How many devices can be powered by optical bay SATA power?

Discussion in 'Mac Pro' started by i-rui, Jan 8, 2014.

  1. i-rui, Jan 8, 2014
    Last edited: Jan 8, 2014

    i-rui macrumors member

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    #1
    I'm just wondering if anyone has crunched some numbers on the power draw from the SATA cables in the optical bay.

    I'm looking to upgrade a few machines with SSD raids. I'm sure i can split the SATA power from 1 to 2, but can i safely split one of them again to run 3 SSDs off of 1 SATA power cable?

    Also, i'm assuming that SSDs will give less of a power drain than a 3.5" HDD, but how do they compare to the 5.25" optical drive?
     
  2. Gav Mack macrumors 68020

    Gav Mack

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    #2
    Think the molex spec is 11 amps for both 5 and 12 volts.

    Spinners on spinning up might cause a problem with that many drives but SSD's should be fine. If you are worried run another long cable from the other ODD molex..
     
  3. i-rui, Jan 8, 2014
    Last edited: Jan 8, 2014

    i-rui thread starter macrumors member

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    #3
  4. MacUser2525 macrumors 68000

    MacUser2525

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    #4
    According to iStat my ssd is .94w my hard drives 3.5w so four ssd ~= one hd going by that you would be driving three drives off two connections pretty sure that is a safe idea.
     
  5. i-rui thread starter macrumors member

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    #5
    my "maxed out" scenario would be 1 3.5" HDD and 2 2.5" SSDs on each sata power cable, for a total of 6 drives (2 3.5" HDD & 4 2.5" SSD).
     
  6. MacUser2525 macrumors 68000

    MacUser2525

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    #6
    Yes that is what I did the calculations on so it would total of three drive or 13.5w, if buddy is correct on the 11a specification then you can draw 55w of 5v power or 132w of 12 v off the cable.
     
  7. Gav Mack, Jan 8, 2014
    Last edited: Jan 8, 2014

    Gav Mack macrumors 68020

    Gav Mack

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    #7
    May want to double check and do the maths of the peak current drain on the spinning disks you are using, some large capacity drives with a lot of platters do draw quite a lot of current at first spinning up, far larger than the drain you see on istat when they are at normal rpm. The large HSGT drives have five platters and will draw more than the 4 platter models from WD. Some drives, mostly server grade have either a jumper or you can run a utility boot cd to set the disks on a delayed start when they power up to stagger the power up current load.

    Think you will be fine though despite that, if you had 3 spinners that might be a problem! The molex spec is from the Seagate ST-506 era, big 5.25 inch full height drives (double optical) the size of a house brick which sounded like a quiet angle grinder and vibrated like hell. 10 megabytes for well over a thousand bucks lol. I had one on my desk with the platters out as a small bin/ashtray when I first started on my IT career path!
     
  8. i-rui thread starter macrumors member

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    #8
    do you think it's changed? would it be specific to the mac pro's PSU?
     
  9. Gav Mack, Jan 9, 2014
    Last edited: Jan 9, 2014

    Gav Mack macrumors 68020

    Gav Mack

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    #9
    Had a quick look at more recent ATX PSU specs and no it hasnt. Spec for the connector was set in 1965 way before Seagate and I was a glint in my dad's eye :eek:

    Will be easy to check, just add the 5 and 12v power ratings for the spinner plus the 2 SSD's. If you are unsure post the drive makes and models but working it out from the Watts=A x V equation it's pretty easy to figure it out.
     
  10. i-rui thread starter macrumors member

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    #10
    initially i was going to use a couple of Crucial m4's for the SSDs (probably eventually replaced with Samsung 840 evo or pros) and a 1tb WD black for the HDD (that will eventually become a 2tb drive, but i can change it to a green model if the peak power consumption is much less as i'm not too concerned about performance for the spinner).
     
  11. Gav Mack macrumors 68020

    Gav Mack

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    #11
    I believe WD are 1Tb per platter so those sized drives should be fine...
     

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