The neutral wire size or cross sectional area requirement in the circuit is a tricky thing to handle.The phase conductor size is decided simply as per ampacity, temperature and load characteristics. We can consider two cases to decide the neutral wire size or cross sectional area requirements-
Concept of neutral wire size or cross sectional area requirement
Case 01In two wire single phase circuit neutral wire is a part of the live circuit. So reasonably size of the neutral wire can be same as the phase.
Case 02But in case of poly phase circuit where neutral is grounded connecting the neutral point such as in the case of transformer in LT side, and in Alternator. This is done to ground the fault current produced due to unbalanced condition of balanced three phase circuit.
So from case 01, it is reasonable to choose the neutral conductor size or cross sectional area same as the phase but in case 02, the neutral conductor has no regular use other than to by pass the fault current in the balanced three circuit. Thus from the scenario of case 2, the question rises here about the size of neutral wire as we have to consider the costing.
In simple words what would be the cross sectional area or sizing of the neutral conductor in three phase balanced circuit if say the phase is 120rm cable. Some engineers use 60% that is 70 rm neutral cable size for 120 rm phase phase conductor. Some engineers do use same size of neutral and phase wire in the circuit.
There are lots of group discussion and forum post about the sizing or cross sectional requirements of the neutral conductor. Discarding the people’s own concept we will stick to the the electrical codes.
Neutral wire size or cross sectional area as per electrical standard
The IEC 60364-5-52 has some straight direction for neutral conductor sizing concept. Below is the summary of the concept depicted in the code-
- The neutral wire will be treated as the live conductor(the precautions).
- The neutral conductor shall have the same cross-sectional area as the line conductor in single-phase, two-wire circuits whatever the section. And in polyphase and single-phase three-wire circuits, when the size of the line conductors is less than or equal to 16 mm2 in copper, or 25 mm2 in aluminium.
- For polyphase circuits where each phase conductor has a cross-sectional area greater than 16 mm2 in copper or 25 mm2 in aluminium neutral conductor may have a smaller cross-sectional area than that of the line conductors only if the neutral conductor has the ampacity of calculated maximum fault current or harmonic current.
- If the harmonic content is greater than 10 % the neutral conductor shall not be smaller than the phase conductors.
Direct quotation from IEC 60364-5-52 about Neutral wire size or cross sectional area
- in single-phase, two-wire circuits whatever the section;
- in polyphase and single-phase three-wire circuits, when the size of the line conductors is less than or equal to 16 mm2 in copper, or 25 mm2 in aluminium.
- the expected maximum 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 conductor;
NOTE The load carried by the circuit under normal service conditions should be practically equally distributed between the phases.
- the neutral conductor is protected against over currents according to the rules of 431.2 of IEC 60364-4-43;
- the size of the neutral conductor is at least equal to 16 mm2 in copper or 25 mm2 in
currents are given in annex D.