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Author Topic: Is my Kill a Watt wrong? says pulling almost 1300W from 1200 W PSU?  (Read 7653 times)
MrBilling
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May 14, 2013, 07:38:22 PM
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I have a 5 card rig.  I am running 3 7950s off of a Silverstone 1200W gold PSU.  And 2 7950s off of another 1200W PSU.





I hooked up my Kill a Watt and it says I am pulling 1260-1290 W off of the one 1200W PSU that is running the 3 7950's.



Is this possible?
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Syke
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May 14, 2013, 07:50:16 PM
 #2

Yes. The 1200w is the output rating, and the 1290w is the input measurement. Due to conversion loss, the 1290w input is roughly 1120w output (1290 x .87). Well under the 1200w output rating.

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May 14, 2013, 07:52:36 PM
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What voltage and clocks are you running? I have an i5 760 rig with one 7950, and 2 monitors hooked up to my kill a watt. If I run it at 1.187V core and 1.625V mem I draw about 400W with the monitors on, and around 340W when they are asleep. This is total power consumption though, so the card itself is probably only using like 220-240W.   
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May 14, 2013, 07:59:11 PM
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I am running 1000/1225 and stock V.  As far as I know stock V.  I am using BAMT and use cgminer config file to OC the cards.  Ill look againbut I dont think I have anything specified for the V.  So I assume its stock.  I do have powertune at 20.
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May 14, 2013, 08:39:35 PM
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I also have powertune to 20.
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May 14, 2013, 09:38:42 PM
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Yep that's normal.

1,200W is the amount of wattage the PSU can supply to the computer.

An 80Plus Gold PSU will be ~87% efficient at 100% load, according to the specification.

This means that for ever 1 watt you pull from the wall (which is what your Kill-A-watt is measuring), you will supply 0.87W to the computer.

A 1200W PSU supplying 1,200W to the computer will pull ~1,380W (1200/0.87) from the wall.

If you're drawing 1,300W, your 5 card rig is really only drawing 1,131W (1300*0.87).

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May 14, 2013, 09:40:57 PM
 #7

It certainly is possible.  PSU are rated for DC OUTPUT power.  Depending on how efficient your PSU is (most modern PSU are 80% to 90% efficient at 100% load) the AC input load will be somewhat higher.

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May 14, 2013, 10:06:37 PM
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Yep that's normal.

1,200W is the amount of wattage the PSU can supply to the computer.

An 80Plus Gold PSU will be ~87% efficient at 100% load, according to the specification.

This means that for ever 1 watt you pull from the wall (which is what your Kill-A-watt is measuring), you will supply 0.87W to the computer.

A 1200W PSU supplying 1,200W to the computer will pull ~1,380W (1200/0.87) from the wall.

If you're drawing 1,300W, your 5 card rig is really only drawing 1,131W (1300*0.87).


no i am running a 5 card rig but I have only 3 cards running off of this PSU.


The other two cards are ru nning off of a different PSU.



I have read what others are drawing running 7950s and this seems very very high.  i have read a few people running 3 cards off of 800W PSUs.   these 7950s should only draw about 200W percard when mining right?
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May 14, 2013, 11:50:54 PM
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Yep that's normal.

1,200W is the amount of wattage the PSU can supply to the computer.

An 80Plus Gold PSU will be ~87% efficient at 100% load, according to the specification.

This means that for ever 1 watt you pull from the wall (which is what your Kill-A-watt is measuring), you will supply 0.87W to the computer.

A 1200W PSU supplying 1,200W to the computer will pull ~1,380W (1200/0.87) from the wall.

If you're drawing 1,300W, your 5 card rig is really only drawing 1,131W (1300*0.87).


no i am running a 5 card rig but I have only 3 cards running off of this PSU.


The other two cards are ru nning off of a different PSU.


I have read what others are drawing running 7950s and this seems very very high.  i have read a few people running 3 cards off of 800W PSUs.   these 7950s should only draw about 200W percard when mining right?

You probably screwed up the connection in bridging the PSUs. Since your single PSU can handle it, why not go buy some SATA/molex adapters and just run everything off the silverstone?
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May 14, 2013, 11:58:43 PM
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Yep that's normal.

1,200W is the amount of wattage the PSU can supply to the computer.

An 80Plus Gold PSU will be ~87% efficient at 100% load, according to the specification.

This means that for ever 1 watt you pull from the wall (which is what your Kill-A-watt is measuring), you will supply 0.87W to the computer.

A 1200W PSU supplying 1,200W to the computer will pull ~1,380W (1200/0.87) from the wall.

If you're drawing 1,300W, your 5 card rig is really only drawing 1,131W (1300*0.87).


no i am running a 5 card rig but I have only 3 cards running off of this PSU.


The other two cards are ru nning off of a different PSU.


I have read what others are drawing running 7950s and this seems very very high.  i have read a few people running 3 cards off of 800W PSUs.   these 7950s should only draw about 200W percard when mining right?

You probably screwed up the connection in bridging the PSUs. Since your single PSU can handle it, why not go buy some SATA/molex adapters and just run everything off the silverstone?
never ever use SATA adapters.

