The classic set-the-office-partition-on-fire situation is to have a
y-connected 3-phase system, with a common neutral, and run a bunch of
PCs off each phase. The PCs pull current at the peak of the AC line,
at six different times each cycle, so even if the three load currents
are equal and have the same waveshape, they don't cancel in the
PCs? Oh..yeah..I hear there's a new Commodore Vic 20 available now. It
has colour, sound, UPPER *and* lower case Fonts..wow.
Actually they did change the rule concerning derating a few years
back, due to what you mention. Before, pretty much only that portion
of the total building load comprising the lighting could NOT be
derated. (and the first 200 amps of non-lighting load)**. The neutral
was generally smaller than hots. Now, it's going in the other direction.
Lighting Ballasts were a big offender, with electronic ones even worse.
All the newfangled stuff seems to cause harmonics, which are additive
in the neural conductor. That could make the Neutral current greater
than the line current.
You inspired to me to find out how bad this stuff is. The following
PDF file has some rather interesting tables.
It appears some electronic ballasts can have UP TO a thirty third
degree Harmonic. Wow. That's more than Freemasonry has.
I don't think it is the rectifiers as much as the chopper in the
switch mode power supply banging that toroid transformer. If they just
converted the AC to DC and used it there would not be a harmonic
It's the interaction between the rectifiers and the storage capacitor.
The current draw only happens for a short period near the peaks of the
mains waveform, as the mains voltage rises above the capacitor voltage,
and the capacitor is topped up to the peak mains voltage again.
(The width/height of this pulse depends how much the capacitor
discharges between each peak, which it its ripple - large capacitor
with low ripple gives a shorter larger pulse.)
You can see this in the last example (Switched Mode Power Supply) on
I don't go on to prove it in that article, but if you do a fourier
analysis of a current waveform consisting mainly of peak draw at the
voltage waveform peaks, you'll find you get mainly 3rd harmonic
components (and multiples, 6, 9, etc). When that happens across all 3
phases, they all synchonise with each other, adding in the neutral
rather than canceling out.
Of course, if you have a modern power factor corrected PSU, then it
will draw a load all through the mains cycle, so that it looks like
a resistive load (as near as possible) with a power factor near 1.
[email address is not usable -- followup in the newsgroup]
From the description given that means essentially rewiring the
building. I doubt many jurisdictions would require that. If they did,
the OP may well decide to keep his classy Zinsco panel and leave
everything alone instead of improving what he has. Kinda
Does "upgrading to the current code" mean, for example, receptacles
have to have the spacing in the current code?
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