Current in Neutral in a 3-Phase System!

Did you ask the designer?

Sizing and Protection of the Neutral Conductor (1) | EEP

My confusion lies in the fact that why isn't there any current in neutral? Leave everything behind. I've searched google ( I knew you would suggest that :P). Consider a balanced system consisting of a generator an 3 lighting circuits. This load is 10 meters from the panel board. The common neutral at the panel board is grounded and it carries 0 A ideally. But all the way from the bulb to panel board, the neutral should carry the same current as the phase. Why isn't it so?

Currents can and often do flow inter-phase. You may find your missing currents consumed by the out of phase legs.

Review your 3 phase theory.

I did not suggest that you search Google.

I performed the search for you and referenced a document which explains how to properly size the neutral conductor.

Speak to Mr Kirchoff, he wrote the law.

Thanks for the tip. It was enlightening.

Check for harmonics in the neutral. The results may give you a small indication that the neutral is indeed being utilized, just not in the way that you think.

10cos(0) + 10cos(-120) + 10cos(120) = 0

If this forum starts answering and making detailed report of such questions ... there need not be an engineering college and no student needs study in the college. no more text and reference books are required ... let there be a forum of this nature and some experts to answer such questions.

Hope the xperts agree with this view.

Hear hear!

There are others here that assume we are here to do the bidding of all those not equipped to read the book or perform a cursory search before coming to the well of knowledge to drink.

I can cite hundreds of examples of those who are simply too lazy to do the work, yet come here expecting us to take the time to educate them, but I won't.

How would you know the experts from the journeymen from the charlatans from the con artists? We've had 'em all! You don't know us from Adam's housecat. Truly.

My advice: stick with a rigorous syllabus at a reputable school and stay abreast of things. Never stop learning.

To make it short, there could be a hidden trick to it which may have forced the designer to have done such...Blah blah blah....

For simple lighting ckt, RYB, Blk, Y/G -all 1.5 mm^2

For heaving lighting ckt, YYB, Blk, Y/G -all 2.5 mm^2

In the absence of 3x harmonics, the maximum current in a 3-phase neutral is 1/√3 (~58%) of the line-to-line current. Cables for this condition are often called 3.5 conductors, because of the reduced neutral size.

GA.

Question: In the absence of harmonics, yes. In the presence of harmonics, what is typical? Worst-case? Just curious. So many loads now contain active electronics, ie, switching power supplies in computers, CFLs and LED bulbs, lamp dimmers, VFDs, the list is endless - and all generate shitloads of harmonics, as you know. What is typical? Worst case (barring failure modes).

A hobby of mine, btw, is ELF/ULF/VLF radio. On spectrograms you can see power-line harmonics all the way to 4.8 kHz and beyond. They're the absolute bane of anyone working at these frequencies.

If the currents in 3 phases are balanced the current in the neutral will be zero,that's why size of N conductor was small to cater for any unbalanced current only. Nowadays due to harmonic generating loads sometimes current in N is more than phase current. Therefore size of N should be bigger than phase conductor. The designer should know what kind of loads will be connected to the system.

Don't jump to the conclusion please. Have the courtesy to read the question carefully. Anyway, thanks for answering.

3 phase +N supply with single phase branch circuits right?

The single phase branch circuits will have the same current in the phase and neutral conductors they should be the same size.

There are 3 single phase circuits in a three phase shystem i.e one in each phse. Should it not tenmtamount to three phse unbalanced load. And only unbalance portion will be in neutral by default. But if a phse fails the load inneutral will change so needs calculation for worsty situation. May be it turns out to be 50% 0f phase load at its worst.

That would be my take on it too. Other posters have referred to the current in the common neutral, but the OP wasn't asking about that.

smi 1989 - is the cable length shorter on the neutral side than on the supply? If so maybe it has been designed for same voltage drop.

If the lighting is of the type that can generate 3rd harmonics then the neutral might need to be bigger.

If the lighting load on 1 phase is fully on, and the other 2 phases switched off then the neutral should be the same size.

If the lighting load is always perfectly balanced then you might not need a neutral at all.

...and the designer might know - if you ask him.

Even though the loads are balanced, how the single phase light will be connected without a neutral ?

As I have said earlier, and I repeat:

1) For normal light lighting circuit: R or Y or B or Blk (N) or Y/G (Earth) are all of 1.5mm^2 of wire needed.

2) For normal heavy lighting circuit: R or Y or B or N (Blk) or earth (Y/G) are all of 2.5 mm^2 of wire needed.

3) Neutral is a MUST.

4) So many questions arises: The neutral is a must which runs from the source DB straight to the POINT (light point)...l

5) OP said RYB where R, Y, B are THREE lighting circuits i.e each single phase run circuit from the 3-phase...Actually consider each a single phase i.e R phse, the second circuit on Y-phase, etc. Mostly MCB are single phase...However, each circuit could be run separate from even a MCB (3-phase)i.e one lighting R circuit, 1 Y circuit, and B circuit...

6) Etc. as I amm getting confused further...Joking to be using 6mm^2 for simple lighting circuit? Unless I am not aware of something...Hahahaha...

Why do I say a "must"...

