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RE: Watts to Amps conversion...

To: <shop-talk@autox.team.net>
Subject: RE: Watts to Amps conversion...
From: "Randall Young" <Ryoung@navcomtech.com>
Date: Mon, 24 Feb 2003 16:33:01 -0800
> 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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