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Can I run 16 HDDs on one Corsair AX1200 PSU? - Page 3

post #21 of 22
Quote:
Originally Posted by gobase2008 View Post

The PSU is a Corsair AX1200, and each HDD is supposed to consume according to the manual, 17 watts during typical operation.
So, 16 HDD would be around 272 watts.

Be careful, spin-up current is not the same as operating current. Those drives might pull twice the operating current during startup, or more - you'll need to find out.

Also, did I read right that you're using 18AWG wire? If all the current is being carried by a single wire, that suggests 270w/12v = about 23A of current through the wire, which is too much for 18AWG according to https://www.eol.ucar.edu/rtf/facilities/isff/LOCAL_access_only/Wire_Size.htm which says 16A is safe through 18AWG.

I also don't know what insulation temperature rating that table is using - you need to decide how hot you're prepared to let your wiring get if lots of current gets drawn, then figure from there.

If we doubled current for spinup, you'd want to carry at least 45A safely in which case that table suggests 11AWG or bigger is suitable.

You want to check the voltage drop as well, looking for example at 18AWG, if you did fully load it to 16 amps of current, the table says it has 6.4 ohms/1000' of resistance, so if you had 6 feet of it in your cable (3 feet to the furthest drive, 3 feet back in the ground wire), that would be about 0.04 ohms, which doesn't sound like much, but by v=ir it means 0.6v of voltage drop along the cable, or 5% voltage drop. The ATX specifications say +12v can vary by +/- 5% or 0.6v so that might be just OK, but you do need to check your numbers, particularly if your wires are longer than the 3 feet I've used as an example, bearing in mind also that the PSU may not output a completely clean +12v, and the drives are also going to drop voltage due to their resistance. In short, the less voltage you lose in your wiring, the better.

Now the SATA power connectors have 3 x 12v pins, but you need to check two things:

1. Your post didn't make clear if you are splitting a single 18AWG cable to all 3 pins, or if you're running 3 wires from the PSU to all of these drives. If the former, you're over the safe current limits, if the latter you are probably OK.

2. Ref "you're probably OK" above - only if the drives pull their 12v draw equally from the 3 +12v pins on the SATA power connectors. If they don't, and instead pull unevenly (or all from one pin) you need to allow for that.

Ref 2: You also need to know which grounds the drives sink that current through. It's no good evenly pulling current through all 3 of the +12v lines, if they then sink it all back through one ground wire. Remember the drive manufacturers will have (hopefully) designed the drives sensibly from an electrical point of view, but they are not necessarily envisioning you chaining so many of them together - there's no reason they should necessarily have spread the current evenly across either the live or ground wires, since for a single drive a single wire would be more than enough.

Unless the manufacturer provide proper specs in a datasheet, I personally would jump a PSU up and spin up a drive with an ammeter in-line to see what they actually draw during spinup, then I'd also double that for safety factor.

Lastly, what happens if something fails and your wiring or a drive shorts out? On an AX1200, you'll be trying to carry more than 100 amps of current through that short until the PSU trips it's overcurrent protection. Worse, you could get a partial short through enough resistance to mean that 90+ amps flows but the PSU doesn't trip - the likely result of that is molten insulation, red hot wire and potentially a fire.

A fuse (or several fuses) in your wiring at sensible places would be a good precaution. I personally, if doing what you're suggesting, would calculate how much current my wiring could take remaining well below the temperature rating of its insulation, then size my fuses accordingly and place them such that they had a factor of 2x the expected current draw at spinup to prevent them nuisance blowing - slow-blow fuses would be a good idea, again to prevent them unnecessarily blowing during short duration power spikes at spinup.

I note that it's not normal practise for single rail PSUs to have any kind of fusing in the wires, so I can only presume that these risks are not considered likely or serious enough to figure in the laws on electrical safety governing products like this. OTOH, having had a PSU fill my room with smoke and weld in it's connectors, I'm inclined towards doing things with a belt-and-braces approach.

As always, I am not an electrician, so take my comments with a pinch of salt - do your own research, and stay safe.
Edited by BorisTheSpider - 5/23/13 at 9:40am
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post #22 of 22
Quote:
Originally Posted by qatwat View Post

If you don't know what you're talking about, don't quote others and pretend you do, it only serves to spread unnecessary FUD when you blow things out of proportion or misinterpret simple statements.
Uh, no? Did you even read the review? The reviewer gave the single rail PSU a 9.8/10. What about it being single rail? He didn't think it was even worth listing under "The Bad" section. Why do you think that is?
The more you know... NOTHING. Any PSU can fry and in the worst case scenario, all of them will take EVERYTHING with them. The more you know...

I blew it out of proportion when I told him a single rail 1200 watt PSU was unsafe?
Really?
Well that's your opinion...which at the end of the day doesn't make it a fact.

OklahomaWolf gave it a 9.8 because the product worked great and the single rail design worked great....in the lab...in the X number of hours he tested it...with a few sets of equipment.
He did make sure to note the single rail in his conclusion:

Can any PSU fry?
A quality PSU will not fry under normal operation......but if you cross the wires a few times sure it will.
I speak from personal experience.

Will a quality PSU it take EVERYTHING with them when they die?
Well after crossing the wires a few times on a PSU myself...the answer is no. (Only the PSU died.)
After seeing a friend drown his PC guts in Antec Kuhler fluid.....the answer is still no. (The mobo and cpu died. HDD and GPU were fine.)

Quote:
Originally Posted by OklahomaWolf View Post

The Mediocre:

single 12V rails are nice... to a point. I think we're getting real close to reaching that point.

Edited by Cancer - 5/24/13 at 9:47am
    
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