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Need an expert to clear up my power issues

Last response: in Components
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June 10, 2011 6:40:35 AM

So, long story short - I recently received a bunch of someones spare components, and have bought the remaining parts needed to complete a system.

I got hold of an OCZ stealthXtreme 600W OCZ600SXS PSU, which is currently powering:
Asus P5k Deluxe
Q6600 OC to 3.5Ghz
Zalman CNPS9900MAX heatsink
4x 2GB OCZ Gold edition PC6400 800mhz
1x DVD writer
1x 7200rpm 500GB HDD

And now onto the video cards. I have 2 PNY 8800GT's, and a PNY 250 GTS card available to use. I either want to run the 8800GT's in SLI, or sell them and buy another 250 GTS for SLI. I can run SLI on this board using the SLIPatch for Windows.
My PSU has 4 rails, each rated at 12V@18amps. 12v1 is for CPU1, 12v2 is for CPU2, 12v3 is for "Motherboard accessories", and 12v4 is for PCI-E 1 & 2.
My problem here seems to be that both 6 pin PCI-E power cables are run off of one rail, limiting me to 18amps for both of my cards.

I have been learning a lot recently about power usage, rails etc, but I need someone to answer my specific problem as I'm still a bit confused. Here are my questions:

1) I have been able to successfully run the two 8800GT's in SLI, does this mean they were underpowered because of the 18amp max? Therefore not safe?
2) The max overcurrent limit on the rail based on the manual is 20.5amps, I'm not sure if that means its safe to go up that high?
3) I have the option of using a molex to PCI-E adapter to get power from the "Motherboard accessories" rail, but I don't know how much is already being used by the mobo and the components. Any suggestions?
4) If the 8800GT SLI is safe, how about 250 GTS SLI?
5) How do I find out the power draw for the components? The manufacture's sites just tell you the overall amps and watts needed for the entire system.

I know the easy answer would be to say "buy a better PSU", which I may end up doing (probably one with a single rail), but I'd like to first learn why the PSU would or would not work before I end up returning it for a better one.

over to you guys...
a b ) Power supply
June 10, 2011 1:45:35 PM

That's a 500w unit sold as a 600w unit.

It's probably pretty maxed out with that hardware.
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June 10, 2011 5:00:32 PM

Thanks for the replies. I've read in various place, including now from that link, that the internals are the same as a couple of other 700w units, which is one reason why I bought it. The review there also says it easily deals with 600w.

There does seem to be two versions of the PSU though, mine is identical to that one in the review but instead of having PCI-E2 on 12v2 and PCI-E on 12v4, mine are on the same rail. My unit is a reconditioned one, if that makes a difference.

http://www.newegg.com/Product/Product.aspx?Item=N82E168...

I read the article, but I still don't know the answer to my questions... if they are in there I apologize but there was a lot of technical stuff to take in that I haven't looked at before!
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a c 243 ) Power supply
June 10, 2011 5:24:32 PM

^ The review states that both PCI-e connectors are on rail 4. Your cards still draw power from 2 rails, some of it comes thru the slot.
Buying reconditioned is a crapshoot, especially with FSP oem'd OCZ power supplies.
You can find them for sale quite often, because they tend to die in warranty more than any other well known brand.
A good quality 500watt psu with 34a or better on the 12v wouldn't have an issue with your specs, and single, dual, or multi rails won't matter.
8800 GT SLI system power consumption
http://www.anandtech.com/show/2553/5
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a c 1167 ) Power supply
June 10, 2011 7:28:21 PM

Up to 75 Watts of power can be delivered through the PCI Express x16 slot to the graphics card. This should come from the +12V1, +12V2 & +12V3 rails in your case. If the graphics card will consume more than 75 Watts of power it will have Supplementary Power Connectors to provide the extra power it needs, in your case from the +12V4 rail.

Your OCZ StealthXStream OCZ600SXS should be able to support two GeForce GTS 250 or two GeForce 8800 GT graphics cards in 2-way SLI mode without exceeding the 18 Amp current limit on the +12V4 rail.
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June 10, 2011 7:30:15 PM

That's great info, thanks. From what I could find, the PCI-E slots give each card 75w, so 6.25amps. The PCI-e power cables provides the same spec.

I assume that the cards will draw the full 75w/6.25 amps from the power cable first, then get the rest from the slot - is that the correct priority?

In the review the label said that one PCI-E power cables was on 12v2 with CPU2, and the other was on 12v4. Does that not mean that they are on different rails then? It also says later in the text that they are on the same rail - I don't get that.

My two 8800GT's take at most 110watts each (I've read some models take less), which is 9.2amps per card. That means the rail with the PCI-E power cables can provide 18amps over the two cards, the remaining .4 amps comes from the PCI-E slots, and the PCI-E slots get the power from the 12v3, which is connected to the motherboard.

Phew. Please tell me I'm right as head == exploding.
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June 10, 2011 7:47:06 PM

ko888 said:
Up to 75 Watts of power can be delivered through the PCI Express x16 slot to the graphics card. This should come from the +12V1, +12V2 & +12V3 rails in your case. If the graphics card will consume more than 75 Watts of power it will have Supplementary Power Connectors to provide the extra power it needs, in your case from the +12V4 rail.

Your OCZ StealthXStream OCZ600SXS should be able to support two GeForce GTS 250 or two GeForce 8800 GT graphics cards in 2-way SLI mode without exceeding the 18 Amp current limit on the +12V4 rail.


Sorry ko888, I was posting as you did.

So what you are saying is that the cards draw 75w/6.25a from the PCI-E slot BEFORE they use the PCI-E power connectors? This would make sense as to how it would work.

