Re: UPS question

["David Buckley" <db@xxxxxxxxxxx>]

--- In ukha_d@xxxxxxx, "matt_miles_uk" <m_miles@b...>
wrote:

> I've managed to get hold of some APC 450VA Powerstack UPS'.
> They run on 110V which is great as most of my networking kit
> will run between 100 - 250 volts.  Probelm is getting power to
> the buggers!  I've asked the nice folks at APC what the input
> load is for these  (ie how much wattage they draw from the mains)
> so I can stick a transformer on them.

As these UPSs are rated at 450VA, the maximum steady state current
they will consume is 450VA plus a couple of VA for their own use,
and a bit more when they charge.

The VA is the same whether you use 110v or 230v, however the current
will be different.

So for three units a 1500VA yellow transformer will be adequate, and
depending on what load you are actually plugging into the UPSs, you
may get away with much less.

VA = Volts multiplied by Amps.

For most purposes, Watts are "equivalent" as to VA.  They are
exactly related by Power Factor, but thats more info than most folks
need :-)

so, roughly, 450VA at 110v = 4A, 450VA at 230v = 2A

When doing your wiring to distribute your 110V, make sure you do the
volt drop calculations.  110V needs much heavier wiring that 230V
for the same load, and it gets worse as the distance goes up.

If you are feeding anything with a switched mode power supply in it
(eg a PC), as the voltage supply drops its current draw will increas.


Heres something I wrote only yesyerday in response to a query about
a data centre that wants to overheat on this very topic, complete
with typographical errors:

One featureette not widely understood with computer loads is that the
power draw is constant, irrespective of voltage.  So the computer
doesn't care if you feed it 400 or 415v, it will draw xKVA, and thus
the current will vary to make up the power draw needed.....   This
is counter-intuitive to Ohms law, which says that for a resistive
element the current flowing is proportional to the voltage applied.
Apply less volts, less current flows, resistor gets less hot.
Reduce power to a computer it doesn't slow down or give off less
heat, it just demands more current.

Same is true of light fittings in buildings these days; all
electronic and dimmable.  Reduce the volts, and up goes the current.





UK Home Automation Meet 2004 - BOOK NOW!
http://www.ukha2004.com

http://www.automatedhome.co.uk
Post message: ukha_d@xxxxxxx
Subscribe:  ukha_d-subscribe@xxxxxxx
Unsubscribe:  ukha_d-unsubscribe@xxxxxxx
List owner:  ukha_d-owner@xxxxxxx