The neutral conductor of a circuit can generate many doubts and the first that must be clarified is that the neutral cable does have current when the circuit load is energized since the neutral is responsible for the return of the charges to the source and for this reason it has current when the load is in operation. Therefore, if the neutral conduct current, it also needs to be properly dimensioned to conduct it.
With that comes the doubt, can the neutral have the same section of the phase? Can the neutral be thinner than the phase? Can the neutral be thicker than the phase? What is the right to dimension the neutral cable? Thinking about it, we decided to make this article that will clear all these doubts, come on guys!
How to dimension the neutral?
There are two situations that are exclusive in three-phase circuits, the first is when the neutral can be thinner, the second is when the neutral has to be thicker than the phase cables, but it is important to understand that this will only happen in three-phase circuits. In this case, if the circuit is balanced, the current in the neutral will be as close to zero as possible.
With regard to a residential electrical installation that is not three-phase, if all circuits are correctly divided and protected, the current will be equal to or very close to the phase current and the neutral cable will always be the same as the phase cable. Understand that the neutral energized in theory can only occur in single-phase circuits, where the neutral is responsible for returning the loads to the source. This makes it easy to understand that the current difference between the phases and the neutral will only occur in unbalanced circuits!
Neutral thinner than the phase, when to use?
When a circuit is composed of only three-phase loads, such as motors, for example, there will tend to be an equilibrium and the neutral will always have current close to zero and in this case, the neutral cable may be thinner than the phase cable, after all only there will be current in the neutral in case of small phase imbalances. But it is important to understand that this thinner cable rule will only apply to cables with a section greater than or equal to 25mm².
The standard NBR5410 shows in table 48 which the reduction of cables and it still stipulates what are the rules for this to occur in item 126.96.36.199.6. In a letter a) it says that the circuit must presumably be balanced. In letter b) it says that the circuit must have third harmonic rates lower than 15% and in letter c) the neutral cable must be protected by overload, that is, the neutral must be protected by a circuit breaker. Therefore, the reduction of the neutral cable will only happen when you meet these 3 items.
Neutral thicker than the phase, when to use?
Annex F of the NBR5410 standard deals exclusively with this case and teaches how to dimension the neutral cable considering harmonic factors. There are more specific cases as shown in Appendix F, in which the neutral cable must be thicker than the phase cable and this will only occur when the third harmonic rate or its multiples are greater than 33%. If you want to know more about harmonics, there is a video on the Mundo da Elétrica channel that talks in detail what harmonics are and how they occur, it is very worth checking out!
The imbalance between the phases and the neutral in a circuit can cause an overload in the neutral, but under no circumstances should this be used as a justification to increase the dimensioning of the neutral. In this case, the phase balance must be corrected, reducing any possibility of overload in this circuit.