[ale] current pulled by computer power supply

Jeff Hubbs jhubbs at telocity.com
Sat Aug 12 23:28:13 EDT 2000


Wandered Inn wrote:

> I'm to the point that I'm concerned with the amount of current I might
> be pulling in my little office here.  I can find the maximum current for
> each computer power supply stamped plainly on each box.   Question is,
> does anyone know what the 'average' current is for a computer?  I
> suspect that the max is only hit when the thing is powering up and/or
> REAL busy.
>
> I'm considering having a dedicated circuited added to the room to supply
> my machines, but I need something to start with.
>
> --
> Until later: Geoffrey           esoteric at denali.atlnet.com
>
> Microsoft != Innovation
> --
> To unsubscribe: mail majordomo at ale.org with "unsubscribe ale" in message body.

Some time ago, I did a lot of research in this area, hooking up a number of
different machines to a digital wattmeter.

The machines I tested tended to pull on the order of 80W for old IBM PS/2s down
to about 35W.  How hard the CPU was working did affect the power consumption, but
not by much.  The rating stamped on the boxes really didn't have a whole lot to
do with how much power they actually consumed but the stamped rating was
invariably more than the real number, IIRC.

There can be a power spike when the machine is first turned on (juicing up the
filter caps) but I expect that measures are taken in the PS design to limit how
bad the spike is.

The formula current * voltage = power isn't quite accurate for computers, but if
you go around and add up powers or add up currents using that formula where
necessary based on the stamped number, you can at least establish an upper bound
for total current, which you can then compare to the rating on the breaker that
serves the circuit.

If that sounds a little complicated, try this:  see if the power outlet can
handle a 1200-watt hair dryer (~10A), and if so, assume that the same outlet can
run roughly ten computers with monitors.

FWIW, I have two APC Back-UPS 500s running off the same outlet, and they power
one computer each plus a DSL modem, a five-port Ethernet switch, two printers, a
scanner, and a third computer that acts as a firewall.  I have to be very careful
about my grounding because the outlet is a two-pronger, so I have to attach the
ground lift's tab to the CATV shield, which is earth-grounded at the back of the
house.  UPSses should never be run with their ground prong floating.


- Jeff




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