Why neutral wire is equal or thinner than phase wire in three phase ?

Thread Starter

meowsoft

Joined Feb 27, 2021
116
Why neutral wire is equal or thinner than phase wire in three phase ?

and in case where all load in three phase is single phase is larger neutral wire than phase wire is required ?, and how to calculate neutral wire capacity (amp) for case where transformer is three phase but all loads is single phase ?
 

Ian0

Joined Aug 7, 2020
1,618
Because the phase currents cancel out. If you put three equal unity-power-factor loads on a three-phase supply there will be no neutral current.
Add up the current vectors of the loads on each phase to give the neutral current:

for example: 10A on brown phase, 5A on black phase, 3A on grey phase. Take brown phase as being 0 degrees.

5A on black phase = -2.5 + 4.33j A
3A on grey phase = -1.5 - 2.6j A
total = 6 + 1.73j A = 6.2A @ 74 degrees.

The maximum neutral current occurs when only one phase is loaded (assuming unity power factor loads).
 
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Why neutral wire is equal or thinner than phase wire in three phase ?

and in case where all load in three phase is single phase is larger neutral wire than phase wire is required ?, and how to calculate neutral wire capacity (amp) for case where transformer is three phase but all loads is single phase ?
In all single phase loads the line and neutral conductors must be the same size.
However, where the single phase loads are drawn from a 3 phase supply the supply neutral may, sometimes, be reduced in size. This is because, as the single phase loads become near to being a 3 phase BALANCED load the return currents from each single phase load return through the other lines and not through the neutral conductor, eg. if three similar 10 Amp single phase loads were connected across the three lines, (and neutrals), the current flowing back on the supply neutral would be zero under these conditions. However, if the three loads were not similar i.e. different current values or different power factors then the neutral current would not be cancelled out to be zero. The size of the neutral conductor is therefore normally either the same size as the line conductors or reduced to a proportion of the line conductor size depending upon how BALANCED the single phase loads are.
However, there is a note of caution! Where the single phase loads produce harmonic currents these are not cancelled out by being balanced. This is especially the case where 3rd harmonics are present. These neutral currents are additive and are at 3 times the fundamental frequency. Such 3rd harmonic currents can cause heating within the neutral conductor! The design engineer needs to know the type of single phase loads that are likely to be present.
Hope this helps and provides you with some other areas of research.
Colin
Why neutral wire is equal or thinner than phase wire in three phase ?

and in case where all load in three phase is single phase is larger neutral wire than phase wire is required ?, and how to calculate neutral wire capacity (amp) for case where transformer is three phase but all loads is single phase ?
 

Thread Starter

meowsoft

Joined Feb 27, 2021
116
Because the phase currents cancel out. If you put three equal unity-power-factor loads on a three-phase supply there will be no neutral current.
Add up the current vectors of the loads on each phase to give the neutral current:

for example: 10A on brown phase, 5A on black phase, 3A on grey phase. Take brown phase as being 0 degrees.

5A on black phase = -2.5 + 4.33j A
3A on grey phase = -1.5 - 2.6j A
total = 6 + 1.73j A = 6.2A @ 74 degrees.

The maximum neutral current occurs when only one phase is loaded (assuming unity power factor loads).
Ok... my question is all load is single phase no three phase load, is I needs bigger neutral wire ?, absolutely no phase-phase load, only phase-neutral, in China single phase is limited in 100a, and my need's is 300a, so I use three phase 3x 100a, but all load is phase-neutral... ABSOLUTELY... so this is an unusual case... and don't think this is usual case
and how to calculate neutral wire capacity ? (in this case all phase is rated at 100a and I think my neutral needs is 300a)
 

Thread Starter

meowsoft

Joined Feb 27, 2021
116
In all single phase loads the line and neutral conductors must be the same size.
However, where the single phase loads are drawn from a 3 phase supply the supply neutral may, sometimes, be reduced in size. This is because, as the single phase loads become near to being a 3 phase BALANCED load the return currents from each single phase load return through the other lines and not through the neutral conductor, eg. if three similar 10 Amp single phase loads were connected across the three lines, (and neutrals), the current flowing back on the supply neutral would be zero under these conditions. However, if the three loads were not similar i.e. different current values or different power factors then the neutral current would not be cancelled out to be zero. The size of the neutral conductor is therefore normally either the same size as the line conductors or reduced to a proportion of the line conductor size depending upon how BALANCED the single phase loads are.
However, there is a note of caution! Where the single phase loads produce harmonic currents these are not cancelled out by being balanced. This is especially the case where 3rd harmonics are present. These neutral currents are additive and are at 3 times the fundamental frequency. Such 3rd harmonic currents can cause heating within the neutral conductor! The design engineer needs to know the type of single phase loads that are likely to be present.
Hope this helps and provides you with some other areas of research.
Colin
Ok... is I need's neutral wire only have same capacity with phase wire, or same capacity with total of each phase wire ?, let's say each phase is 100a, and is I need's wire with 300a capacity ?
 

