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Antec CP-850 - Not a TRUE 850W PSU?

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July 17, 2011 8:13:51 AM

Hey everybody, I'm building a new rig and I'm putting an Antec CP-850 850W PSU in my rig (case is Antec DF-85) but I had a look at some of the specs in a review and saw that it could only deliver a max of 768W on the 12v rails. This bothers me cause when I go for a SLI setup I think that I will really need the extra wattage and I know that GPUs run on the 12v Rails.

Look here, towards the bottom of the page: http://www.jonnyguru.com/modules.php?name=NDReviews&op=...

Does this mean it can't actually deliver what a 850W PSU should be able to do? 768W will be enough for what I'm setting up but I just need confirmation that my facts are right.

Cheers.
July 17, 2011 8:26:42 AM

Keep in mind that the 850w rating includes everything, not just the GPU's. Modern CPU's are in many cases 95w or more, and that's not on the 12v rail.

Often times a Power Supply's 12v max will be almost identical to it's total maximum, but it really doesn't have to be to be a true 850w unit. In actual usage it'd have to be putting out far more than 850w to reach an 850w draw on the 12v rail alone.
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July 17, 2011 9:00:07 AM

rolli59 said:
768watts on the +12volts is in the range (90%) of what to be expected from a 850watt PSU. The rest of the voltages provide the 82 missing watts and it will provide its rating just look at the conclusion http://www.jonnyguru.com/modules.php?name=NDReviews&op=...


Yes, but when compared to another 850W PSU like the Corsair 850-TX V2 (http://www.pccasegear.com/index.php?main_page=product_i...) the TX is shown to be able to pull up to 840W on the 12v rail but almost the same as the CP can put on the 3.3 and 5 volt rails. That makes a whole lot of difference to me.

Unless.....unless other 850W PSUs shouldn't be pulling a their full wattage on the 12v rail all at once even if they say they can, then it would make sense why it doesn't matter if it can't pull it's full weight on the 12v rail alone.
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July 17, 2011 9:03:38 AM

Yargnit said:
Keep in mind that the 850w rating includes everything, not just the GPU's. Modern CPU's are in many cases 95w or more, and that's not on the 12v rail.


Wait, if the CPU isn't put on the 12v rail, then where is it supposed to be go? I thought it would be put on the 12V2 rail which is where the 8 pin EPS12V and the 6 pin ATX12V are.
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a c 85 ) Power supply
July 17, 2011 9:26:31 AM

It is on the 12V rail. Only the 12V AFAIK.

You're missing that In the case of the V2, its using DC to DC tech. This means that for each W you put out on the 5 or 3.3V rail, it gets subtracted from the 12V. So if your system needs a total of 50W on the 3.3 and 5V rail, that's ~ 4A less that the 12V will be able to use. I would say the CP-850 is a true "850W" unit, but its probably based on older tech by now.
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July 17, 2011 9:32:31 AM

Being able to put near 100% of their rated output on the 12v rail is only beneficial during lab testing, where they'll load only one rail at a time.

I didn't look through the specs of you particular power supply in great detail, but as rolli59 echoed, there's no problem with the ratings on your power supply.

Often times when tested, PSU's rated with nearly 100% of their available wattage on the 12v rail can actually exceed the rated draw for short periods of time if they are well made units. (The Corsair AX1200, one of the top power supplies available, for instance has been tested with a draw of around 1350w on the 12v rail)

If you're going to be loading your power supply that close to it's rated maximum capacity it's probably best to step up to the next larger unit anyways. Spending it's entire life working at over 90% capacity is just going to make it wear out faster.

That said, what video cards are you planning to use with this unit? As long as you aren't running SLI GTX580's, GTX590's, or XFire HD6990's an 850w PS should be more than sufficient. (SLI GTX570's, or Xfire HD6970's are pushing it, but will be fine as long as you aren't running Futuremark 24 hours a day) ;) 
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July 17, 2011 9:39:40 AM

4745454b said:
It is on the 12V rail. Only the 12V AFAIK.

You're missing that In the case of the V2, its using DC to DC tech. This means that for each W you put out on the 5 or 3.3V rail, it gets subtracted from the 12V. So if your system needs a total of 50W on the 3.3 and 5V rail, that's ~ 4A less that the 12V will be able to use. I would say the CP-850 is a true "850W" unit, but its probably based on older tech by now.



That's another question I need answered; what actually goes on the 3.3V and 5V rails?
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a c 85 ) Power supply
July 17, 2011 10:01:02 AM

These days, not much. In days of old the CPUs and ram used to use the 3.3v rail. Only things I know of that use it now is the logic circuitry. A harddrive for example will use the 12V rail to spin the motor, but the 5V rail for the PCB.
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July 17, 2011 10:13:14 AM

4745454b said:
These days, not much. In days of old the CPUs and ram used to use the 3.3v rail. Only things I know of that use it now is the logic circuitry. A harddrive for example will use the 12V rail to spin the motor, but the 5V rail for the PCB.



I don't suppose that you would know how on average many watts the 3.3+5 volt rails would need? I'm going to go ahead and guess maybe 50W at the most which would mean that it wouldn't matter if the Corsair could deliver a max of 840W to the 12V rail if the 3.3V+5V rails need the power too.

I'm planning on SLIing two GTX 570s and maybe some overclocking on a i7 2600K. Guru3D suggested that two 570s would consume about 410W and I know the 2600K requires 95W. Add three hard drives, a DVD drive and the seven fans inside my case and I seriously doubt that my hardware will draw close to 768W on load.
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a c 85 ) Power supply
July 17, 2011 10:30:17 AM

As always it depends on the parts. Case fans use the 12V rail. I was reading a spec sheet on a WD harddrive the other day. I think it said something around 250mA when reading. This comes out to around 1A per drive. (from the 5V rail.) 1A x 4 drives and you have less then 5A needed. 50W seems like a fine figure to use.

Your looking at 410W + 95W, or 500W for the base system. Lets say 50W for the board and drives. If you'll be OCing the CPU it will go over that 95W. 600W or so would be what you want to make sure you have.
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July 17, 2011 11:06:42 AM

4745454b said:
As always it depends on the parts. Case fans use the 12V rail. I was reading a spec sheet on a WD harddrive the other day. I think it said something around 250mA when reading. This comes out to around 1A per drive. (from the 5V rail.) 1A x 4 drives and you have less then 5A needed. 50W seems like a fine figure to use.

Your looking at 410W + 95W, or 500W for the base system. Lets say 50W for the board and drives. If you'll be OCing the CPU it will go over that 95W. 600W or so would be what you want to make sure you have.


I also read that USBs utilise the 5V rail too. So I guess the CP-850 can deliver 850W but because the 3.3V+5V rails need breathing room, it doesn't necessarily need to be able to deliver most/all 850W to the 12V rail like more expensive power supplies do.
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a b ) Power supply
July 17, 2011 11:15:15 AM

your being extremely picky about where your watts go. you dont need 100% of the power on the 12v rail. If that was all you needed, they would not even bother with the 5v and 3.3v rails on psu's. If you want an Antec PSU that can load all its power on the 12v, then get the trupower new.
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July 17, 2011 11:17:29 AM

iam2thecrowe said:
your being extremely picky about where your watts go. you dont need 100% of the power on the 12v rail. If that was all you needed, they would not even bother with the 5v and 3.3v rails on psu's. If you want an Antec PSU that can load all its power on the 12v, then get the trupower new.


I'm not being picky I'm just trying to sort out something someone said to me on Whirlpool:

http://forums.whirlpool.net.au/forum-replies.cfm?t=1738...
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