Harmonic Distortion Limit and Neutral Cable Sizing 2

[Edelma_1]

Hello Everyone,

I found out from IEC 60364-5-52 that for a multiphase circuit you can actually reduce the size of the neutral conductor than it`s active size if the harmonic distortion current is less than 15%. I would like to know by how many percent can one reduce the neutral.

 

[waross]

While this has no standing in IEC land, it may be of interest:
The law in Canada:
(1) The neutral conductor shall have sufficient ampacity to carry the unbalanced load.
(2) The maximum unbalanced load shall be the maximum connected load between the neutral and any one
ungrounded conductor as determined by Section 8 but subject to the following:
(a) there shall be no reduction in the size of the neutral for that portion of the load that consists of
(i) electric-discharge lighting; or
(ii) non-linear loads supplied from a three-phase, 4-wire system; and
(b) except as required otherwise by Item (a), a demand factor of 70% shall be permitted to be applied
to that portion of the unbalanced load in excess of 200 A.
(3) The size of a service neutral shall be not smaller than the size of a neutral selected in accordance with
Subrule (1) and shall
(a) be not smaller than No. 10 AWG copper or No. 8 AWG aluminum; and
(b) be sized not smaller than a grounded conductor as required by Rule 10-204(2), except in service
entrance cable or where the service conductors are No. 10 AWG copper or No. 8 AWG aluminum.
(4) In determining the ampacity of an uninsulated neutral conductor run in a raceway, it shall be considered
to be insulated with insulation having a temperature rating not higher than that of the adjacent circuit
conductor

That said, it is common in Canada to have breaker panels supplied with a 200% neutral bus.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Edelma_1]

I actually read the standard half way, the full standards goes thus:

for multicore circuits the neutral conductor size can be smaller than the active size if the Harmonic distortion current is less than 15%.

When the Harmonics are between 15% to 33% the neutral conductor size shall be same size as the active conductor.

meanwhile the current THD should not exceed 20% of the fundamental frequency, in other words the neutral size should be same as conductor size for a nominal THD current of 20%.

 

[che12345]

Edelma_1 (Electrical)(OP)25 Jul 24 15:10
"..I found out from IEC 60364-5-52 that for a multiphase circuit you can actually reduce the size of the neutral conductor than it`s active size if the harmonic distortion current is less than 15%....I actually read the standard half way, the full standards goes thus:....I would like to know by how many percent can one reduce the neutral".
1. I am an octogenarian, the IEC 60364-5-52 (2001) I have in my collection could be outdated/superseded. Please refer to the latest.
2. Per 2001 publication , in brief ... 523.8.3 ... if the harmonic current is greater than 10% the neutral conductor shall not be smaller than the phase conductors...and the corresponding reduction factors for higher harmonic currents are given in annex D.
Che Kuan Yau (Singapore)

 

[Parchie]

@EdeLma_1,
The formula I got is In/Ip = 3 X (per unit THD)/(sqrt(1+(per unit THD)^2) for circuits laden with 3rd harmonics. In = neutral current, and Ip = phase current.
I got 45% with 15% THD

 

[Edelma_1]

thanks Parchie,

asides using instruments to find out the THD, is it possible to get it by manual calculation first before proceeding to find voltage and current values of the harmonic orders?

 

[che12345]

@ Mr Parchie (Electrical)26 Jul 24 06:43
"...The formula I got is In/Ip = 3 X (per unit THD)/(sqrt(1+(per unit THD)^2) for circuits laden with 3rd harmonics. In = neutral current, and Ip = phase currentI got 45% with 15% THD"
1. I am skeptical of the simplistic formula on the In/Ip value =..... Reason: the magnitude of In and Ip (unless further qualified) do NOT indicate the harmonic content.
2. To illustrate, consider a Y with N system:
a) 3 de-similar linear loads connected across AN, BN , CN. There will be current in the Neutral. The neutral current is the fundamental, with harmonic = 0.
b) 3 de-similar non-linear loads connected across AN, BN , CN. There will be current in the Neutral. The neutral current is the fundamental, + the arithmetical sum of the third harmonic currents etc., where harmonic current would NOT = 0.
3. Logically, neither the In nor Ip indicates the value of the harmonic current in the Neutral line.
Che Kuan Yau (Singapore)

