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Current rating and dimensioning of cables


The dimensioning of nominal conductor cross-sections to obtain the current rating in relation to the load in uninterrupted operation is a very complex matter.
When selecting, dimensioning and using cables according to their intended purpose, various influencing factors must be taken into account in calculating the nominal conductor cross-section. These are generally normative provisions for the installation types, individual usage conditions, and operating states at the installation site.


As a manufacturer of cables and other system-relevant products, for reasons of insurance law, LAPP is not permitted to interpret the diverse and customer-specific requirements. Accredited planning agencies must be involved here to confirm acceptance of the installation on the basis of official documents.


Nevertheless, with this guide, we would like to support you by assisting in the safe use of our products.

Standards


The basis for calculating current loads and cross-sections of cables is the international standard IEC 60364-5-52 (International Electrotechnical Commission). This standard deals with “Selection and erection of electrical equipment – wiring systems”.


In Europe, this standard has been transposed into harmonization document HD 60364-5-52, Electrical Installations of Buildings.


In Germany, the original text of the HD has been adopted in DIN VDE 0100-520. In addition, national amendments not included in the original version of the HD have been added.


The permissible current ratings and installation types were later combined in DIN VDE 0298-4. This therefore represents a mixture of national and international directives for Germany.


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 rating and the corresponding nominal conductor cross-section.


This standard is based on European harmonization document HD 603 or the IEC 60287 series.

Note

Different values may appear in other countries and regions due to differing national regulations. As a result, DIN VDE 0298-4 cannot be applied to other countries in general, but must be individually checked by the customer.

Environmental influences and reduction factors


Temperature


NOTE: The ambient temperature must always be below the conductor temperature, otherwise there is no heat exchange.

  • The operating temperature is the maximum permissible temperature at the conductor in uninterrupted operation (specified in the data sheet).
  • The ambient temperature is the temperature of the surrounding medium. The base load capacity for installation in air is an ambient temperature of +30°C

Influencing factors


  • Overcrowding of cables and circuits
  • Number of loaded conductors
  • Insulating compound
  • Voltage class
  • Ambient temperature differing from +30°C
  • Coil cords

Example cross-section calculation


When determining a suitable nominal conductor cross-section, taking reduction factors into account, the operating current of the plant is taken as the starting point for calculation. You divide the operating current by the respective reduction factors. The result represents a fictitious current load, with which you select the next higher value in the table of basic current loads  and thus arrive at an approximate nominal cable cross-section.

Table of basic current loads

Given


ÖLFLEX CLASSIC 110 (conductor temperature for permanent installation: 80°C)

  • Installation type selected: permanent installation
  • Operating current: 10 A
  • Number of conductors under load: 3
  • Number of cables in installation pipe: 3 (Table 12-6 factor 0.70)
  • Differing ambient temperature: 40°C (Tabelle 12-2 factor 0.89)

Calculation


10 Ampere ÷ 0.70 ÷ 0.89 = 16.1 Ampere (fictive)


According to Table 12-1  (DIN VDE 0298-4 Table 11), this value of 16.1 Ampere (with 18 Ampere in the table) would result in a nominal cross-section of 1.5 mm2


For a given cross-section, the reduction factors must be multiplied by the current rating of the nominal cross-section according to Table 12-1  (DIN VDE 0298-4 Table 11).


NOTE: If single-conductor, touching or bundled cables are overcrowded on surfaces, an additional reduction factor must be used before applying the reduction factors. DIN VDE 0298-4 Table 10