TT and TN-S schemes a-Single-phase circuits or those of c.s.a. < 16 mm2 (copper) 25 mm2 (aluminium): the c.s.a. of the neutral conductor must be equal to that of the phases b-Three-phase circuits of c.s.a. > 16 mm2 copper or 25 mm2 aluminium: the c.s.a. of the neutral may be chosen to be: – Equal to that of the phase conductors, or – Smaller, on condition that: – The current likely to flow through the neutral in normal conditions is less than the permitted value Iz. The influence of triplen harmonics must be given particular consideration or – The neutral conductor is protected against short-circuit – The size of the neutral conductor is at least equal to 16 mm2 in copper or 25 mm2 in aluminium TN-C scheme The same conditions apply in theory as those mentioned above, but in practice, the neutral conductor must not be open-circuited under any circumstances since it constitutes a PE as well as a neutral conductor. IT scheme In general, it is not recommended to distribute the neutral conductor, i.e. a 3-phase 3-wire scheme is preferred. When a 3-phase 4-wire installation is necessary, however, the conditions described above for TT and TN-S schemes are applicable. Reference to sizing the neutral conductor in case of non-sinusoidal currents can be found in IEC 60364-5-524. Clause 524.2 indicates that the neutral conductor shall have at least the same section as the phase conductors: -in two-conductor single-phase circuits and for all conductor cross-sections -in multi-phase circuits and in three conductor single-phase1 circuits when the cross-section of the phase conductors is equal to or less than 16 mm2 for copper or 25 mm2 for aluminium. Clause 524.3 states that, for other multi-phase circuits, the neutral conductor may have a reduced cross section if all the following conditions are met: – the maximum expected current, including harmonics, if any, in the neutral conductor during normal service is not greater than the current carrying capacity of the reduced cross-sectional area of the neutral – the neutral conductor is protected against overcurrent – the size of the neutral is at least 16mm2 in copper or 25mm2 in aluminium. These clauses are normative