How Much PSU Wattage Headroom Should I Leave?

[Charitzo]

So after picking up a second XFX 7970 DD Edition, I've started running in to stability issues.

After reading some reviews of the cards (One's a DD edition, the other a Ghz edition), I've come to the conclusion my system is drawing around 818W at peak. I am currently using a Corsair TX850 PSU. This PSU was purchased new last October.

How much head room between my total power consumption and power supplied should I leave? Is it likely this small margin of 32W is causing system instability?

The CrossFire setup was working fine for about a month, then stability became an issue.

Thanks,
Charlie

 

[xTempered]

The PSU is a year old along with running Crossfire near max wattage for a month could've caused the components in the PSU to slightly decay causing the stability issues although im not exactly sure. PSUs dont stay super high performance their whole life they die out like everything else. This is my take on the problem but I'm pretty sure someone else might know the exact reason.

Its possible (although again im not sure) that PSUs are like HDDs were they need some room to work at full capacity. An HDD needs atleast 10% open storage to work at full potential maybe PSUs are the same

I've never let my Wattage get within 75w of the max wattage.

 

[coastie65]

So after picking up a second XFX 7970 DD Edition, I've started running in to stability issues.

After reading some reviews of the cards (One's a DD edition, the other a Ghz edition), I've come to the conclusion my system is drawing around 818W at peak. I am currently using a Corsair TX850 PSU. This PSU was purchased new last October.

How much head room between my total power consumption and power supplied should I leave? Is it likely this small margin of 32W is causing system instability?

The CrossFire setup was working fine for about a month, then stability became an issue.

Thanks,
Charlie

Hi. Normally I just mostly look at what is on the +12v rail. That is a bit thin in my opinion and you might want to consider a 950w Corsair, Seasonix, XFX, PC Power & Cooling, or Silverstone.

 

[xTempered]

Oh yea look at what Coastie65 said. I forgot about that. Your PSU might have multiple +12v rails and make sure the amps can support the GPUs. The specific amount needed is in the GPU specs.

 

[Charitzo]

Hi. Normally I just mostly look at what is on the +12v rail. That is a bit thin in my opinion and you might want to consider a 950w Corsair, Seasonix, XFX, PC Power & Cooling, or Silverstone.

It has a single 12v rail - I'll have to check it tomorrow, I just packaged it and a GPU up for RMA, then came to the realisation I was working on very thin margins. I'll rebuild tomorrow and let you know.

 

[Charitzo]

Oh yea look at what Coastie65 said. I forgot about that. Your PSU might have multiple +12v rails and make sure the amps can support the GPUs. The specific amount needed is in the GPU specs.

Amps as in current or amplifiers? Haha. In terms of wattage, it's odd. On XFX's spec sheet for the GPU's it only says "Minimum 500W supply". I thought this was a bit high, so I looked up some reviews with real world figures. The Ghz version came out at 360W whilst the DD Edition came out at 275W. Using those figures and some calculations from pcpartpicker, I realised the wattage I was actually pumping out.

 

[xTempered]

Oh yea look at what Coastie65 said. I forgot about that. Your PSU might have multiple +12v rails and make sure the amps can support the GPUs. The specific amount needed is in the GPU specs.

Amps as in current or amplifiers? Haha. In terms of wattage, it's odd. On XFX's spec sheet for the GPU's it only says "Minimum 500W supply". I thought this was a bit high, so I looked up some reviews with real world figures. The Ghz version came out at 360W whilst the DD Edition came out at 275W. Using those figures and some calculations from pcpartpicker, I realised the wattage I was actually pumping out.

Current. Some GPUs might need something like +12v Rail @45amps (random number) and your PSU might only provide something like a +12v Rail @35amps. You might not find the specific amount you need on the specs that come with it. It'll most likely say like you mention just "minimum 500w" or something like that because most psus now will usually have enough amps to support any GPU that it might power. You can find the specific amount you need on the GPU manufacturers site

 

[coastie65]

What tempered said in the above post is dead on point. That is why I pay more attention to what is on the rail. Incidently, a single rail is better. Anyway, this rig is pulling some 425w, so it would run fine on a 500w PSU as long as I have enough on the rail for the GTX 680 I have in here. I have planty on the rail as well as an 850w PSU which is overkill. Should I go to a second 680, then I would have enough PSU for the sli.

 

[Charitzo]

Current. Some GPUs might need something like +12v Rail @45amps (random number) and your PSU might only provide something like a +12v Rail @35amps. You might not find the specific amount you need on the specs that come with it. It'll most likely say like you mention just "minimum 500w" or something like that because most psus now will usually have enough amps to support any GPU that it might power. You can find the specific amount you need on the GPU manufacturers site

Couldn't find anything on XFX's website about necessary current. However, on top of the PSU I saw that 70A is the max load on the 12V rail. On another note, it took me 2 minutes to throw my PSU in and check the voltage rails.

3.3V rail - 3.360V - 30A
5V rail - 5.080V - 30A
12V rail - 12.192V - 70A

Surely though, I = P/V, so;
Ghz Edition: I = 360W / 12.192, I = 29.53A.
DD Edition: I = 275W / 12.192, I = 22.56A.
29.53A + 22.56A = 52.09A.

I say 360W since that's what reviewers showed to be the peak load current of the Ghz edition, and 275W for the same reason.

That leaves a 17.91A headroom. Doesn't sound like current is the issue to me.

 

[Charitzo]

What furthers my suspicions that I may have too much power draw is the fact that both cards work perfectly on their own - issues only arise when in CrossFire (if we exclude seemingly constant driver crashes..).

