How many HDD on 750w PSU

Hi all. I have a farming rig with B450M/16gb RAM/3400G/SSD 1TB M2/ PCIE SATA 24port
I have A-Data XPG CORE REACTOR 750W Gold
There is 22A for 5V rail and 70A for 12V
I really dont understand how the load is distributed between 12v and 5v and dont understand, can be 24 HDD conntected safily? And what can happen if the load (5v rail) will exceed the allowable values, for example ~ 10% ?
Will the protection on the power supply work in the worst case, or are there more chances that som of 24 disks just break down?
I wish you all a good harvest and a peaceful sky over your head

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Quite unintentionally, I managed to start approx. 32 HDDs on a 350W Fortron power source. After the start, the input was 700W and the source survived. Some disks only occasionally clicked and were not visible in the system. I still have a regular 650W Seasonic and it doesn’t even start half as fast, even though it has better parameters according to the label. The problem is at the start, it would be ideal to turn on the drives in several phases.

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Short answer: yes, 24 disk is ok on 22A 5V (5V is the limiting factor here) but it is also getting to the limit of official recommended specs.
You can look at the specsheets for the hdd’s, they will list peak and typical amperage for both 5V and 12V

For instance Seagate X18 Sata:
12 TB startup (peak) DC is 0.85 on 5V and 1.56 on 12V
18TB startup (peak) DC is 1.01 on 5V and 2.02 on 12V

So for 24x12TB, you would be on 20,4 A on the 5V rail. And this is peak DC, so only a very shot time (miliseconds). During farming it will drop down to 0.3A per drive for both 12V and 5V.

For 12x18TB you would be on 24,24A on the 5V rail, so technically over the limit. But even that should not be an issue as it is only a short peak that the PSU should be able to handle.

Personally I prefer to stay within all the limits, even if you can push more. Better safe than sorry. But you could probably hook up 30-40 drives and still get away with it. It’s just the short startup peak that is the issue after that the power draw is very minimal. This is why SAS controller have the option to spin up the drives in sequence. Some sata controllers have that option as well but not so common. Some hdd’s also have the option to start in idle mode so they dont have the high startup current all together.


I would like to add to what @Voodoo wrote that you also need to consider what controls those HDs.

If you have just a few extra PCIe SATA boards, all what he wrote is accurate as all drives will start spinning at the same time.

However, if you have an HBA, those usually stagger disk startups, so there is usually no more than 1 HD pulling the full power. In such case your PSU should be good enough.

Also, PSUs are not digital, so that 750 W and 22 A are considered good as long as they are in the “allowable” ranges (same for your power line voltage). That’s one more reason to rather “stay in range” and not overbook the loads as you need to consider that all those other factors may accumulate on the low end while you will try to overbook it slightly.

By the way, 12 V powers only the main spindle, so some mechanical variations will slightly distribute the initial power draw. On the other hand, 5W is the board and the stepper, and those fluctuate less, as the board power up latency is really small and stepper doesn’t need to rev up.


I would like to add a question, how can I recognize on SAS controllers that they can spin up disks sequentially? Everyone can do it, can it be set somehow in the bios of the controller? Thanks!

Thank you for comments
good question! I wanna add also
How to know, is my FPU support spin up disks?

I have 17 3.5" internal drives running for over a year, stable. Went up to 20 as far as I remember, and it was still fine. Even back when I was plotting and the farmer had a 3900x cpu.

17 is not much cause most PSU have 20A 5v rail.
Intresting case with 25+ HHD on 20A 5V rail

here’s 28 drives on 22A

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Just be careful, please when using Y-splitters for HDD power. The cheap ones can cause fire!