I’m planning of setting up a file server and will be using 72 2.5” drives and having a few concerns about powering them.
I have done some rough estimations on power consumption:
http://www.seagate.com/www-content/datasheets/pdfs/mobile-hddDS1861-2-1603-en_GB.pdf~2W per drive load: 2 * 72 = 144W
http://www.anandtech.com/show/8747/samsung-ssd-850-evo-review/10Some use up to 4W.
Due to the planned use of SAS Expanders the maximum data bandwidth of each drive will be significantly reduced so lets say a more generous maximum power consumptions of 3W per drive.
3 * 72 = 216W
Interesting that SSDs have higher load power consumption but their idle is much lower in the mW range vs about half a Watt.
For 2.5” drives it looks like the 3.3V and 12V rails are never used looking at the data sheets of a few drives, so only the 5V rail is needed. I was planning on using a Seasonic Prime 750W power supply but the 5V rail can only provide 100W, when I need more than double just for drives not to mention the motherboard’s need for 5V.
There is plenty of power on the 12V rail and I’m wondering if using two 12V to 5V DC-DC would be solution to this problem.
Looking at this converter
http://www.current-logic.com/shop/index.php?main_page=product_info&cPath=4&products_id=179 The manufacture says not to run >80% continuously for more than 30 minutes. I plan to run it continuously with fan cooling if needed so I should really consider this to be a 120W unit. Two would provide 240W of power and if at full continuous load at 90% efficiency would draw 267W from the 12V rail.
I was thinking of using a cable like this:
https://www.moddiy.com/products/Corsair-Style-8-Conductor-Flat-Ribbon-Cable-Wire-%2818AWG-Black%29.html but it is only 18AWG.
According to this site
http://www.solar-wind.co.uk/cable-sizing-DC-cables.html 18AWG can only handle 5V 2A at 50cm. So I’ll need 1 cable per 3 drives?
This drives will be in banks of 8 and 1 set of 4 per DC-DC converter.
Is this a good approach? Any better ideas and how is my logic thanks?