.5 A in this installation, compared to 120 A in free air.
11. Common Mistakes and How to Avoid Them
| Mistake | Consequence | How to Avoid |
|---|---|---|
| Ignoring grouping factors | Overheating, cable failure | Always apply grouping factors when cables are installed together. |
| Using free-air ampacity for buried cables | Cables may overheat | Use correct derating factors for the installation method. |
| Not considering soil thermal resistivity | Underestimate ampacity reduction | Check soil type and apply Cs factors. |
| Installing cables in thermal insulation without derating | Severe overheating | Avoid running cables through insulation or use larger cables. |
| Not applying ambient temperature correction | Derating not applied correctly | Always check the ambient temperature and apply Ca factors. |
12. The Role of Standards
The following standards provide detailed installation methods and derating factors:
IEC 60364-5-52 – Electrical installations, selection and erection of wiring systems.
IEC 60502 – Ampacity tables for power cables.
NEC (US) – National Electrical Code, with tables and correction factors for different installation methods.
BS 7671 (UK) – IET Wiring Regulations, appendix 4.
Always refer to the relevant standard for your region and application.
The installation method is not just a logistical detail—it is a critical design variable that directly affects the cable's ability to carry current safely. A cable sized correctly for free air may be dangerously undersized when installed in a conduit, bundled with other cables, or buried in dry soil.
By understanding how different installation methods affect heat dissipation and applying the appropriate derating factors, you can ensure that your cables operate safely, efficiently, and reliably for their intended service life. The installation method matters—treat it as seriously as the cable itself.