> Does anyone happen to know the magic formula for converting Watts to Amps
> for AC current? I know that it's Watts = Voltage * Amps for DC, but I
> believe there's a "factor" in there for AC. I can't seem to find any
> place to get a definite answer...
>
> By the way, I'm trying to spec out the UPS systems (along with the load
> distribution) that I need for some new servers we are getting at work.
Tim, I think you're talking about "power factor", which is the cosine of the
phase angle, and varies depending on what the load is. An resistive load
will have a 100% power factor, an inductive or capacitive load will have a
lower number. AC Watts = Volts(rms) * amps(rms) * power factor.
However, the right way to size an UPS is by volt-amps, which is simply
volts(rms) times amps.
If this is a large installation, you need to talk to both of your vendors
(UPS and server) about harmonic currents; but for run-of-the-mill PC servers
just make sure the VA rating of your UPS exceeds the sum of all the devices
that will be connected to it. I like to be very conservative here, both
because devices tend to get added later, and because a bigger UPS will
typically give you a longer run time.
Randall
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