How to size neutral line in three phase system with single phase load

Thread Starter

Tinsae

Joined Jan 8, 2015
46
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
 

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MaxHeadRoom

Joined Jul 18, 2013
19,058
Your single phase pairs should be the same gauge as each other, IOW each phase has its accompanying neutral.
Is there any reason you cannot do this?
If combining them, then use a gauge that is the combo of the three phase feeds.
Max.
 

Thread Starter

Tinsae

Joined Jan 8, 2015
46
Your single phase pairs should be the same gauge as each other, IOW each phase has its accompanying neutral.
Is there any reason you cannot do this?
If combining them, then use a gauge that is the combo of the three phase feeds.
Max.
Your single phase pairs should be the same gauge as each other, IOW each phase has its accompanying neutral.
Is there any reason you cannot do this?
If combining them, then use a gauge that is the combo of the three phase feeds.
Max.
Max.
First of I would like to appreciate your fast response.
You are right untill the main cable comes. As you said each single phase load with the same thickness of phase and neutral come to the main breaker. But after the breaker there will be power cable as you know most power cables neutral thickness is smaller this what I afraid for
 

Thread Starter

Tinsae

Joined Jan 8, 2015
46
Not sure I understand, why are the neutral cables smaller after the breaker, what circuits are they feeding?
Max.
I mean most cables neutral thickness is smaller for example 3x120mm2 +70
This cable has the 120mm2 3wires for the phases but only 70mm2 for neutral. Since I should use big cable like after the breakers board that is why I face the problem
 

MaxHeadRoom

Joined Jul 18, 2013
19,058
In a three-phase linear circuit with balanced loads, the neutral carries no current.
The neutral carries current if the loads on each phase are not identical. This is how the the neutral wire can be made thinner than the phase wires, saving some cost, if the neutral conductor is smaller than the phase conductors, it can be overloaded if a large unbalanced load occurs.
Are these fluctuating loads.?
And do you know what the exact loads are?
Max.
 

Thread Starter

Tinsae

Joined Jan 8, 2015
46
The loads are coolers single phase 220v motors about 500w but many of them . And we can assume that at any instant we have the same amount of coolers on each phase hence balanced load can be considered.
 

MaxHeadRoom

Joined Jul 18, 2013
19,058
You should be close if you size the neutral for the highest current of any single load.
Check if any local regulations exist for this condition of unbalanced 3ph and N and required sizing.
Max.
 

BR-549

Joined Sep 22, 2013
4,936
It sounds that you are being fed power that expects 3 phase loads. So the neutral is sized for imbalance current, not load current.

Check the drop across the neutral leg. It has to carry all 3 phase currents when wired like that.

Edit: The ideal solution, is to use 380 V heaters.
 

Thread Starter

Tinsae

Joined Jan 8, 2015
46
Yes you guys are right. When I closely look my loads configiration they form star connected load so there will be no current flowing in the neutral cable after the main breaker
 

mvas

Joined Jun 19, 2017
538
Yes you guys are right. When I closely look my loads configiration they form star connected load so there will be no current flowing in the neutral cable after the main breaker
But ...
Since you do not have 3-Phase Loads, you cannot design for the "Best Case" scenario - ie perfectly balanced loads.
Since you have single phase loads you must design for the "Worst Case" scenario.
With only Single Phase loads connected, you should NOT exceed the AMPACITY Rating of the Neutral Wire by any single phase.

Example Free Air ...
70mm^2 = 245 Amps = Max on the Neutral Wire
120mm^2 = 344 Amps = Max on any Phase Wire for 3-Phase Loads

In this case, you should not exceed 245 Amps on any single phase due the smaller Neutral Wire
and the total load on any Phase should not exceed 80% of most Circuit Breakers, whichever is LESS.

Example: If you use 300 Amp circuit breakers then 300 Amps x 80% will limit each phase to only 240 Amps max.

You need to read and understand how your Local Electrical Codes limit the total amps of single phase loads on a 3-Phase 4-Wire Wye service

If you turn OFF all motors on Phase R and Phase S then how many amps will flow through the Neutral Wire?
The same as Phase T ?
 
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Thread Starter

Tinsae

Joined Jan 8, 2015
46
Yes mvas, any unbalance creat more current load on the neutral line. I will use 70mm2 for neutral and 120mm2 for the phases also to prevent significant unbalance load I will arrange the sockets alternatively phase by phase. By the way my max current is about 180amps and cable will burried in the ground.And for this scenario I am thinking 120mm2. do you think this is too big for this current?
 
Yes mvas, any unbalance creat more current load on the neutral line. I will use 70mm2 for neutral and 120mm2 for the phases also to prevent significant unbalance load I will arrange the sockets alternatively phase by phase. By the way my max current is about 180amps and cable will burried in the ground.And for this scenario I am thinking 120mm2. do you think this is too big for this current?
The maximum current in the neutral can not exceed the maximum current in which ever phase has the highest current. This is because any imbalance is carried by the other two phases, also subtracting from their neutral currents. You need to only size according to the electrical code; which likely is the same size as the phases.
 
In summary it would appear that all four wires should be the same gauge.
Please research triplen harmonics in three phase neutrals. I understand that theatre installations where many single phase lamps on dimmers are operated off three phase supplies sometimes have to have double size neutrals. (correction - I guess it is only 1.73x) In cases where many computer and electronic devices (single phase) are used on a single phase supply (like large office buildings) there may be special considerations for the neutrals. You will need to determine if your loads have significant triplen harmonics.

Some places I found with a google search
http://www.electrical-installation.org/enwiki/Sizing_the_neutral_conductor
https://forums.mikeholt.com/showthread.php?t=57249&page=2 (see post 13/14/15)
https://www.mindconnection.com/library/electrical/case_harmonics_01.htm

But really you need to be asking the electrician or inspector in your area who is familiar with three phase and triplen harmonics. They are the ones you will deal with if anything goes wrong.
 

MaxHeadRoom

Joined Jul 18, 2013
19,058
In a balanced star connected system, neutral current is always zero.
The OP says his loads are all sized equally.
The local service company should be consulted as to policy of sizing such a supply neutral.
Max.
 
In a balanced star connected system, neutral current is always zero.
"should be" and "are" sometimes collide in the real world. Where at least part of the issue lies is with devices like uncorrected computer power supplies that do not draw power equally, but grab a surge at the peak of the wave form (when the diodes conduct to top up the storage capacitors). As I understand it, this causes a harmonic in the supply that adds up (instead of cancelling each other out) in the three phase neutral and can overload a regular neutral. The search term is "triplen harmonic."
 
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