(Yet another) "Will this PSU support this Setup?"

Hello everyone, just a quick question that I feel I need to ask here, as doing my own research through randomizer's Sticky at the top of the page didn't lead me to a conclusive answer.

I'll give you my current specs quickly and then outline my predicament:

- Intel Core i5 750, OC'ed to 4.0 GHz (200 MHz bus, x20 Multiplier)
- 4GB Corsair DominatorGT Dual Channel DDR3, OC'ed to 1600 MHz (LINK @ Scan)
- GigaByte P55A-UD3R motherboard (LINK @ NewEgg)

Important bits:

- 750W CoolerMasterGX PSU (Link @ NewEgg)
- 1GB XFX Radeon HD5850 Black Edition, (Factory OC'ed?) to 765 MHz / 1125 MHz (LINK @ Scan)

Now, I want to upgrade my Setup so that I have 2 of the HD5850's in CrossFire. Reading randomizer's Sticky, he (or she - sorry, I'm new here, don't want to offend anyone!) says that:

* Recommended is a 500watt PSU with two 75 watt 6 pin PCI express connectors. 600 watt PSU with four 6 pin connectors to run crossfire is recommended but I suggest a 650 watt for some head room.

* Recommended 42 Amps on the 12 volt rail for single card and 50 Amps for Crossfire.

All looking good, as the GX750 PSU meets all these (it has 4x 6pins, and specs at 60A on the 12v rail). The problem is, this review by jonnyGURU (in particular, Page 3) shows this PSU failing at loads that (I think?) would be similar to running 2x 5850s, so I'm quite worried about forking out ~£180 for a 2nd Graphics Card if my PSU won't be able to power it.

I built this rig myself a few months ago (my first ever), but I'm still a complete newbie and, as I said, I've been unable to find myself a conclusive answer doing my own research, and so I kindly put it to you guys: Do you think this Power Supply will support 2x HD5850s in CrossFire? Might I have to lower the overclocks of my CPU/RAM etc to achieve it?

Thanks for your time :-)

PS: First post, yay!
10 answers Last reply
More about another will support setup
  1. Tough call
    Delivers the wattage but fails at ripple and noise for higher loads. But in the real world the video cards don't get to the maximum TDP in games so if you want to use it don't do benchmarks. From some CF reviews I see that the systems (i7) take a bit under 500W on load. Wait, that's an OCed 5850 so nope.
  2. Hello, cheers for the replies :-)

    Just to clarify mosox, do you mean that I'd be able to use 2x 5850s with my current PSU, if I downclock them back to what a "normal" 5850 would be at?

    And thanks for the suggestions obsidian86 - If this PSU definitely can't handle 2x 5850s then I'll grab one of the ones you listed!

  3. If you downclock them and get the CPU back to normal you're looking at some 40A. The PSU looks fine at that but it's at it's limit for being in the specs. I would get the XFX 750 Black Edition.

    Wait, you can't crossfire on that mobo, your second PCIex16 slot is running at x4, you need x8 or x16. You can sell your card and get a HD 6950/6970 and use that on your current PSU.
  4. Oh my, I hadn't thought of that! I'm an idiot :sol:

    Yeah that's a really good idea, will probably do this as soon as my next pay day hits. Cheers for the help mate!

  5. You don't "need" x8 or x16 on the 2nd slot to CrossFire. There have been several past discussions on this very forum that showed x8/x8 vs x8/x4 CrossFire/SLI showed little difference in performance (a few percent). In games that fully support them, you'll still see significant increases in performance in x8/x4 CrossFire or SLI configurations.

    As for the PSU, a quality ~500W PSU would handle your setup with only a single GPU installed, just as the spec "guideline" suggests. If you intend to CrossFire, then the 700W range is what I'd suggest. They key is to find something with not only enough 12V amperage, but with proper distribution. Single-rail PSUs have little trouble with distribution, while some multi-rail PSUs do. OCZ's ModXstream Series comes to mind as a multi-rail PSU that has issues because they isolate the 12V1 rail to only the mobo, while the 12V2 rail has to power to ALL other 12V devices ( HDD, GPU, Optical drive, etc.).
  6. Hmm, that's interesting - Well I think I'll have to get a better PSU first, and then install a 2nd 5850 on the motherboard I already have once I get enough money. Or, if it ends up being cheaper, I'll grab a beefier Graphics Card that can be supported by the PSU I already have.

    Cheers for the help everyone, really appreciate it :-)
  7. Opinions are different on this, read this thread
    I still think x4 isn't very good for fast cards like the one you have.
  8. Mosox, good of you to reference that discussion. Lmeow's finding that the 3-way SLI test THG performed was hampered due to the x4 slot being 1.0 would appear to indicate that the key to whether or not an x4 slot is sufficient enough for Multi-GPU configurations depends upon whether the x4 slot is PCI-E 2.0 or 1.0/1.1. If such a slot were 2.0, then obviously the bandwidth limitation is not nearly as severe as if it were 1.0/1.1.

    I based my previous statement on CrossFire discussions (and personal experience) with AMD 785G boards, in which the x4 slots were PCI-E 2.0. When I personally compared results of CrossFire tests on my own 785G and 790X boards, the differences in performance were small, with the 785G losing only a couple frames within each of the games I tested. (Tested using a Phenom II X3 720 unlocked @ X4 3.3GHz with two HD4890's.)

    According to specs from Gigabyte, only the x16 slot of the GA-P55A-UD3R "conforms to PCI-E 2.0 standard." http://www.gigabyte.com/products/product-page.aspx?pid=3438#sp
    Same thing with the GA-P55A-UD3. http://www.gigabyte.com/products/product-page.aspx?pid=3439#sp
    (Gotta use the global site to see both. The US site excludes the "UD3R" board, as well as many other boards which are not available in the US.)

    I never considered the possibility the x4 slot could still be a 1.0 slot, thus limiting performance. Since the x4 slot on these boards is 1.0, I now find myself unable to recommend CrossFire on this board. While it should still show some benefit over a single-GPU solution, it's likely the 2nd card would be hampered due to PCI-E bandwidth restriction. If you truly wish to CrossFire on a P55 board, I'd suggest getting one that supports a full 16 PCI-E lanes in an x8/x8 split, in which both slots would be PCI-E 2.0. Unfortunately, such boards are not as common as one would hope. Newegg has only eight P55 x8/x8 capable boards. And while they start as low as $110 (Biostar T5 XE CFX-SLI), most are $150-$250. (Interesting there's not a single Gigabyte among them.)
  9. Yep, AMD chipsets are more generous with the bandwidth. Even the new 1155 is hampered. For Intel, only the 1366 has lots of PCIe 2.0 lanes (36 but AMD leads with 42 on the 890FX). The 1156 has a maximum of 16 I think.
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