 

[xTempered]

Current. Some GPUs might need something like +12v Rail @45amps (random number) and your PSU might only provide something like a +12v Rail @35amps. You might not find the specific amount you need on the specs that come with it. It'll most likely say like you mention just "minimum 500w" or something like that because most psus now will usually have enough amps to support any GPU that it might power. You can find the specific amount you need on the GPU manufacturers site

Couldn't find anything on XFX's website about necessary current. However, on top of the PSU I saw that 70A is the max load on the 12V rail. On another note, it took me 2 minutes to throw my PSU in and check the voltage rails.

3.3V rail - 3.360V - 30A
5V rail - 5.080V - 30A
12V rail - 12.192V - 70A

Surely though, I = P/V, so;
Ghz Edition: I = 360W / 12.192, I = 29.53A.
DD Edition: I = 275W / 12.192, I = 22.56A.
29.53A + 22.56A = 52.09A.

I say 360W since that's what reviewers showed to be the peak load current of the Ghz edition, and 275W for the same reason.

That leaves a 17.91A headroom. Doesn't sound like current is the issue to me.

Huh well sorry if thats not the problem I'm not sure what is. Honestly it could be a simple thing like the games your running your Crossfire on simply arent supported well

 

[coastie65]

Current. Some GPUs might need something like +12v Rail @45amps (random number) and your PSU might only provide something like a +12v Rail @35amps. You might not find the specific amount you need on the specs that come with it. It'll most likely say like you mention just "minimum 500w" or something like that because most psus now will usually have enough amps to support any GPU that it might power. You can find the specific amount you need on the GPU manufacturers site

Couldn't find anything on XFX's website about necessary current. However, on top of the PSU I saw that 70A is the max load on the 12V rail. On another note, it took me 2 minutes to throw my PSU in and check the voltage rails.

3.3V rail - 3.360V - 30A
5V rail - 5.080V - 30A
12V rail - 12.192V - 70A

Surely though, I = P/V, so;
Ghz Edition: I = 360W / 12.192, I = 29.53A.
DD Edition: I = 275W / 12.192, I = 22.56A.
29.53A + 22.56A = 52.09A.

I say 360W since that's what reviewers showed to be the peak load current of the Ghz edition, and 275W for the same reason.

That leaves a 17.91A headroom. Doesn't sound like current is the issue to me.

Those readings are at idle more or less. You are running two 7970's in Crossfire. You start put a load on those bad boys and that 17a are going to disappear. For example. I had a machine with a 500w PSU that had 18a on the rail. The card I was using at the time required a minimum of 18a on the rail. so all is fine. Not really, I was playing Oblivion and thing were fine untill I got to a Graphically intensive part. There wasn't enough power and the Game stuured and lagged. I went and changed out the PSU to one with nearly double the 18a ( 32a ) on the rails, and never had a problem afterwards. Whatever the Minimum the card calls for, I usually go about 20a over that ( or a lot more in my present case ). That is just on a single card, sli and Croassfire changes the rules a bit and you really need some headroom for that. It may run fine under light apps, but once the heat is turned up on those cards, it will tell. Better to have a little overkill than to find out you have enough. Having said that, there are some games that do not support sli or Crossfire well ( they are not optimized for it ).

 

[Charitzo]

Current. Some GPUs might need something like +12v Rail @45amps (random number) and your PSU might only provide something like a +12v Rail @35amps. You might not find the specific amount you need on the specs that come with it. It'll most likely say like you mention just "minimum 500w" or something like that because most psus now will usually have enough amps to support any GPU that it might power. You can find the specific amount you need on the GPU manufacturers site

Couldn't find anything on XFX's website about necessary current. However, on top of the PSU I saw that 70A is the max load on the 12V rail. On another note, it took me 2 minutes to throw my PSU in and check the voltage rails.

3.3V rail - 3.360V - 30A
5V rail - 5.080V - 30A
12V rail - 12.192V - 70A

Surely though, I = P/V, so;
Ghz Edition: I = 360W / 12.192, I = 29.53A.
DD Edition: I = 275W / 12.192, I = 22.56A.
29.53A + 22.56A = 52.09A.

I say 360W since that's what reviewers showed to be the peak load current of the Ghz edition, and 275W for the same reason.

That leaves a 17.91A headroom. Doesn't sound like current is the issue to me.

Those readings are at idle more or less. You are running two 7970's in Crossfire. You start put a load on those bad boys and that 17a are going to disappear. For example. I had a machine with a 500w PSU that had 18a on the rail. The card I was using at the time required a minimum of 18a on the rail. so all is fine. Not really, I was playing Oblivion and thing were fine untill I got to a Graphically intensive part. There wasn't enough power and the Game stuured and lagged. I went and changed out the PSU to one with nearly double the 18a ( 32a ) on the rails, and never had a problem afterwards. Whatever the Minimum the card calls for, I usually go about 20a over that ( or a lot more in my present case ). That is just on a single card, sli and Croassfire changes the rules a bit and you really need some headroom for that. It may run fine under light apps, but once the heat is turned up on those cards, it will tell. Better to have a little overkill than to find out you have enough. Having said that, there are some games that do not support sli or Crossfire well ( they are not optimized for it ).

Those power draw figures I used (360W and 275W) are the numbers for full load, supposedly.

DD Edition:
- Idle 40W
- Full Load 275W

Ghz Edition:
- Idle ?
- Full Load 360W

 

[Charitzo]

Okay so I actually dug up an old 700W OCZ PSU, and wired it up to my Corsair TX850. The results were still not great - FurMark locked the system after 10 or so minutes. Would you guys say this rules out the PSU as the issue?