Date   : Sat, 11 Mar 2006 00:33:57 GMT
From   : Pete Turnbull <pete@...>
Subject: Re: Them pesky case stickers (was 1MHZ SCSI/ATA board.)

On Mar 10 2006, 11:59, David Hunt wrote:
> > 12kW (not kWh :-)) is 12000W, at a power factor of around 85% is
about
> > 14000VA, at 240V that's over 58A.  Most houses only have a 60A
fuse.
> >  How did you manage that?
>
> The main fuse is 100A, the 12kW was calculated as the difference
between the
> two bills the year before, same period (a guesstimate) - not sure
what you
> mean by "power factor" are you talking about AC loading and
instantaneous
> currents?

It's unusual for electrical loads to be purely resistive, so the
current is rarely exactly in phase with the voltage.  This means that
just multiplying voltage and current readings doesn't give you true
wattage; the current is slightly less when the voltage is at its peak,
and vice-versa. Power factor is a measure of that difference.  Most
computer equipment is somewhat reactive and has a power factor of about
80% - 90%.  However an elctricity meter does measure true wattage -- or
rather it measures the energy consumption, which is wattage times time.
 So to work out the current correctly when dividing by voltage, you
apply the power factor as a correction.

> As I took the reading from the meter, that's what I paid for, I'm
> not quite sure about the correlation between what I actually use and
what I
> end up paying for. I knew it was a lot, I guess, when the breaker
went on
> the circuit I was using, all I did was to distribute the servers to
other
> circuits of the house, I have two separate circuits in my "computer
room",
> the rest sat on the table downstairs.

A domestic ring main is normally 30A (sometimes 32A) so your 58A would
need to be over at least two ring mains.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York