1. Under the following conditions, the neutral conductor should have the same cross-section as the phase conductor:
1) Single-phase two-wire circuit of any cross section;
2) In three-phase four-wire and single-phase three-wire circuits, the cross-section of the phase conductor shall not exceed 16mm2 (copper) or 25mm2 (aluminum).
2. In a three-phase four-wire circuit, the cross-section of the neutral conductor can be smaller than the cross-section of the phase conductor when the cross-section of the phase conductor is greater than 16mm2 (copper) or 25mm2 (aluminum) and all the following conditions are met:
1) During normal operation, the expected maximum current of the neutral conductor is not greater than the allowable current carrying capacity of the reduced neutral conductor section.
2) For TT or TN systems, where the neutral conductor section is smaller than the phase conductor section, an overcurrent protection corresponding to the conductor section should be installed on the neutral conductor. This protection should cut off the phase conductors but not necessarily disconnect them. Sexual conductor. When the following two conditions are met, the neutral conductor does not need to be installed with overcurrent protection:
——The protective device of the loop phase conductor has been able to protect the neutral conductor;
——The maximum current that may pass through the neutral conductor during normal operation is significantly less than the current carrying capacity of the conductor.
3) The cross section of the neutral conductor is not less than 16mm2 (copper) or 25mm2 (aluminum).
3. The protective conductor must have a sufficient cross-section, and its cross-section can be determined by one of the following methods:
1) When the cut-off time is between 0.1 and 5s, the cross-section of the protective conductor should be determined as follows:
In the formula, S——cross-sectional area (mm2);
I——The fault current (root mean square value) when a fault with negligible impedance occurs (A);
t——the time (s) for the protection appliance to automatically cut off the power supply;
K—depends on the material of the protective conductor, insulation and other parts, as well as the coefficient of initial temperature and final temperature. It can be calculated and calculated according to the current national standard “Electrical Equipment Selection and Installation Grounding Configuration, Protective Conductor and Protective Bonding Conductor” GB16895.3 Select.