Sizing Neutrals for Switching Power Supply Loads -Guy Macon

bud-- wrote:


You are incorrect. The above is only true in the special case where the loads have current waveforms that cancel, such as resistive loads, matched inductive loads, or power-factor-corrected power supplies as loads (often found in servers). The above claim is *NOT TRUE* for cheap switching power supplies sharing a neutral. In that case each phase has a huge current spike at the peak of the voltage waveform (think capacitor being kept charged through a bridge rectifier).
I strongly recommend that you read section 7 ("Why do 3rd harmonic currents overload neutral conductors") of the following FAQ: [ http://www.mirusinternational.com/pages/faq.html ]
There are harmonic standards (USA IEEE Standard 519-1992, EU Standard EN61000-3-2) that attempt to put the burden of keeping these harmonic currents at a safe level on equipment manufacturers, but the person who buys a couple of hundred cheap personal computers and installs them in an old buildings to replace electric typewriters often doesn't know anything about this and ends up overheating the neutral.
Also see:
[ http://www.springer.com/west/home?SGWID=4-102-45-131253-0 ] [ http://www.dataforth.com/catalog/pdf/an108.pdf ] [ http://www.smartdev.com/pdf/ac_power.pdf ] [ http://www.mirusinternational.com/downloads/hmt_faq06.pdf ] [ http://www.mirusinternational.com/downloads/hmt_faq07.pdf ] [ http://www.mirusinternational.com/downloads/hmt_faq08.pdf ] [ http://www.mirusinternational.com/downloads/hmt_faq09.pdf ] [ http://www.mirusinternational.com/downloads/hmt_faq10.pdf ] [ http://cms-emc.web.cern.ch/cms-emc/pdffiles/PhDfiles/grounding.pdf ] [ http://www.thecircuitdetective.com/twocircuit.htm ]
-- ( G o o g l e F o o d . . . ) www.guymacon.com Guy Macon Guy Macon Guy Macon www.guymacon.com Guy Macon www.guymacon.com Guy Macon Guy Macon Guy Macon www.guymacon.com Guy Macon www.guymacon.com Guy Macon Guy Macon Guy Macon www.guymacon.com Guy Macon www.guymacon.com Guy Macon Guy Macon Guy Macon www.guymacon.com Guy Macon
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Guy Macon wrote:

I would say the general case is neutral balance with harmonics being the special case.
I considered adding a comment on harmonics. The thread was residential and harmonics are not a major problem. If one fully qualifies all possibilities in answers they can become unreadable.
Harmonics certainly can be a problem, but probably not even on most commercial/industrial 15/20A circuits. My impression is they are more of a problem on 3 phase because of triplen adding.
--
bud--

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wrote:

That is true but in commercial applications and even some large multi family dwellings like high rise condos, the supply is 208 3 phase. The recomendation is that you upsize the neutral 2 wire sizes if there is a significant harmonic load.
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snipped-for-privacy@aol.com wrote:

...or if you are unsure as to whether there will be a significant harmonic load some time in the future. It is a Good Thing if the fellow who decides to put a really big arc welder in his garage, turns his house into a marijuana farm, or to run racks and racks full of computers (Google started by renting space in a residential garage, IIRC) manages to do so without setting the wiring on fire.
--
Guy Macon
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