Harmonics, Electronics, Neutrals

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brother

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Just trying to get a better understanding of Electroncis Harmonics and Shared Neutrals. Im told when dealing with mostly with electronics, even though there you have a 'full boat, (ie black red blue, 3 phase) you still need/should UPSIZE the neutral because of the harmonics on electronics. In just resistive loads, it ok to leave the neutral the same size.

Is this true, been searching for more info, but having trouble. Im not much of an electronics guy.
 
I don't have the knowledge to provide the details but I can tell you this.

If that 'full boat' supplies non-linear loads it is possible that the neutral can carry more current then any one single phase conductor.

Howevever.....

Unless you designing the circuit to be maxed out from day one IMO up sizing the neutral is worthless.

Lets say it's a 20 amp full boat wired with 12 AWG and a 20 amp breaker. Unless your loading each leg to 15 to 20 amps with non-linear loads you will not approach the 25 amp rating of the 12 AWG neutral.

In my experience we do not max out circuits supplying electronic equipment.
 
Do you run MWBC's and same size neutrals to large computer lab's? Thus, why we run designated neutrals to the such. As Bob pointed out, why would anyone design a circuit and expect it to be loaded to the max? It is done, and can cause problems, but if you are talking design and layouts, than circuits should be run accordingly..........
 
I did a large office building with telemarketer cubicles filled with rows and rows of computer terminals. "Super neutral" mc cable that had # 12 black, red, blue, and green with a #6 neutral was specified. It was my first experience with odd mc configurations and I was a little dubious but now I'm hooked. I'm running my long homeruns in a 6 story Hotel with 10-16 mc (12 circuits and 1 extra white). It's a real labor saver and the derating works out just right. Also easier to run when ceiling space is at a premium.
 
I am with you on the use of the larger MCs for home runs. :smile:


Still not buying the usefulness of a 6 AWG neutral with 12 AWG hots for a 20 amp branch circuit. Some engineer is being more then conservative.

It would be interesting to see the calculated load for each of theses circuits. I bet it is well under 10 amps per leg.
 
iwire said:
Still not buying the usefulness of a 6 AWG neutral with 12 AWG hots for a 20 amp branch circuit. Some engineer is being more then conservative.

It would be interesting to see the calculated load for each of theses circuits. I bet it is well under 10 amps per leg.
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I agree with this. There was a time years ago when the common thought that pervaded the industry was to provide a "super neutral". I do not believe that is the pervading thought these days.
 
I am with you on the use of the larger MCs for home runs.
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100%

The number of jobs I have worked on where customers had equipment operational issues, Mostly UPS's beeping, some printer problems...almost all were VD issues.
 
brian john said:
100%

The number of jobs I have worked on where customers had equipment operational issues, Mostly UPS's beeping, some printer problems...almost all were VD issues.
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I have had this discussion with many electricians, and it is surprising how few guy's know at what point ( in distance ) conductor size has an effect on voltage drop. I would say most guy's think it is about 4 to 5 times the length that it really is.
 
I bet the material savings from using the mc cable with the oversized neutral. Less copper less material for the sheath.
 
brother said:
Even with a short distance, they claim that the 'triplen' effects is what causes the neutral to carry MORE current than the hots, even on short distances.
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That is entirely true, but just because the neutral may carry more current then the hots does not mean the neutral must be made larger.

Meaning if the each of the three hots are carrying 8, 12, 15 amps and the neutral is carrying 20 amps it is still less then the 25 amp rating of the conductor.
 
iwire said:
That is entirely true, but just because the neutral may carry more current then the hots does not mean the neutral must be made larger.

Meaning if the each of the three hots are carrying 8, 12, 15 amps and the neutral is carrying 20 amps it is still less then the 25 amp rating of the conductor.
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So are you saying there is no chance for it to be carrying 30 amps on the neutral in this scenerio??
 
brother said:
So are you saying there is no chance for it to be carrying 30 amps on the neutral in this scenerio??
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I believe that only in theory the neutral current can be as high as 1.73 times the highest phase current. So to get 30 on the neutral you would need to have 17 amps on one of the phase conductors and loads that caused the highest possible distortion.

I think that PC power supplies are actually getting better as in Europe part of the listing requires better designed power supplies.

Brian has a lot of experience with all this from his years of testing. Hopefully he will jump in. But he has said in the past that he has only found problems where facilities have not been well designed overall, not just a lack of a super neutral.
 
I just read the article link from M.D. and what is everyones opinion of the seperate ground issue.



Power quaility said:
All 20A receptacle branch circuits serving computers were replaced with appropriately sized THHN cable in order to include a separate neutral conductor for each phase conductor and a separate grounding conductor for each branch circuit. In addition, the existing No. 12 conductors were upsized to No. 10 to accommodate high crest factors and minimize voltage drop. While this might seem overly conservative, the branch circuits were now fully protected from high neutral currents and were, in addition, equipped with a reliable ground path.
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I would say it is definitely conservative ,...overly so is a matter of opinion.
I doubt they will experience power quality issues.. voltage drop or harmonic
 
brother said:
Also, at what point, or how many electronics devices does it take to really cause a concern?? I know i got my computer, printer, fax, copier etc all on the same circuit and i never had a problem.
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brother, for some more research, query on switch mode power supplies and go from there. That will address the issue better than such a basic search. Do you own a scope?
 
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