<please note the term "more usual" below...usual means regular or habitually or typically occurring or done; customary...>
Cut 'n paste:
It is more usual however, to connect dissimilar loads to each phase of a star system. This will result in differences in phase currents which in turn will create some 'leftover' current which will flow back along the 4th wire (neutral) to the star point of the supply origin. Such a system is known as an unbalanced system and usually arises as a result of connecting separate single phase loads to each phase of a three phase star connection.

Find the word "simple" in the original post.

We have no idea how that lighting circuit is implemented. We know nothing about it *at all.*

We ask our OPs to be clear and concise, and many are, thankfully, but when we add to or change what the OP has written, it is no longer the OP's question we are answering, but our own.

With a 3-phase load connected in star the neutral wire carries the unbalanced current.

If each of the 1-phase loads are exactly the same there will be no imbalance and hence no current flowing through the neutral wire. You can take it away.

In fact you could run the lights without a neutral even when unbalanced. Funny things will happen to the brightness of the 'on' lights as as the phase currents re-balance themselves to compensate for a lack of neutral wire.

Yeah...That's the quesiton. If there is no neutral wire, how is the circuit complete? At the star point, the vector sum is zero. But when I am talking about single phase ligthing cicuits connect to different phases R,Y,B. Since the lights are single phase, so the current has to return back to the Panel board via something ????????

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OP is asking about the current in each neutral leg before the common neutral point.

As it's lighting, each of the 3 systems is likely to consist of a number of outlets which guys can switch on and off as required. So I think a common neutral wire is needed.

The simplest way for you to understand that is to actually draw out how the cabling is laid and the lengths involved. Do not mix up neutral at a socket or anywhere on a single phase circuit, with the neutral going to the substation...

It is difficult to be certain as code is different in various countries and I don't know them all!!!

The center connection of the three phases, where the neutral comes from each single phase circuit, is the "Central-Neutral-Point" so to say. It is usually in the building concerned, but possibly might not be....I assume that it is in the building....

This is also connected to the neutral/ground from the substation.......this is the point at which, if all loads are identical on all three phases, that there will be no current flow from that point to the substation vial the neutral/ground cable. (This is usually grounded to some form of ground spear or similar....) Power will only flow down each of the three phases in such (unusual!) circumstances.....

It is a theoretical effect that will seldom be seen in real life......but it is important to understand how it happens and that the single phase circuits (neutral & phase), only connect to a phase and that neutral point.

The neutral that the OP is talking about (if I understood him correctly!), is the one going from the "Central-Neutral-Point" to the substation, The one I call the "Neutral-Ground" wire.......

I personally (got to allow for local code!!), would aim to allow this to be at least as large as each phase wire....as in many things in this life, I feel that bigger is also better!!!

If a fuse blew for one phase and all the single phase loads on the other two phases remained in operation, the angle of the phase currents would be at 120° to each other, but it could be possible that the total current in the neutral, exceeds the current of a single phase.......though if correctly sized, this should not be at a dangerous level.....

I hope that this helps a little......have a great day.

You wrote "The neutral that the OP is talking about (if I understood him correctly!), is the one going from the "Central-Neutral-Point" to the substation, The one I call the "Neutral-Ground" wire......."

I don't think you did understand him correctly! His original post says ".....when we look into the circuit, the current reaching the load has to go somewhere, I mean the the current must flow in neutral between the load and the common neutral grounded point."

It's mentioned in several posts, including mine.

Serious error in understanding, you didn't look to see who I was replying to.

Look now and you might make more sense of my post!!! His name is Mustafa if I remember correctly...

I was trying to help HIM!!!

But you said "The neutral that the OP is talking about (if I understood him correctly!)......." which sounds to me like you're referring back to the OP (and his original post) even though the reply is to Musthafa.

But don't let's fall out over it, it's not important .

My personal take is still that the OP is talking about the neutral/ground wire, not the neutral at the load.....but I have been wrong before, so I was just doing a "CMA" move......just in case that was not quite true, but I was replying to Mustafa.....not difficult to check....

Mustafa is not alone with confusing these two "Neutrals", so I tried to distinguish them, one from the other for him and any others who are confused......its not easy for people who have never studied Electrics to understand I feel.....or who has not studied logics either!!

Have a great day.

OP is not questioning the size of the common return neutral, he is questioning the neutral/white wire coming out of the other side of a single phase load before the connection to the system neutral. If the designer has #12 as the black wire and #14 as the white, then OP is correct, it should be the same size as the hot leg/black. If a light bulb draws 15 amps he rightfully expects 15 amps coming out and both wires sized to carry that current, it doesn't matter whether it's a three phase or single phase system.

exactly

In a star connected balanced load the neutral has no current flow. That is why cables are normally "3 1/2 core". The smaller section of the neutral conductor is because only the unbalanced current will flow in it.( so cost can be reduced )

It is advisable to connect the "neutral" line to ground.

This is advisable as a break in the neutral wire does not expose the equipment to Phase to phase supply voltage.

Until such place as any idividual neutrals for each phase are combined, they need to be full size. If they are connected together at some junction box away from the distribution panel, the "home run" may, under certain circumstances, be smaller, depending on the vectorial sum of the combined currents, and on the presence of harmonics.

Wow...that pretty much *sums* it up!

common knowledge...less common than common sense

....and often much more correct...