Also, 12v1 and 12v2 are CPU1 and CPU2 connectors, but you are saying my cards will still have access to these rails? Is that because they are plugged into the motherboard? I assumed they were only for the CPU, and that I would only be able to include the 12v3 ("motherboard accessories") rail when calculating power for my graphics cards.
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a c 1167 ) Power supply
June 10, 2011 8:29:11 PM

chindoza said:
So what you are saying is that the cards draw 75w/6.25a from the PCI-E slot BEFORE they use the PCI-E power connectors? This would make sense as to how it would work.

It's probably more evenly split between the PCI Express Slot and the 6-pin PCI Express Supplementary Power Connector until the PCI Slot power limit is reached and then anything more comes through the Supplementary Power Connector.

chindoza said:
Also, 12v1 and 12v2 are CPU connectors, but my cards will still have access to these rails because they are plugged into the motherboard? I assumed they were only for the CPU, and that I would only be able to include the 12v3 rail.

From the power supply's perspective all four virtual rails obtain their power from the one physical +12Volt rail. This is done so that each virtual rail can have current limiting applied to each of them.

12V1 - CPU1
12V2 - PCI-E2 / CPU2
12V3 - M/B accessory
12V4 - PCI-E1

The original design of that power supply was that it was only suppose to have the one PCI-E1 connector. OCZ kludged the design to add the PCI-E2 connector and to power it they had to leech the power from another rail, 12V2.

I had to reread the part about the three +12 Volt wires to figure out what they were attempting to say.

The first +12 V wire on the cable is connected exclusively to the connector labeled as “PCI-E1”

The other two wires are split into five wires distributed over the two connectors PCI-E1 & PCI-E2

In the original design all three +12 Volt wires would have been connected to the PCI-E1 connector and there would be no PCI-E2 connector at all.
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June 10, 2011 9:07:22 PM

ko888 said:

From the power supply's perspective all four virtual rails obtain their power from the one physical +12Volt rail. This is done so that each virtual rail can have current limiting applied to each of them.

12V1 - CPU1
12V2 - PCI-E2 / CPU2
12V3 - M/B accessory
12V4 - PCI-E1


I have the reconditioned version, which according to the label is slightly different. I don't get why. You can see the label here
http://images10.newegg.com/NeweggImage/productimage/17-...

So in my unit, PCI-E 1 and PCI-E 2 power connectors appear to be on one rail, right? +12v4. I guess this is not as good, but still should let me run the 8800GT or 250GTS SLI's fine. Just wanted to check.

I really appreciate everyones time btw. I look forward to giving back to the forum.
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a c 1167 ) Power supply
June 10, 2011 9:45:46 PM

chindoza said:
I have the reconditioned version, which according to the label is slightly different. I don't get why. You can see the label here
http://images10.newegg.com/NeweggImage/productimage/17-...

So in my unit, PCI-E 1 and PCI-E 2 power connectors appear to be on one rail, right? +12v4. I guess this is not as good, but still should let me run the 8800GT or 250GTS SLI's fine. Just wanted to check.

I really appreciate everyones time btw. I look forward to giving back to the forum.

Yes, the label on yours seems to indicate that it's a previous version of that power supply.

My thinking is that they ran into current limit problems with both connectors on the same rail so they revised it to share 12V2 in an attempt to distribute the load and then modified the label for the revised version.
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June 11, 2011 12:25:28 AM

ko888 said:
Yes, the label on yours seems to indicate that it's a previous version of that power supply.

My thinking is that they ran into current limit problems with both connectors on the same rail so they revised it to share 12V2 in an attempt to distribute the load and then modified the label for the revised version.



Right. So my final questions:

My PCIE-E slot can take 75W/6.25a of power. Do you think this come from 12v1 (CPU1), 12v2 (CPU2) and 12v3 (M/B Accessories), or just from 12v3? I ask because if it's all coming from 12v3 I will want to keep a check of what else I run from that rail.

Am I also right in thinking that I have nothing to worry about that my PCI-E1&2 connectors are on the same rail unless I run much better cards in SLI?
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Best solution

a c 1167 ) Power supply
June 11, 2011 1:14:02 AM

chindoza said:
My PCIE-E slot can take 75W/6.25a of power. Do you think this come from 12v1 (CPU1), 12v2 (CPU2) and 12v3 (M/B Accessories), or just from 12v3? I ask because if it's all coming from 12v3 I will want to keep a check of what else I run from that rail.
Based on your power supply's label I would think 12V3 for the motherboard's PCI Express slots.

That rail would also power such things as hard disk drives, DVD/CD drives, cooling fans, SSDs, case lighting, etc. Those are relatively low power devices.
chindoza said:
Am I also right in thinking that I have nothing to worry about that my PCI-E1&2 connectors are on the same rail unless I run much better cards in SLI?
Yes.
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June 11, 2011 1:16:32 AM

Best answer selected by chindoza.
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a c 243 ) Power supply
June 11, 2011 11:50:43 AM

ko888 said:
It's probably more evenly split between the PCI Express Slot and the 6-pin PCI Express Supplementary Power Connector until the PCI Slot power limit is reached and then anything more comes through the Supplementary Power Connector.

It's about how the card's pcb is designed to draw power.
An 8800 GT doesn't use much more power than the slot is "officially" capable of providing and wont come close to maxing the slot's limit.
Like most cards that require external power, it will draw the bulk of it from the external connection.
http://www.xbitlabs.com/misc/picture/?src=/images/video...

Another example, 8800 GTS
http://www.xbitlabs.com/misc/picture/?src=/images/video...
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