Neutral wire

[raman00084]

I have 3phase line in my house
The wire used in the main board are
R phase 4sqmm
Y phase 4sqmm
B phase 4 sqmm
Neutral 4 sqmm

All my loads are single phase
For example in R phase I have 10 amps load
In Y phase I have 8 Amps load
In B phase I have 5 Amps load
Since the loads are single phase the return path is neutral . in neutral the current will be 10+8+5 total 23 amps ?
My doubt is the neutral wire must be 3 time more 12sqmm?

 

[KlausST]

Hi,

No. It´s AC. Thus it´s all about phase shift.

For symmetrical load (like 10A, 10A, 10A) the neutral current is about zero.

All three phase voltages are with 120° angular shift with respect to each other.
And the load may cause additional phase shift of current w.r.t. voltage.

So if you know the phase shift (angle) and the RMS value (length) then you may (graphically) add the vectors.

****

All considered pure sine currents. If you have lot´s of overtones it becomes even more difficult. Then in worst case 3rd overtones truely add up in a symmetrical system.

Klaus

 

[Easy peasy]

if all the loads are resistive - then the currents do not add in the neutral in a simple way, but some of the difference will flow.

however if all the loads are a cap input filter after a bridge rectifier then the currents can and do add simply in the neutral - as they are discreet non overlapping pulse currents.

This was a big issue in US high rise offices when PC's were first introduced and caused a lot of smoking neutral wires and overloaded transformers.

So you need to know the nature of the loads and their current draw waveshapes

 

[cupoftea]

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