If the idea is that you have a distribution panel that has the 3 phases coming in and single phase circuits going out then if I understand you correctly, you are correct. The neutral of a single phase circuit should be the same size as the active (or hot). When all the single phase circuits combine and connect to the incoming 3 phase then at that point, no, the neutral is usually sized smaller than the phase conductors because the neutral has only to carry the unbalanced portion of the loads on the three phases. If the 3 phases all had identical loads the neutral current would be a theoretical zero Ampere. The balance is never perfect so the neutral is included where loads can be single phase. Unless you had one phase fully loaded and the other 2 with no load at all there should not be a problem.Dear all.
I am to install a three phase system for load consisting mainly single phase. I have divided the single phase loads in to three and fed each group with one hot from the three hots. But each group use the same return line the neutral, the problem is, shouldn't I tripple the thickness of the neutral because it cary all the phases currents? To worsening my problem standard cable neutral thickness is even smaller than the phase. I have attached sketch of this
Needless to say that in this distribution panel that takes the 3 phase to make a bunch of single phase circuits that the single phase circuits should all have a circuit breaker each which is sized and of the type to suit the cable and installation conditions of each of the single phase circuits. The main switch and main circuit breaker (if you use a breaker at that point) should be 3 or 4 pole types and not 3 individual switches or breakers.
Not knowing enough about the regulations in your part of the world nor the application I am not sure if this is required or kosher but I would consider an MEN (Earth to Neutral) link at the panel that breaks the 3 phase into single phase circuits.
Hope that helps in some way and isn't an insult to your intelligence