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Whats my upper limit on graphics cards

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a b ) Power supply
February 26, 2013 3:58:30 AM

my build on January 28th 2012, over a year ago. btw all purchased at the local frys electronics


ddr3 4x4 16 Gig: http://www.newegg.com/Product/Product.aspx?Item=N82E168... $79.99 2 day sale
antec hcg-750: http://www.newegg.com/Product/Product.aspx?Item=N82E168... $84.99 2 day sale
evga gtx 560 1 gig: 169.99 2 day sale
i5 2500k: 224.99 @4.6ghz $224.99
asus p8z68-v/gen3: http://www.newegg.com/Product/Product.aspx?Item=N82E168... $184.98
cm haf 912: $54.99

added a hyper 212 cooler and crucial m4 120 gig when it dropped below $100

so i have been helping answer questions on the forum and i have noticed quite a few issues with amperage and new cards and especially SLI. what is the most graphics power this machine could still push. Some of these 2 day sale components were not what I went to pick up but the prices were good at the time. the antec said high capacity gamer on it and I just went with it without doing much research beforehand since I already had another one picked out. The excitement of a new build made me do it. I think I would be okay but power/amps and graphics cards are not my specialty.

February 26, 2013 5:09:50 AM

If that Antec PSU is at all accurate on its ratings you have plenty of head room to go SLI. You would have to be seriously pushing your overclock limits on both CPU and GPU with a nice liquid system to get to the point where 750W won't cut it. If you were at 500W I would have second thoughts, but 750 is a lot of juice.

Just make sure you have enough cooling. Things get pretty toasty with 2 GPUs.
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a c 1203 ) Power supply
February 26, 2013 5:13:04 AM

The Antec HCG-750 has a maximum combined +12 Volt continuous current rating of 62.5 Amps and has four (6+2)-pin PCI Express supplementary power connectors.

If you don't overclock the CPU or GPU's you can power a system using two GeForce GTX TITAN's in 2-way SLI mode or any of the following:

A single GeForce GTX 690

Two GeForce GTX 680 graphics cards in 2-way SLI mode

Two GeForce GTX 670 graphics cards in 2-way SLI mode

Two GeForce GTX 660 Ti graphics cards in 2-way SLI mode

Two GeForce GTX 660 (non-Ti) graphics cards in 2-way SLI mode

A single Radeon HD 7990

Two Radeon HD 7970 GHz Edition graphics cards in 2-way CrossFireX mode

Two Radeon HD 7970 graphics cards in 2-way CrossFireX mode

Two Radeon HD 7950 graphics cards in 2-way CrossFireX mode

Two Radeon HD 7870 (Boost Edition) graphics cards in 2-way CrossFireX mode

Two Radeon HD 7870 GHz Edition graphics cards in 2-way CrossFireX mode
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a b ) Power supply
February 26, 2013 6:15:51 AM

So that I can help others the only way I can see that you can get that 62.5 amps is by
watts= amps*volts or amps = watts/ volts. 750/12= 62.5.

i have 4 12volt rails @ 40 amps each. this equates to 480 watts per rail max? Is the power just first come first serve after that for who gets the amperage.

i5 2500k 110 watts at load and 1.425 v(vcore) = 77 amps not possible sooo
120 watts over 12 volt rail = 10 amp
140 watts for OC 560 12 volt= 12 amps

is my understanding of this correct?
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a b ) Power supply
February 26, 2013 7:18:30 AM

The formula is P = VI (Power = voltage*current).

Your PSU is solid enough but it is quite confusingly labeled. It might have 4 40A 12V rails, but it is only capable of 750W of 12V load http://www.hardwaresecrets.com/fullimage.php?image=3033...
As you correctly deduced, 750/12 gives you the 62.5A figure.

CPU voltage is powered from the 12V supply too, so that 62.5A figure has to cover that as well as graphics.
A reasonable figure for your current system would be at most somewhere around 300W of 12V, or 25A.

As for what stuff gets what rail, it really depends on the PSU. Yours is distributed like this-
12V1 - Main motherboard, sata, peripheral
12V2 - ATX12V/ETX12V
12V3 - PCIe cables
12V4 - PCIe cables
Some of those are clearly never going to pull 40A, not in a million years.
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a c 1203 ) Power supply
February 26, 2013 3:31:09 PM

At any point in time you cannot draw more than 62.5 Amps from any combination of +12V rails being used.

According to Antec the OCP Set Point on each +12V rail on the HCG-750 is set at 53 Amps.

If, for example, you happen to draw more than 53 Amps from any one of the +12V rails, the OCP should be triggered shutting down the PSU.
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a b ) Power supply
February 26, 2013 3:31:47 PM

so what you are saying is by the specs I have roughly 450 watts left to use, plus another 100 watts when I switch out the graphics cards.

does anybody know roughly what the increased risk of damage is when you go above spec'd demands. In other words this power supply seems to run stable up to 1,000 watts. Is it completely foolish to push these limits
below is a quick link to the overload tests
http://prntscr.com/u91qo
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a c 1203 ) Power supply
February 26, 2013 3:51:52 PM

The Antec HCG-750 has:
- Over current protection (OCP)
- Over voltage protection (OVP)
- Short circuit protection (SCP)
- Over power protection (OPP)
- Over temperature protection (OTP)

If any of those protections are triggered the PSU is suppose to shut itself down.

The table that you posted are the limits that, when any one of them are exceeded, will cause the PSU to shut down.
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a b ) Power supply
February 26, 2013 6:33:53 PM

thanks for that info ko888. with your assistance and reading your other posts I really am getting a better grasp on power supplies and I hope to further educate users about the fallacy of judging power supplies by watts and bronze, silver, gold status.

also hardware secrets had some very nice articles to read on what all the protections mean(shunts and OCP and OTP) and other technical power supply knowledge.
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a b ) Power supply
February 26, 2013 6:34:11 PM

Best answer selected by fkr.
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