Selection Based on Continuous Operating Current
Determine the continuous operating current of the cable by considering factors such as the installation method and ambient temperature, then select an appropriate cable cross-section. This selection can generally be made by referring to a cable ampacity (current-carrying capacity) table. For example, in a three-phase, four-wire low-voltage power distribution system-assuming an ambient temperature of 25°C and direct-burial installation-the cable cross-section should be chosen based on the load current.
Verification of Short-Circuit Thermal Stability
To ensure that a cable is not damaged during a short-circuit fault, its short-circuit thermal stability must be verified. Calculate the minimum allowable cable cross-section based on the system's short-circuit current and the duration of the short circuit; the selected cable cross-section must be no less than this calculated minimum.
Verification of Voltage Drop
In cases involving long-distance power supply or heavy loads, it is necessary to verify the voltage drop across the cable. Ensure that the voltage drop at the load end does not exceed the permissible limit during normal operation. Generally, for low-voltage power distribution systems, the voltage drop should not exceed 5%.
