What is the recommended size for a neutral conductor in a 3 phase 4 wire system?

[Zubair8]

Hi,
Frankly, I'm new to this forum. So please do guide me if I posted wrongly.

I need some help in sizing the neutral wire. For a 3 phase 4 wire system (3Ph + N), what is the recommended size of the neutral conductor. Should it be the same as the phase conductor or should it be 50% of the phase conductor. Let say a power cable that is sized up to 300sqmm, does the neutral cable also need to be sized to 300sqmm or can it be less?

Is there any specific standard, IEC or NEC that mentioned the requirement for neutral conductor size. Appreciate if you guys can mention the IEC or NEC number.

Thanks.

 

[Averagesupernova]

Do you mean neutral or ground? There may be a difference depending on what you are running power to. When running 3 phase power to a motor for example, what you are calling 'neutral' isn't really neutral at all. It is not supposed to carry any current except in the case of a fault. It is to be grounded to the case of the motor. However, when running 208 or 240 volt power into a building the 4th conductor (combination neutral/ground) will carry some current because of lights and other 120 volt loads. You are always safest making the 4th conductor the same size as the other 3.

 

[dlgoff]

I agree with Averagesupernova. Use the same size conductor if in doubt.

Three phase systems may or may not have a neutral wire. A neutral wire allows the three phase system to use a higher voltage while still supporting lower voltage single phase appliances. In high voltage distribution situations it is common not to have a neutral wire as the loads can simply be connected between phases (phase-phase connection).

http://www.3phasepower.org/3phasewiring.htm"

 

[Zubair8]

With motor its pretty straight forward, but main concerns are when dealing with LV distribution where unbalanced load exist.

For LV distribution, normally the power supply from LV Switchgear to LV MCC utilizes big cables (150sqmm - 300sqmm). The same case also applies to LV Switchgear Busbar Rating. To save copper, some people opt for lower neutral busbar rating compared to the phase busbar.

As I understand, recently either IEC or NEC requires the neutral busbar to be of the same size as the phase busbar. Can anybody point out to which clause in IEC or NEC that mentioned this.

Appreciated it. Thanks.

 

[m.s.j]

You can refer to Grounding Riddle No.12 from http://electrical-riddles.com for a short discussion.