What are the rules for selecting neutral and protective conductor cross sections?

1. The neutral conductor shall have the same cross-section as the phase conductor when there are the following conditions:

(1) Any cross-section of single-phase two-wire circuit;

(2) three-phase four-wire and single-phase three-wire circuit, the phase conductor cross-section is not greater than 16mm2 (copper) or 25mm2 (aluminum).

2, three-phase four-wire circuit, the phase conductor cross-section is greater than 16mm2 (copper) or 25mm2 (aluminum) and meet all of the following conditions, the neutral conductor cross-section can be smaller than the phase conductor cross-section:

(1) In normal operation, the expected maximum current of the neutral conductor is not greater than the permissible current-carrying capacity of the reduced neutral conductor cross-section.

(2) For TT or TN systems, where the neutral conductor cross-section is smaller than the phase conductor cross-section, overcurrent protection corresponding to the cross-section of the conductor is required to be installed on the neutral conductor, which shall cause the phase conductor to be de-energized but without disconnecting the neutral conductor. Overcurrent protection is not required on the neutral conductor when the following two conditions are met:

--The protection of the circuit phase conductor is already capable of protecting 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, the cross-section can be determined by one of the following methods:

(1) when the cut-off time in 0.1 ~ 5s, the cross-section of the protective conductor should be determined in accordance with the following formula:

Where S - cross-sectional area (mm2);

I - the fault current (square root mean value) when a fault with negligible impedance occurs (A);

t - the time for the protection appliance to automatically cut off the power supply (s);

K - depends on the protective conductor, insulation and other parts of the material as well as the initial and final temperature coefficients, can be calculated and selected in accordance with the current national standards, “the selection of electrical equipment and installation of grounding configuration, protective conductors and protective coupling conductor” GB16895.3.