How many hard drives on one PSU rail?


How many 1.5 TB Seagate SATA hard drives could a 12V rail on a power supply be able to power? The power supply is an Antec EarthWatts EA 750W Power Supply w/ Quad +12V. Each 12V rail is rated with a max load of 25.0A.


~ Mark
9 answers Last reply
More about hard drives rail
  1. More than you can fit inside a FULL size tower case.

    If that one rail is dedicated to just HDDs and CD/DVD drives then the limit is is around 20 IF you could write to all 20 at the same time.

    Other factors - If that rail also supplies power to the pci-e video card connector you would have to factor that out. ie if Rail provides 10 Amps (120Watts) to the video card then that would limit the available current for HDDs/CD/DVD drives to 15 Amps - that would limit you to about 15 Drives. Next factor is total power where you would factor out MB/CPU/Video card from total.

    Didn't see power for seagate readily, but probably simular to WD Black - 2 TB drive = approx 10.7 W (approx 0.9 Amps - minus what is drawn from the +5 V line) when writing
  2. Hi.

    Thanks for the reply. I just got two new Enermax Galaxy Evo 1250W power supplies in a custom built case. Here is a spec sheet:

    Each unit has six, 12V rails. On the first power supply, I am using two of the 12V rails (two black modular connectors) to power 20 drives (10 drives per 12V rail) plus the motherboard which is on a third 12V rail (The processor is an i5 and the motherboard has 2 PCI SATA cards and two PCI-e SATA cards). There is no video card. The second power supply is using two of the 12V rails (two black modular connectors) to power 25 drives. I am just using standard molex connectors. The spec sheet for the PSU says each rail can handle 30A and all six rails can handle a total of 104A or 1248W. One of the 12V rails on the second power supply is powering 15 drives, while the other one is only powering 10.

    So do you think I will be okay?


    ~ Mark
  3. PSU1,Rail 1 10 x11 = 110W Rail rated @ 360 W no problem there
    Psu1, Rail 2 Same same

    Psu 1 combined 220 Watt for Rail 1 and 2 + MB rail 0 which is less than 360 Watts (My guess since no GPU wattage would be less than 200. Thats only 420 Watts

    PSU2 Rail 1 with 15 drives, ballpark 165 Watts (Way under 360) and 2nd rail with only 10 is less.

    I see no problem. A single one should handle 45 drives + a I5.
    IF all 45 drives were to write at the same time, 45 x 11 = 500 + 200 for I5 Plus MB (MINUS GPU) say 200 thats only 700 Watts
  4. Okay, so I should be okay on the watts. How about amps? I read somewhere that Seagate 1.5 TB drives are 2.8 amps. 15 x 2.8 amps is 42 amps, which is more then one 12V rail is rated for. Is that going to be a problem?

    ~ Mark
  5. 2.8 Amps @ 12 V = 33.6 WATTS, that is very high for a 7200 RPM HDD, so I dought it.

    If wattage is listed, just divide by 12 for currrent, and if Current is listed just multiply by 12 for wattage.

    You did not specify which model as they do very alittle between models. But for the Seagate 1.5 Tbyte -11 here is what you want to look for: (8.9 Watts ( I = 0.75 A) avg running a benchmark) I Ball parked 11 Watts (just under 1 Amp),2032-8.html

    Try doing a google search for your model and look for reviews.
    Note total power for a 3.5 inch drive is the sum of the power delievered on both the 5 Volt line (small) and the 12 Volt line (majority)
  6. Hi,

    I had a look at the drives and they are Seagate Barracuda 7200.11 1.5 TB drives. On the drive, it shows +5V 0.72A and +12V 0.52A. So what does that mean?

    ~ Mark
  7. It means that the drive draws 0.72A from the +5 V rail (5 x .72 = 3.6 Watts, and
    On the +12 V rail it will draw 12 x .52A = 6.24 Watts. While total power is 9.84 Watts, do not use that for determining power need from the + 12 V rail - that is the 6.24 Watts per HDD attached to that rail.

    Basiclly this means power derived from +12 V is even less than I stated.

    HOWEVER (More power is required from the +5 than I anticipated.) and the +5v is much closer than the +12V as a limiting factor.
    Note: you normally only have one +5 V rail @ the stated power, so 25 HDDs @ 3.6 W = 90 watts (Must add in any other 5 V requirements for total +5 V Rail limit.

    Just looked, Your 5V rail is 25 Amps (125 Watts), close
  8. Hi,

    The one power supply with the 25 drives, only had drives on it, nothing else. The second one has 20 drives, the motherboard, CPU, six 120 mm fans, and one boot drive. Should I be looking at using different power supplies here? What do you recommend?

    ~ Mark
  9. You should be fine with the 1250 W PSUs (you already have).
    You could have used a pair of Corsair 750 Watt PSUs. Very good PSU w/28 Amps on 5 V and 60 Amps on 12 - Single rail.
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