One-third neutral and full neutral

This is my first question here; if its not good enough, please let me know. Thanks

For underground cables, we can choose “concentric neutral cable” or “tape shielded cable”.

For the first one, it contains 1/3rd sized neutral or full size neutral.
According to the following website:

Power Cable Neutral and System Grounding

1/3rd sized neutral is used for three phase supply.
full sized neutral is used for single phase supply.

What is the difference of the structure between them?

Sorry, I do not see a full explanation on website as far as I can find.




Also see:… .
– Li-aung Yip
Jan 25 at 12:43


1 Answer


In single phase circuits, the neutral conductor carries all the return current of the phases, and must be sized appropriately.

In a three phase circuit, the neutral conductor instead carries the imbalance current of the phases and third harmonics, which are in phase with each other. In a well-balanced system, it is possible to derate the neutral conductor on the assumption that it will be carrying considerably less current (because a balanced three phase system, ideally, has no need for a neutral). However, this must be done with care, because if the system becomes unbalanced or the harmonics get out of hand, you will quickly exceed the rating of the neutral conductor.

The “difference in structure” is merely a difference in the size (area) of the neutral conductor.



In the diagram above it may be as simple as 3 x more neutral strands of wire in the single phase version.
– KalleMP
Jan 23 at 16:37



@KalleMP That exactly is the difference, the “Full Neutral” can carry the full rated current of the actual cable, and uses more concentric wires. For the record, both the strand (conductor) shield and insulation shield in the above drawing are composed of a semi-conducting rubber… they are not pure insulators.
– R Drast
Jan 25 at 16:54

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