Tor Olav Kristensen wrote:
> Tim Attwood wrote:
> ...
>>> 2. Is maximum load proportional to diameter or cross section area? (The 
>>> latter is quadratically propertional to diameter.)
>> It's proportional to the surface area. Thus all the stranded and braided 
>> wire. 
> 
> No, this is not true for DC currents and AC currents at low frequencies.
> 
> (Maybe your are thinking about the "skin effect" that appears at high
> frequencies ?)

Well, it seems there are two relevant phenomina here.

1. Heat production, which is proportional to current and inversely 
proportional to cross-section area.

2. Heat dissapation, which is proportional to surface area.

I'm not sure which one is dominant...

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4736c72d$1@news.povray.org...
> 2. Heat dissapation, which is proportional to surface area.
>
Heat dissipation is propotionnal to surface area only if this area is free 
of insulation.

Marc

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Tim Attwood wrote:
>> 1. A car runs on 12V electrics, not 250V. Does that make a difference?
> 
> Yes, lots. Voltage is the size of the elephant, amperage is how fast he's
> going when he runs you over. I think many cars will pull more than 100 A
> cold starting though.

No, not for the sizing of the cable. If the cable has resistance of
0,01ohms and the current is 100 A, the cable will heat up with
100^2*0,01=100 W (P=I^2*R) and hold up 100*0,01=1 volt (U=I*R) of
voltage (this 1V is called the voltage loss of the cable). Rest of the
voltage (11V or 249V) will get to the final device and it'll have matter
there.

Practically everyone/-thing (lets say house heating, 10kW) needs power
(P, measured in watts). While P=U*I, we can produce 10kW of heat eg.
with either 1A*10kV, 10A*1kV, 100A*100V, 1kA*10V or 10kA*1V. If the
transmission lines to the house adds up with 1ohm, we'll heat the
transmission lines up with anything from 1W (1A^2*1ohm) to 100 000 000 W
= 100MW (10kA^2*1ohm). This is why the main transmission lines do not
have 230-400V voltage, but ~10-400kV voltages instead - to minimize the
power loss, which only heats up the outside air.

> 100 A * 12 V = 1200 W (~1.6 hp)
> 100 A * 220 V = 22 KW (~29.5 hp)

True, at the final device.

-- 
Eero "Aero" Ahonen
   http://www.zbxt.net
      aer### [at] removethiszbxtnetinvalid

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Invisible wrote:
> I used to own an old HP line plotter. (Do they still make those?) 

Every time I see my wife writing chinese by hand, it reminds me of fond 
hours watching the HP line plotter label stuff on the output page.


-- 
   Darren New / San Diego, CA, USA (PST)
     Remember the good old days, when we
     used to complain about cryptography
     being export-restricted?

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Tor Olav Kristensen wrote:
>> 2. Is maximum load proportional to diameter or cross section area? (The
>> latter is quadratically propertional to diameter.)
> 
> Cross section area.

Also depends on AC vs DC.

-- 
   Darren New / San Diego, CA, USA (PST)
     Remember the good old days, when we
     used to complain about cryptography
     being export-restricted?

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>> Roughly double your standard 13A cable would do...  Look at the cable 
>> going to the starter motor on your car, that's usually 100A or so.
>
> 1. A car runs on 12V electrics, not 250V. Does that make a difference?

No, the wire has to be the right thickness for the *current*, voltage 
doesn't make any difference to its current carrying capability.  That's why 
the power grid is at a stupidly high voltage, so that current is lower (for 
the same power) and hence the cables can be thinner and less power wasted by 
heat.

> 2. Is maximum load proportional to diameter or cross section area? (The 
> latter is quadratically propertional to diameter.)

Cross-sectional area.  IIRC the definition of resistance is something like 
material_resistivity*length/x-sec_area.  If the resistance of a cable is too 
high, then it will get too hot and melt.  Heat dissipated is I^2*R...

> Apparently the planners have access to an A0 printer. I don't even want to 
> imagine what that must cost... ;-)

Not *that* much, they just print on a roll of paper and work like an 
inkjet - just a bit wider :-)

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>>> Like I said, when they hook up the external generator, it comes with a
>>> cable well over 10 cm thick (!)
>>
>> A lot of that thickness will be insulation, shielding and armor.
>
> Probably.

It will probably be 3-phase as well if you are running large bits of lab 
equipment...

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>> Yeah, but that's little compensation if as a result businesses stop using 
>> you and you fold
>
> I don't think you understand what I mean by "absurd amounts of money". ;-)
>
> [As in, like, if companies *did* stop using us, we'd *still* have so much 
> money it wouldn't even matter...]

But what are you going to do with all the money if nobody uses you anymore 
because of your reputation of shoddy back-up plans?  How long will that 
money last paying everyones salaries? ;-)

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scott wrote:
>>> Yeah, but that's little compensation if as a result businesses stop 
>>> using you and you fold
>>
>> I don't think you understand what I mean by "absurd amounts of money". 
>> ;-)
>>
>> [As in, like, if companies *did* stop using us, we'd *still* have so 
>> much money it wouldn't even matter...]
> 
> But what are you going to do with all the money if nobody uses you 
> anymore because of your reputation of shoddy back-up plans?  How long 
> will that money last paying everyones salaries? ;-)

Well, it's not really my field, but presumably the people responsible 
for this stuff have looked into it quite thoroughly. ;-)

FWIW, we had a freezer fault over the weekend... Interesting coincidence.

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Phil Cook wrote:
> And lo on Fri, 09 Nov 2007 16:08:36 -0000, Invisible <voi### [at] devnull> did 
> spake, saying:
> 
>> Well, I'm sure somebody will have looked at that and decided it's not 
>> worth it. (I would think that for more than 3 seconds of cover, you'd 
>> need a battery larger than our entire office...)
> 
> You'd think.

Well, 6 mass spectrometers using 450 V electrics... That's gotta eat 
juice like candy. If you just wanted to power the freezers, that's 
probably quite reasonably - the compressor doesn't even have a 100% duty 
cycle, and it's only a small electric motor after all.

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