Cable temperature calculation: formulas, tables, and practical guide
In the electrical sector, the correct sizing of cables is essential to ensure the safety of installations and energy efficiency.
An often overlooked but highly important aspect is verifying that the expected current does not cause the maximum operating temperature of the cable to be exceeded, which is defined based on the characteristics of the insulation material.
The correct methodological approach is structured as follows:
A direct calculation of cable temperature is therefore not performed.
This parameter directly affects current-carrying capacity, insulation lifespan, and the prevention of short circuits or malfunctions.
In this in-depth guide, we will examine what a cable’s maximum operating temperature is, how it is calculated, relevant standards, and useful tools for safe and compliant design.
The temperature of an electrical cable represents the heat generated internally due to the Joule effect (I²R) and the heat absorbed or dissipated to the surrounding environment. Excessive temperature rise can compromise insulation integrity, reduce cable lifespan, or create fire hazards. CEI standards define precise limits for maximum allowable temperature depending on the insulation material:
| Cable type | Maximum operating temperature |
|---|---|
| Heat-resistant PVC | 90°C and 105°C |
| Silicone | 180°C |
| PTFE | 260°C |
Complying with these limits is essential to avoid overheating and ensure proper system operation.
Several factors contribute to the final temperature of a cable. The main ones are:
Cable temperature calculation is based on formulas describing thermal equilibrium between generated and dissipated heat. One commonly used formula is:
T = Ta + (I^2 × R × K)
Where:
It should be noted that this formula is a theoretical simplification, useful for illustrating the concept of thermal balance, but it is not used in real sizing calculations according to CEI or IEC standards.
In practical design, current-carrying capacity tables provided by CEI standards or manufacturers are used, together with correction factors for different installation conditions. In more complex cases, especially in industrial applications, advanced thermal models or FEM (Finite Element Method) calculations may be used.
CEI-UNEL standards, in particular CEI 64-8 and CEI UNEL 35024, provide reference tables for cable current-carrying capacity based on temperature and installation method.
The values shown in the table below refer to PVC-insulated cables (maximum operating temperature of 70°C) and are for illustrative purposes:
| Cross-section [mm²] | Current capacity in air (70°C) [A] | Buried capacity (70°C) [A] |
|---|---|---|
| 1.5 | 18 | 21 |
| 2.5 | 24 | 28 |
| 4 | 32 | 37 |
| 6 | 41 | 47 |
In addition, correction factors must be applied in cases of:
These factors are essential to determine the actual cable capacity under non-standard conditions.
For a simplified estimate of cable temperature in free air, the following empirical equation can be used:
T = Ta + (I^2 × R × 0.017)
This method is valid only for unshielded, non-grouped cables with good ventilation. Resistance values (R) are found in the manufacturer’s technical datasheets. For greater accuracy, results should be compared with CEI tables.
To avoid cable overheating, it is recommended to:
Cable temperature calculation is a crucial step to ensure the efficiency and safety of every electrical installation. Understanding the factors affecting temperature, correctly using formulas, and consulting CEI standards tables allows for more informed and reliable design.
Caviterm supports you in thermal cable sizing with professional tools, technical consulting, and compliance with CEI standards. Avoid overheating risks and ensure safe, compliant installations. Contact us for more information.
