[Beowulf] Re: HVAC and room cooling... wires

Jim Lux James.P.Lux at jpl.nasa.gov
Fri Feb 6 13:12:49 EST 2004


At 09:30 AM 2/6/2004 -0500, Robert G. Brown wrote:
>On Fri, 6 Feb 2004, Nathan Littlepage wrote:
>
> > > That's not crazy, that's actually rather sane.  What would be crazy
> > > would be grounding the neutrals and/or ground wire in
> > > different places.
> > > Can you say "ground loop"?
> > >
> >
> >
> > Grounding loops.. truly a bane. I remember one instance where someone
> > wired a telecommunications switch to two different grounds. The -48v DC
> > power had it's own ground, and someone had grounded the chassis to a
> > different feed. I little lesser know fact was the lightning rod on the
> > tower next to the building was linked to the same ground as the power.
> > When lightning did strike, nothing but smoke as the charge rolled from
> > one ground to the other on each bay.
>
>There is also a memorable instance of powered racks with incoming two
>phase power split into two circuits having a polarity reversal so its
>neutral wire on one circuit was 120V above chassic ground and the
>neutral on the other circuit.  When somebody plugged a single unit with
>components on both lines -- I think it was more like "meltdown and
>fire".  Not really a ground loop, of course...

The classic error is wiring two sets of receptacles (e.g two racks full of 
gear) on the two sides of the 220, with neutrals properly connected, then 
having the neutral conductor fail, so the two 110V loads are in series 
across 110V.  Works fine as long as the loads are balanced, but when you 
start to turn off the loads on one side, the voltages don't balance any more.


>...but plenty of people have been electrocuted or fires started
>because there was a lot more resistance on the neutral line to a remote
>"ground" than there was to a nice, local, piece of metal.

The notorious MGM Grand fire in Las Vegas, for instance, was caused by a 
ground/neutral/resistance thing.

>  Basically,
>AFAICT there is really nothing in the NEC or CEC that is "stupid".  In
>fact, I think that most of the code has undergone a near-Darwinian
>selection process, as in electricians who fail to wire to code (and
>often their clients) not infrequently fail to reproduce.
>
>I don't think code is conservative ENOUGH, if anything, and like to
>overwire for any given situation.  12-2 is just as easy and cheap to
>work with as 14-2, for example.

Not if you buy your wire in traincarload lots when wiring a 
subdivision.  That extra copper adds up, not only in copper cost, but 
shipping, etc.  Consider that the wiring harness in an automobile weighs on 
the order of 50-100kg, and you see why they're interested in going to 
multiplex buses and 42V systems.  Ballparking for my house, which is, give 
or take 50 feet long, 20 feet wide, and 20 feet high, I'd say there are 
wiring runs comparable to, say, 3000 feet.  That's 9000 total feet of 
conductors (Black,White, Ground).  12AWG is 19.8 lb/1000 ft, 14 is 12.4 
lb/1000ft. Using AWG14 instead of AWG12 saves the contractor 70 pounds of 
copper. Copper, in huge quantities, is about $0.70/lb, so by the time it 
gets to the wire maker, it's probably a dollar a pound, so it saves the 
contractor $70 (not counting any shipping costs, etc. which could be 
another $0.10/lb or so)

$70/house is a bunch o' bux to a builder putting up 500 homes in a 
tract.  They make a profit by watching a thousand little details, each of 
which is some tiny fraction of the overall price ($70 on a 2000 ft house is 
0.035/square foot, compared to $70-100/ft construction cost).  It's much 
like automotive applications, or mass market consumer electronics, where 
they obssess about BOM (bill of materials) cost changes of pennies. (Do you 
really, really need that bypass capacitor?  Does it have to be that big? 
How many product returns will we get if we leave it out?)

This is why aluminum wiring was popular: the density of Al at 2.7 vs Cu at 
8.96, even after you factor in the fact that you might need more aluminum 
(because it's lower conductivity), it's still better than 2:1 weight 
difference. (Aluminum and Copper are about the same price these days, but 
copper has bigger fluctuations... back in the 70's copper was expensive and 
aluminum cheap (about 2:1))  So, 2:1 mass, 2:1 price.. changes the cost of 
the wire alone from $200/house down to $50/house...


Consider an office building with 20-30 floors, of 10,000 square feet 
each.  AWG12 vs AWG14 can be a BIG deal.  There was a lot of arguing about 
the heavier neutral wire needed in light industrial office 208Y/120 wiring 
with all the poor power factor loads (i.e. computers with lightly loaded 
switching power supplies).






>   10-2 unfortunately is not, but it
>gives me comfort to use it whereever I can.  And I kinda wish that all
>circuit breakers were GFCI by code as well, not just ones servicing
>lines near water and pipes.  However, these are still available as user
>choices -- code permits you to go over, just not under.
>
>Anybody curious about wiring should definitely google for the electrical
>wiring FAQ site.  It explains wiring in relatively simple terms.
>
>    rgb
>
>--
>Robert G. Brown                        http://www.phy.duke.edu/~rgb/
>Duke University Dept. of Physics, Box 90305
>Durham, N.C. 27708-0305
>Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at phy.duke.edu
>
>
>
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James Lux, P.E.
Spacecraft Telecommunications Section
Jet Propulsion Laboratory, Mail Stop 161-213
4800 Oak Grove Drive
Pasadena CA 91109
tel: (818)354-2075
fax: (818)393-6875

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