neutral cable cross section?

[emadshaaban]

dear elecrical experts;
-in all of 4 cores cables the neutral is less by 50% of each phase core,whay is 50%?
- for (3ph-N) power supply feeds single phase loads so the neutral should have the sum of currents of the 3 phases,whay that isn't true.
plz help.

 

[dpc]

If all the phase currents are equal, there is no neutral current.

Most codes allow reduction in neutral size on three-phase circuits based on the fact that the neutral current should be less than the phase currents. (Triplen harmonics are a complication that sometimes requires an increased neutral.)

 

[emadshaaban]

thx dpc ,but if yuo put an ampmeter across neutral of single phase load you ganna measure the same current in phase.!!!!!

 

[dpc]

Yes, but your orignal question was talking about the neutral of a three-phase feeder. That is entirely different.

You need to decide what question you are asking.

 

[emadshaaban]

thanks alot dpc, i think my both equestion for the same point.
1ph supplty is a part of 3ph power supply,
so if you i've got 3ph-N power supply, feeds several single phase loads ,at load side amps in phase equal to amps in neutral ,but 3ph supply side the neutral current doesn't eaual the sum of currents.
that was my equestion . please reply

 

[SparkyLarks]

Are you familiar's with Kirchoff's Current Law.

If not you should google it and read up.

Basically if you have a single phase system the current flowing into a device, in the phase conductor , will be equal to the current flowing out on the Neutral

For a balanced three phase system , we essentially we have current flowing in on three phase conductors and flowing out on one neutral

so the current on the neutral In is equal to the sum of the three phases, i.e Ip1 +Ip2 +Ip3.
So In=Ip1 +Ip2 +Ip3
However we have to look at the instantaneous current as opposed to the average and as the current is typically a sine wave, and the ampliture of the waves are equal and each phase is a 120° phase shift to each other you get

In= Ip1(Sin(a+0°)) +Ip1(Sin(a+120°)) + Ip1(a+240°)
Which If you do the maths come out at
In=0

so hence the ability to use a lower neutral

P.s. the explanation was done from memory so If I have left something crucial out please add it in

 

[DRWeig]

emadshaaban,

If you expect to have one of the three phases fully loaded while the other two have zero load, you can't reduce the neutral size. this would mean you have no 3-phase loads in the building.

I can't imagine an instance in which this would happen. We put 3-phase services in to drive 3-phase loads. Every balanced 3-phase load on that service of yours will reduce the neutral current. When totally balanced, it's zero.

The concept you seem to be missing is: If all three phases have the same current (and harmonics are not an issue), the neutral current will be zero.

Note: As dpc allowed, the triplen harmonics can confuse the issue.

Good on ya,

Goober Dave

Good on ya,

Goober Dave

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[waross]

This is often a case by case issue. If you load two phases equally, the neutral current is very close to the phase currents. If there is a lot of lighting ballasts the triplen current may become an issue.
In some configurations, ground fault current may return to the transformer on the neutral and in the event of device failure, fault current may return on the neutral. Some panel boards have 200% rated neutral bus rating.
There are exceptions to everything that I have said. There is not a simple answer to this question.
And by the way,

-in all of 4 cores cables the neutral is less by 50% of each phase core,whay is 50%?

I don't agree with this. I have seen too many tens of thousands of meters of 4 core cable with four equal conductors.
Are you confusing the grounding conductor in a three conductor cable with a neutral? Many cables have an equipment grounding conductor included in the construction and this grounding conductor is not counted as one of the cores.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[FreddyNurk]

As an aside to dcp's statement regarding code, in Australia the latest wiring rules just about mandate a full size neutral for consumer side installations (there is a performance based section as well, hence the just about statement).

 

[emadshaaban]

thanks alot, all of you for help.

.sparkylarks the currents equation that you wrote ,helpped me.is the same idea of the equation used in the core balance ct?.


.waross the calbes that i face 4core colored (black/black/brown/blue)
Ex : black,black&brown=70mm2 blue=35mm2 and so on in all other ranges

 

[waross]

It sounds like you're in IEC land. In NEMA land only the equipment grounding conductor is reduced in size in a cable.

Bill
--------------------
"Why not the best?"
Jimmy Carter