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
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.
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 – in other words they provide regulations that must be followed in order to comply with the Standard. However, complying with these clauses requires knowledge of the type and number of loads that will be in use after the installation is put into service – unfortunately, this information is rarely available. The Standard also includes an informative annex – information provided to help the designer in the form of guidance and recommendation rather than regulation – that provides a methodology for sizing cables correctly.