Sizing Neutrals for Switching Power Supply Loads -Guy Macon

[Guy Macon]

A true neutral - that carries only the unbalanced current of the 2 phase
conductors in this case - does not count. If there is a 60A load in any
combination between A-B, A-N, B-N there will only be 60A 'going' and 60A
'returning'.

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 ]







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[bud--]

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 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.

 

[[email protected]]

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.

--

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.

 

[Guy Macon]

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.

....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.