Lighting

I think the worst case is all the lighting on in two largest phases and the third off, i.e. If it's balanced load then it's the same as the full load current in either of the two phases, so cable should be equal to the phase cable size.

If only one phase is out. The neutral would see the vector sum of the other two phases and not the sum of the phases.

The vector sum of two equal phase currents, at 120 degrees phase difference is in fact equal to either of the two phase currents.

We should consider single line to neutral fault as well as double line to neutral fault too

to assess the maximum current that will flow in the neutral.The fault clearing time of the protective device(fuse/CB) too should be considered.

In 3Φ system [4wire distribution in Star with a N wire; Question under discussion]

If 2 or 3 phases have balanced no current flows through N. N gets only the unbalanced differance; while in only 1phase on load full current will flow through the N. This is well explained by Poster in #1 if we have a sense of humor.

Nanananananana.

Let the OP sort it out.

Wiring is not set by the load conditions. Wiring is set by your local code standards combined with the rating of the circuit breaker. I know that this is not the answer expected by a physics professor but it is the proper way to do electrical wiring. Oh and this should be done by an electrician licensed in the community that then gets inspected by the local inspectors.

Quite right, I think this guy was asking about proportion or ratio of the main to the neutral conductors. We can only assume that he knows what he is doing as far as regulations, local codes and ampacity.

Agreed. I'm certain that the real question was what will be the amperage found on the neutral wire from the source and not what was actually asked. But there are times that we should answer the actual question and not the likely intent. Besides, we don't know how the lighting load is actually getting distributed or sequenced from the source. So we cannot guess what the neutral current will be along the way. If nine lights were strung AAABBBCCC or ABCABCABC or AABCCBACB then there will be completely different neutral currents along the way even though no neutral current would exist from the source. That is if the lamps are single phase loads attached to a three phase system.

ASSUME???

The "intent" is as was stated.

The information applies to a Delta Star connection of three transformers for a low-voltage generator fed three-phase four wire system.

Full size cable for the neutral whether as a four core three phase and neutral main feeder or single phase and neutral subcircuits. This covers all possible combinations of lights on and off including the case of two phases on and one off. This is the way we do street lighting in my part of the world, no reduced neutrals.

The particular load, amps, cable sizing, ambient temperatures, type of cable mounting, conduit, NEC, IEE, normes francais, Building Regulations, Health & Safety, and common sense, all off topic. Whether we "assume" or "don't assume" that Anonymous #1 needs to know more on these points is also off topic.

I'm off to another thread, nice chatting.

Yes...thank you. I was considering doubling the neutral due to the prolific triplen induced THD. I am seeing some types of distortion on the current readings when security doors are opened and closed. I am thinking that the improper wiring of the interior transformer (ground not delivered to panel) is having some effect here, but thought I should check all avenues. Analysis is so costly I would like to have all ducks in a row before calling a consultant, to ensure no smoke blowing.

In addition to unbalanced load conditions neutral current increases due to harmonics too.Due to harmonics in some buildings neutral is sized twice the phase conductor.You should identify the harmonic generating loads first.

That is right. You should measure the currents including the harmonics. If your loads are mostly switching supplies (like computers and fluorescent tubes or LED lights), the harmonics add up to a very high value on the neutral. There are transformers/chokes available commercially, that leaves fundamentals thru, but enforce balance on harmonics, reducing such currents on the neutral. Codes regulate the harmonics leaving your premises and entering the power grid. Such a solution is much cheaper, than Power Factor Control in every device, but in a few years it may come to that.

If all else fails, try British Standard 7671.

If you mean 2 Phases and a Neutral common to both:

1- The Cable size to the distribution board (before distributing to the load) will have to be the same size as the biggest of the two single phase cables (Usually the 2 phases are the same size).

2- The cables to the loads: This will depend on the locations, distances and configuration of the circuits. The best practice is to use Separate Neutral wires for each run (or phase) emanating from the distribution board.Then the size of the cable will be the same as the phase cable.

3- Harmonics: Your question seems to involve lighting only. Depending on the type of lighting (electronic chokes, transformers with PWM etc), you might have some harminics. But if you use a 4 wire system (1 neutral line /phase), you will be OK to size the neutral to the phase cable size.

In any case, the local regulations will supercede any calculations if they dictate a bigger size cable than what is calculated. Also the practice of whether to use a common neutral in the circuit you are designing. Check that first with a local qualified electrician.