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Author Topic: PSU Ampere Calculation (on the wall with 230V) ??  (Read 5740 times)
DeathAndTaxes
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Gerald Davis


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June 05, 2012, 02:13:48 PM
 #21

One thing that wasn't mentioned is power factor.
Volts * Amps is not Watts (well not on AC circuits).   It is VoltAmps.
VoltAmps * Powerfactor = Watts.

A power factor of exactly 1 (where voltamps = watts) only exists for resistive loads (like a old style light bulb).  For capacitive loads you will always have a PF of <1.  Modern Power supplies are pretty good and will tend to have a PF of 0.95 or higher (don't believe that 99.9% PF marketing crap).

The good news is that you only pay for Watts.  So if you are pulling 5A @ 230V = 1150 VA but the PF is 0.95 you are actually charged for 1150*0.95 = 1092W. 

The bad news is the wire is rated on amperage so that "cost savings" doesn't make your wire handled more juice.  Yes if the circuit (outlet, wire, & breaker) is only rated for 16A and you are pulling 5.5A you can't fit more than 2 units.  Still with some modest power reduction (lower memclock, reduce cpu clock/voltage in bios, turn off unecessary functions in bios, use linux, use usb key, etc) you likely can get it under 5A and thus can fit 3 rigs per circuit.

To measure watts you need a more sophisticated meter.   You need a "wattmeter" which measures both amperage and voltage simultaneously and continually.  Your power meter installed by the power companies does this.  A handheld meter which can universally measure wattage on any circuit is expensive (usually $500+) but dedicated meters you wire into a circuit or sub panel are much cheaper (say ~$150).
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June 05, 2012, 07:56:15 PM
 #22

Modern Power supplies are pretty good and will tend to have a PF of 0.95 or higher (don't believe that 99.9% PF marketing crap).

That's why independent testing such as this shows that AX1200 as having 0.99PF above 600W right?

Quote
you likely can get it under 5A and thus can fit 3 rigs per circuit.

No, breakers for circuits are only rated for 80% continuous load, so you need to get below 4.26A, otherwise you'll get trips while mining.

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To measure watts you need a more sophisticated meter. You need a "wattmeter" which measures both amperage and voltage simultaneously and continually.  Your power meter installed by the power companies does this.  A handheld meter which can universally measure wattage on any circuit is expensive (usually $500+) but dedicated meters you wire into a circuit or sub panel are much cheaper (say ~$150).


Kill-A-Watts can measure Power Factor (and thus VA and Watts). You dont need a super-expensive device to do so.




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June 07, 2012, 12:25:00 AM
 #23

Kill-A-Watts can measure Power Factor (and thus VA and Watts). You dont need a super-expensive device to do so.
KAW doesn't work on 240V.  A portable wattmeter tends to be expensive. 




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