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May 15, 2013, 01:26:44 AM
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Yep that's normal.

1,200W is the amount of wattage the PSU can supply to the computer.

An 80Plus Gold PSU will be ~87% efficient at 100% load, according to the specification.

This means that for ever 1 watt you pull from the wall (which is what your Kill-A-watt is measuring), you will supply 0.87W to the computer.

A 1200W PSU supplying 1,200W to the computer will pull ~1,380W (1200/0.87) from the wall.

If you're drawing 1,300W, your 5 card rig is really only drawing 1,131W (1300*0.87).


no i am running a 5 card rig but I have only 3 cards running off of this PSU.


The other two cards are ru nning off of a different PSU.


I have read what others are drawing running 7950s and this seems very very high.  i have read a few people running 3 cards off of 800W PSUs.   these 7950s should only draw about 200W percard when mining right?

You probably screwed up the connection in bridging the PSUs. Since your single PSU can handle it, why not go buy some SATA/molex adapters and just run everything off the silverstone?



What do you mean my Silverstone can handle it?  It cant handle 5 GPUs.  According to this kill a watt this thing is maxed out on 3 GPUs.  And I didnt know you have to bridge the PSUs.  I just thought you had to rig the ATX connector so ithe second GPU would turn on.


MrBilling
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May 15, 2013, 01:42:14 AM
 #12

Yea After diong some more research I think these 7950s should not be using more than 300W.  Alot of people are saying 200W.


I do have the kill a watt plugged into the surge protector.  Does this make a difference?  Should I plug the kill a watt directly into the wall then plug the surge protector into it?
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May 15, 2013, 02:50:50 AM
 #13

Are you sure you are reading WATTS and not VA (volt-amps) off your meter?

I have 3 7970, stock speeds, and I pull 875W according to my Kill-A-Watt with a Gold rated 1200W PSU.

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MrBilling
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May 15, 2013, 03:12:25 AM
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Yea.  I have these PC Power & Cooling 500W single rail PSUs that I was planning on running my 5 card off of and I cant even run 1 card off of those.


But yea the above pic is just 3 cards.  I have the other two cards running off of another identical 1200W PSU for the time being.  My original plan was to run 4 off of one 1200W PSU and the other card off of one of the 500W PSUs.

And this is no overclock on these 3 cards.

But I cant get it to run like that because the cards are taking so much power.



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May 15, 2013, 03:58:28 AM
 #15

1141Watts with only 3 cards? Ya, 3x 7950s should run on less than 800Watts. You should be able to run 5 7950s for BTC mining with lower memory clocks with no problems.

If you've got 2 of those 1200W PSUs, try swapping them? Maybe it's a bad Kill-a-watt?

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May 15, 2013, 05:15:03 AM
 #16

One way to check for a bad kill-a-watt is to plug something into it which has a defined wattage (hairdryer, lightbulb, etc).  If nothing else a multi-plug extension cord, and as many lamps w/ 120W bulbs as you can find makes a good test load. Make sure to find out the max wattage the k-a-w can handle before you melt it trying to plug in an arc welder.
MrBilling
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May 15, 2013, 06:07:46 AM
 #17

Can it be the way I have the dual PSU setup?


All I did was short the green wire to ground on the ATX connector on the second PSU.



I saw another guide that showed splicing the green wires from both PSU ATX connectors and also both grounds.  It says to do this and also to splice pins 7 & 8 together which is the power ok and ground.


I don't see how this at all could make the cards draw this much power.  But I dunno.


I will test the kill a watt tom and report back.
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May 15, 2013, 08:54:24 AM
 #18

Off topic: I used to have a high school teacher called Mrs. Watt. What makes it even better is that she was an IT teacher Tongue

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May 15, 2013, 11:03:28 AM
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Testing the KAW with a known load (bulbs) that someone above suggested is a good idea.

Other questions: give us full details of your system hardware (motherboard, CPU, how many SSD/HDD, etc). Does the wattage also reflect a monitor that you have hooked up? I presume not since you are plugging the PSU straight into the KAW right?

Also, are you using the CPU for anything (CPU mining, etc)?

As others have said, you should not be pulling that much wattage with just 3x 7950s.

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MrBilling
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May 15, 2013, 02:51:29 PM
 #20

Testing the KAW with a known load (bulbs) that someone above suggested is a good idea.

Other questions: give us full details of your system hardware (motherboard, CPU, how many SSD/HDD, etc). Does the wattage also reflect a monitor that you have hooked up? I presume not since you are plugging the PSU straight into the KAW right?

Also, are you using the CPU for anything (CPU mining, etc)?

As others have said, you should not be pulling that much wattage with just 3x 7950s.



This is the system.


ASRock 970 EXE4
Athlon II 270
8GB DDR3 (4 x 2GB)
1200W PSU running 3 7950s  Another 1200W at the moment is powering the other 2 7950s.
No HDD just a USB stick with BAMT on it.
The W is being measured with just the PSU plugged into it.



I am about to test the meter.  I will report back what I find in a few mins.
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