Thread Starter

meowsoft

Joined Feb 27, 2021
116
In all single phase loads the line and neutral conductors must be the same size.
However, where the single phase loads are drawn from a 3 phase supply the supply neutral may, sometimes, be reduced in size. This is because, as the single phase loads become near to being a 3 phase BALANCED load the return currents from each single phase load return through the other lines and not through the neutral conductor, eg. if three similar 10 Amp single phase loads were connected across the three lines, (and neutrals), the current flowing back on the supply neutral would be zero under these conditions. However, if the three loads were not similar i.e. different current values or different power factors then the neutral current would not be cancelled out to be zero. The size of the neutral conductor is therefore normally either the same size as the line conductors or reduced to a proportion of the line conductor size depending upon how BALANCED the single phase loads are.
However, there is a note of caution! Where the single phase loads produce harmonic currents these are not cancelled out by being balanced. This is especially the case where 3rd harmonics are present. These neutral currents are additive and are at 3 times the fundamental frequency. Such 3rd harmonic currents can cause heating within the neutral conductor! The design engineer needs to know the type of single phase loads that are likely to be present.
Hope this helps and provides you with some other areas of research.
Colin
And how to calculate neutral wire capacity for case where 3rd harmonics are present ?, it's very likely in my project's case 3rd harmonics is present, because majority load is data center and CNC machine, and yes... I think 3rd harmonics is usually in IT case
 

crutschow

Joined Mar 14, 2008
26,775
In your other post you asked about using three transformers to isolate the phases to get single phase.
The problem is, the transformers don't change the phase of the signal, so you still have three signals that are out of phase.
If you combine their outputs, it will create a short circuit.
 

Ian0

Joined Aug 7, 2020
1,618
Ok... my question is all load is single phase no three phase load, is I needs bigger neutral wire ?, absolutely no phase-phase load, only phase-neutral, in China single phase is limited in 100a, and my need's is 300a, so I use three phase 3x 100a, but all load is phase-neutral... ABSOLUTELY... so this is an unusual case... and don't think this is usual case
If you put three single-phase loads one between each of the three lives and neutral on a three-phase supply, then you have a three-phase load. It is the most usual case.
Think of a street with several houses. The electricity company supplies the street with a three phase supply. Each house gets a single phase. First house gets brown phase, second house gets black phase, third house gets grey phase, fourth house gets brown phase etc. The neutral currents add as vectors (see post #2). If there are many houses, all using a random amount of current, it is most likely that all the currents will cancel out, and there will be no neutral current.
The neutral current will be at its maximum if houses 1, 4 and 7 are using electricity, and the other houses are using none.
 

schmitt trigger

Joined Jul 12, 2010
418
The problem are the harmonics. Other poster has already mentioned the triplets (3rd, 6th, 9th.....) which they add, not cancel, on the neutral.

Unfortunately to make an exact assessment, you would have to measure the harmonic content of each phase up to the 40th (which is 2Khz bandwidth) displace them 120 degrees and perform a vector sum.
Then perform a vector sum of the fundamental reactive currents.
Quite elaborate calculation.

A simplified and safe assumption would be that only the fundamental’s real component will cancel out. Everything else will be carried by the neutral.
 

crutschow

Joined Mar 14, 2008
26,775
A simplified and safe assumption would be that only the fundamental’s real component will cancel out. Everything else will be carried by the neutral.
And I think it would be also safe to assume that the sum of the harmonic currents of the three phases will vector add to be less than the maximum current of one phase, which the neutral should be designed to carry.
 

Thread Starter

meowsoft

Joined Feb 27, 2021
116
In your other post you asked about using three transformers to isolate the phases to get single phase.
The problem is, the transformers don't change the phase of the signal, so you still have three signals that are out of phase.
If you combine their outputs, it will create a short circuit.
Very wrong answer... OK... Can I use three independent-separated transformer with same output paralelly ? answer : YES. Let's say I have one single-phase transformer connected to Moscow city grid at Phase R - Neutral and run about 200 kilometers of wire to Vladimir city... and I have one more single-phase transformer in Vladimir city grid connected at Phase S - Neutral , can I connected them paralelly, answer : YES. I Question this to my technician Also ANSWER IS YES. You Are Not Understand My Question... Because... I Think... This Practice Isn't Usual In Your Country, Specifically Western Countries., Question Is : "Can I Connected Single - Phase Transformer Paralelly ?", If You still not understand... don't answer my question !.
 

Thread Starter

meowsoft

Joined Feb 27, 2021
116
In your other post you asked about using three transformers to isolate the phases to get single phase.
The problem is, the transformers don't change the phase of the signal, so you still have three signals that are out of phase.
If you combine their outputs, it will create a short circuit.
Is That Configuration Possible ??
 

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Thread Starter

meowsoft

Joined Feb 27, 2021
116
NO.!
Plus any 3ph transformer consists simply of three 1ph transformers mounted on the same frame.
What's your definition about Transformer 1, 2, 3 ?, that's transformer is SINGLE-PHASE not THREE-PHASE, don't think about previous question !
 
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Thread Starter

meowsoft

Joined Feb 27, 2021
116
What's your idea of what will happen?
Read My Question... Is connect multiple transformer in parallel possible ? Yes or Not ?, not with this question nor context, but generally, If you still answer No, Is better for you to not answer my question !
 

Delta prime

Joined Nov 15, 2019
831
Hello there :)
In no way is anyone trying to beat you up this is the conveyance of information point counterpoint constructive! Allowing your imagination to take flight and see from a different perspective.;)
 
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