 

[Parchie]

@che12345,
Goggle is friend. It could be your too. Go figure.
@Edelma_1,
The best way to go is hire the services of a third party doing harmonics frequency sweep on your system. We did that. That neutral to phase current ratio hold true if the dominant harmonics in your system is the 3rd harmonics, like what we have.
Or, you can look up to the ceiling and imagine how much harmonics each of your loads possibly injects into your system and then start from there. Verify if your estimates is correct by measuring the parameter (voltage or current).

 

[7anoter4]

If you are referring to IEC 60364-5-52 Annex D Effect of harmonic currents on balanced three-phase systems Table D.52-1 (C.52-1) – Reduction factors for harmonic currents in four-core and five-core cables then if the harmonics' level is more than 15% you have to increase the live conductor's area by amplifying the current calculated with the factor shown in the table. That means you have to take more current into consideration. If the TDH is up to 15% you have not to increase the current.
By the way, Che Kuan Yau, I have my self 2001 edition- as I am 86 and I am not so proud with it.
As I know, in the 2009 edition-the last edition, it seems to me-the Annex D is not modified.

 

[7anoter4]

By the way, it is not the neutral conductor area you have to increase, but you have to increase all the live conductors-the TDH is measured only in neutral.

 

[waross]

Ampacities in the Canadian Code are based on 3 current carrying conductors in a raceway or cable
When there is significant current in the fourth conductor, ie: the neutral, ampacities are derated to 80%.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[7anoter4]

The same: according to NEC [2023 ed.] Art.310.15 Ampacity Tables.
Par.(E) Neutral Conductor
(3) On a 4-wire, 3-phase wye circuit where the major portion
of the load consists of nonlinear load- harmonic currents
are present in the neutral conductor- the neutral conductor
shall therefore be considered a current-carrying conductor.
According to Table 310.15(C) (1), for 4 loaded wires the ampacity shall be reduced to 80%. However, no referring is noted about percent of harmonics load.

 

[7anoter4]

Sorry Che Kuan Yau, for my crude joke. I really appreciate your comments on all the threads. Sure, wish I was only 65 years old when I retired and the only meds, I was taking was half a glass of whiskey and a well-done steak. But 21 years have passed quickly since then.

 

[7anoter4]

In my day, any three-phase power cable, manufactured to an IEC standard, had a reduced-section neutral conductor. In my day, there were no inverters and no harmonics in the current delivered.
Now if you intend to use a reduce cross section neutral the problem is complicated.
First, take a look on IEC 60364-5-52 Regulation 523.6.3 (523.5.3). and 524 Cross-sectional areas of conductors [524.3]
Then check the size of the neutral conductor is at least equal to 16 mm2 in copper or 25 mm2 in
aluminium.
Then, the harmonics -in all three live conductors-shall be add to the actual neutral current
resulted from unbalanced phase currents-if any. Then with the resulted current go to
calculate the cross-section area according to Annex A and D.
Third-you need a protection on this neutral -according IEC 60364-4-43 431.2 Protection of the neutral conductor.
A neutral conductor protection?

 

[waross]

In my day, any three-phase power cable, manufactured to an IEC standard, had a reduced-section neutral conductor. In my day, there were no inverters and no harmonics in the current delivered.

Older than fluorescent ballasts and mercury vapour ballasts?

Then, the harmonics -in all three live conductors-shall be add to the actual neutral current
resulted from unbalanced phase currents-if any.

Beware;
Line to line connected loads will cause unbalanced phase currents but will not cause neutral currents nor neutral harmonic currents.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Bronzeado]

Just to think.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1725071394/tips/Neutral_Current_xhzo4a.pdf[/url]

Cheers,

Herivelto S. Bronzeado
Brasília, Brazil

https://www.linkedin.com/in/heriveltobronzeado/

https://www.researchgate.net/profile/Herivelto_Bronzeado