Calculation of cable section and conversion factors for cables
Current carrying capacity of electric cables
For the cable dimensioning the nominal cable section of each conductor has to be chosen in a way that the current carrying capacity is not smaller than the max. continuous current that traverses the conductor under normal conditions. The limiting temperatures the current carrying capacity refers to, shall not be exceeded for the insulation and sheath material of the respective cable types. Also the laying of used cable belongs to the defined conditions. This is important for the determination of the admissible load current.
Conditions that have to be considered are among others:
- allowed operating temperature at conductor
- ambient temperature
- accumulation, bundles and laying of the cable
- number of cores
- wound up cables
- current frequency
- impacts of ripple currents (deviating from 50 Hz)
Calculation of cable section:
The cable section shall not only be chosen according to the current carrying capacity. Furthermore, also requirements for the protection against dangerous body currents, overload and short circuit currents as well as voltage drop have to be considered. In case that cables are operated above the recommended application temperature for a longer time, huge damage can be caused that possibly leads to an early failure and to a considerable deterioration of the cable characteristics.
for help: conversion tables
Examples for the calculation of the maximum current carrying capacity
according to VDE 0298-4:2023-06
1. Example for the calculation of the current carrying capacity
| cable type: | CC 500 |
| item no.: | L0200-0315 |
| temperatur range: | up to +70°C |
| nominal voltage | Uo/U: 300/500V |
| construction: | 3 G 1,5 mm² |
| application: | For this installation cable a current carrying capacity at + 50°C ambient temperature, two charged cores and an installation with 3 other cables on the floor shall be calculated. The four cables have the same dimension and are in contact with each other. |
Tables of VDE 0298-4: table 10, 11, 17 and 22 have to be considered:
- current carrying capacity for two cores with a section of 1,5 mm² each charged at the same time at a temperature up to + 30°C acc. to table 10 and 11/5: 18A
- factor for deviating ambient temperature +50°C acc. to table 17: 0,71 (18A x 0,71 ≈ 12,8A)
- factor for the laying of four of these cables on the floor in contact: 0,75 (12,78A x 0,75 ≈ 9,6A)
The current load for the described cable shall not exceed a value of 9,5A.
2. Example for the calculation of the current carrying capacity
| cabel type: | DR 720 P |
| item no.: | L0720-0640 |
| temperature range: | up to +90°C |
| nominal voltage: | Uo/U: 0,6/1 kV |
| construction: | 6 x 4,0 mm² |
| application: | For this reeling cable a current carrying capacity at + 65°C ambient temperature, five charged cores and an installation in three layers on a motor reel shall be calculated. |
Tables of VDE 0298-4: table 10, 11, 18, 27 and 28 have to be considered:
- current carrying capacity for a nominal section of 4,0 mm² at a temperature up to + 50°C acc. to table 10 and 11/5: 34A
- factor for deviating number of cores (five cores) acc. to table 27: 0,75 (34A x 0,75 = 25,5A)
- factor for deviating ambient temperature +65°C according to table 18: 0,79 (25,5A x 0,79 ≈ 20,1)
- factor for the installation in three layers on a motor reel acc. to table 28: 0,49 (20,1A x 0,49 ≈ 9,9A)
The current load for the described cable shall not exceed a value of 9,8A.
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