Sizing Neutrals Single Phase Distribution

Think about it. The neutral current can never be greater than the largest of the three phase currents.

Follow the wiring regulations applicable to your country and don't waste time trying to reinvent the wheel.

I think you have not experienced the problem of Neutral over heating due to Triplen Harmonics in Single Phase circuits where computer or other linear loads are predominant and we have to provide 3P+2N cables and Busducts as the currents in Neutral circuit can be as high as 1.732 times phase currents.

And I think you're not talking about residences any more. Instead of coming here, why don't you convince someone your country's wiring regulations need amending instead? Your ball.

Actually it can. Triplen or zero sequence harmonics due to the use of non-linear loads such as switch mode power supplies, electronically ballasted fluorescents, VFDs, etc. can result in high currents circulating in the neutral.

Zero sequence harmonics, ie, 3rd, 6th, 9th, are in phase and therefore add up in the neutral conductor. The neutral current can thus be 2 to 3 times higher than the largest line current, and if the neutral conductor is of insufficient capacity, the resultant voltage drop can cause overheating of the conductor, burning of switch contacts and a potential difference between neutral and earth.

Balancing of loads between phases does little to alleviate this problem and the use of voltage optimisers, zig zag transformers, filters, etc. can be necessary.

OK. So I've got three single phase breakers of 32A rating, and two of them are turned off. There is 32A in the phase, meaning that the breaker is at the point of tripping. What is the neutral current?

I then turn on the other two breakers, which each load to the same 32A. What is the neutral current now?

Provided there is a neutral current, there will always be a voltage between it and ground.

Your ball.

Perhaps I should have said a "greater" potential difference or even a "dangerous" potential difference between neutral and earth, but I thought the implication was obvious when taken with the context of my accompanying statement re. neutral conductor sizing.

Depending on the dynamics of the loads, the neutral current can be considerably higher than the 32 amps due to those harmonics that I mentioned.

As for being "my ball"...I have stated plain facts, whether you choose to believe them concerns me not.

I have not had a problem with high neutral currents that I know of. No problems with neutral wires or connections. But to be ready to check for them, can they be measured with normal clamp on amp meters, or is a power quality analyser needed? -- JHF

A digital analyser is the best option, but if your clamp meter is a true RMS type then it can be used. Average measurement AC ammeters - even those calibrated to display RMS values - will only satisfactorily measure sinusoidal waveforms and are pretty much useless on non-linear ones which will be the case for neutral harmonics.

Thanks. I know my DMM is true RMS, but I'll have to check the speccs on my clamp meter. -- JHF

Dear Mr.Crabtree,

You are right, the Neutral Current can never be more than the highest current in any one of the phase.

But the Non-Linear Load which is responsible for Harmonic Disturbances, the amplitude of the different Harmonics will add up ARITHMATHICALLY and NOT VECTORIALLY, as against the conventional circuit where the unbalanced current will flow through Neutral for which the resultant current is VECTORIALLY ADDED, and hence neutral conductor size will e less.

Hence it is essential to have higher size Neutral Conductor, prepferably independendant earthing for each non-linear loads. This topic came for discussions some time back.

DHAYANANDHAN.S

A contractor can always put in heavier gauge wire of phase, neutral or ground than the code specifies as minimum standard. Remember though that the nominal conducting material, copper, is not cheap. It is so valuable that most coins are no longer made with copper. If the customer/contractor can afford and desires heavier gauge wire than the minimum standard, great.

Upsizing the neutral would seem very prudent it seems.....

The present, code sanctioned practice of having downsized neutrals in nominally balanced 3ph distribution networks may need reviewing.

I haven't seen the insulation on the smaller neutral conductor on the local utility's ABC that runs past my balcony blistering yet. There may be a neutral CT and commandable breaker at the transformer hut.....(per recommendation 4 of the presentation)

Residential loads are presenting themselves as more industrial in nature more and more now.

Upsize neutral now....

Learnt something today. Thanks Naha.

Now I can spec a neutral CT and commandable circuit breaker for some of my installations, or instal bigger neutrals. Margin just got a bit better.

Also be considering the acquisition of some true RMS instruments.

Wal, gr8 link thank you Sir!

BTW both the US and British codes mention sizing neutrals due to actual loads and heating. This comes right when i was getting comfortable with the idea of downsizing neutrals, now thats obviously not a good idea.

Everyone should read Wal's link carefully and think about it.

Thanks for the acknowledgement 1c.

A day doesn't pass where I don't learn something new.

APFCs and Harmonic filter installations could be a more common place installation requirement too...like I said, good for the margin.

But mate, "Sir!"??? Knighthoods are for gay minstrels and jesters....

Good article. It does not however suggest that increasing neutral cable size is the best solution since the transformer still has to deal with the adverse effects of the harmonics and so has to be derated. It seems to favour other approaches for dealing with harmonics.

Considering that this article was written about 14 years ago, it is also good to see what advances have been made and are being incorporated into non-linear loads. This article from GE discusses the solutions they provide alongside their VFD's.

the article suggests k-factor rated transformers over de-rating regular transformers. but its also suggests derating cables or upsizing neutrals. IEEE 519 table 10.3 requires (suggests ???) THD to be kept to 5%. Here

http://www.mirusinternational.com/downloads/Interpreting-IEEE-Std-519-and-Meeting-Harmonic-Limits-VFDs-PCIC-2003-15.pdf

we have an example of a THD which uses 182 amp/3p at 60hz,
using 233a/3p RMS because of its harmonics = (51 amps harmonics/182 amp fundamental) = 28% THD