Residental mains wiring questions (USA)



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 neutral.
John
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John Larkin wrote:

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.
http://tinyurl.com/2u9u6a5
It appears some electronic ballasts can have UP TO a thirty third degree Harmonic. Wow. That's more than Freemasonry has.
**(Canadian rules.)
mike
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On Jul 3, 5:12 pm, John Larkin

Rectifiers generate harmonics (no surprise).
The "triplen" harmonics - 3rd, 6th, 9th, ... - add instead of canceling on a 3-phase wye neutral.
-- bud--
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wrote:

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 problem.
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     snipped-for-privacy@aol.com writes:

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 http://blogs.sun.com/agabriel/entry/ac_power_power_factor_explained 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.
--
Andrew Gabriel
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Thanks, i was about to try expressing that clearly. You already did it well.
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250.4-B requires "simultaneously disconnect all ungrounded conductors". That does not necessarily require a multipole breaker - it can be multiple breakers and a listed handle tie.
-- bud--
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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 counterproductive.
Does "upgrading to the current code" mean, for example, receptacles have to have the spacing in the current code?
-- bud--
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In actual practice, it means you negotiate with AHJ based on what the existing construction (all properties, not just the wiring) is and which fire / life safety properties and cost get balanced.
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