Current carrying capacity and dimensioning of cables and lines
The dimensioning of nominal conductor cross-sections for the current-carrying capacity with regard to the load in undisturbed operation is a very complex issue.
When selecting, dimensioning and using cables and lines as intended, various influencing factors must be taken into account when calculating the nominal conductor cross-section. As a rule, these are normative bases for the types of installation, individual conditions of use and the operating status of the installation.
As a manufacturer of cables, wires and other system-relevant products, LAPP is not allowed to interpret the diverse and customer-specific requirements for insurance reasons. For this purpose, accredited planning offices are to be involved, which confirm the acceptance of the installation with official documents.
Nevertheless, we would like to support you with this guide to help you use our products safely.
Norms
The basis for the calculation of current loads and cross-sections of cables and lines is the international standard IEC 60364-5-52 (International Electrotechnical Commission). This standard deals with the "Selection and erection of electrical equipment - wiring systems".
In Europe, for example, this standard was transferred to the harmonization document HD 60364-5-52, Electrical Installations of Buildings.
In Germany, the original text of the HD was adopted in DIN VDE 0100-520. In addition, national additions that are not contained in the original version of the HD have been added.
The permissible current-carrying capacities and laying types were later bundled in DIN VDE 0298-4. It is therefore a mix of national and international guidelines for Germany.
NOTE: In other countries and regions, different national regulations may result in different values. The DIN VDE 0298-4 cannot generally be transferred to other countries, but must be checked individually by the customer.
For power distribution cables with a nominal voltage of 0.6 / 1 kV (e.g. NYY), DIN VDE 0276-603 is the normative basis for calculating the current carrying capacity and the corresponding nominal conductor cross-section.
The basis for this standard is the European harmonization document HD 603 or the IEC 60287 series.
Environmental influences and reduction factors
temperature
- The operating temperature is the highest permissible temperature on the conductor with undisturbed operation (information in the data sheet).
- Ambient temperature is the temperature of the surrounding medium. The basic load capacity when laying in air is an ambient temperature of +30 ° C
NOTE: The ambient temperature must always be below the conductor temperature, otherwise no heat exchange can take place.
Influencing factors
- Accumulation of lines and circuits
- Number of loaded cores
- Insulating compound
- Voltage class
- Ambient temperature deviating from + 30 ° C
- Coiled cables
Example cross-section calculation
When determining a suitable nominal conductor cross-section, taking reduction factors into account, the system's operating current is the starting point for a calculation. They divide the operating current one after the other with the reduction factors. The result shows a fictitious current load, with which you select the next higher value in the current load basic table and thus arrive at an approximate nominal cross-section of the line.
Given:
| ÖLFLEX CLASSIC 110 (fixed conductor temperature 80 ° C) | |
|---|---|
| Operating current | 10 A |
| Number of loaded cores | 3 |
| Number of lines in the conduit | 3 (Table 12-6 factor 0.70) |
| Deviating ambient temperature 40 ° C | (Table 12-2 factor 0.89) |
Bill :
10 amps ÷ 0.70 ÷ 0.89 = 16.1 amps (fictitious)
According to Table 12-1 (DIN VDE 0298-4, Table 11), this value of 16.1 amperes would result in a nominal cross-section of 1.5 mm² with 18 amperes.
In the case of a given cross-section, the reduction factors must be multiplied by the current carrying capacity of the nominal cross-section according to Table 12-1 (DIN VDE 0298-4 Table 11).
NOTE: If there are accumulations of single-core, touching or bundled cables on surfaces, another reduction factor must be used before the reduction factors are used (DIN VDE 0298-4, Table 10).