The nominal cross section of each conductor has to be chosen that the current-carrying capacity is not smaller than the max. constant current, passing the conductor under normal conditions. The limit temperatures to which the current-carrying capacity refers to, shall not be exceeded for the insulation and sheath of the corresponding cable types. A defined condition is also the type of laying of the used cable. This has to be considered for the determination of the allowed load currents. Conditions that have to be considered are among others:
- Ambient temperature
- Cable accumulation and types of excess-current protection
- Heat insulation, wound up cables
- Current frequency
- Effects of harmonic waves
The current-carrying capacity is not the only criteria for choosing the cable section; furthermore, the requirements for the protection against harmful body currents, overload, short-circuit, currents and voltage drop have to be considered. In case that the cables are used for a longer period with temperatures exceeding the allowed values, they can be damaged considerably leading to an early failure and an important deterioration of its characteristics.
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