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How many drives can you power with 1 molex

J

Jeroen1000

Limp Gawd
Joined
Sep 17, 2010
Messages
266
As some of you may know, I've got a norco case with 5 SAS backplanes and there are 4 drives per backplane.
Now, I have 2 modular PSU cables with 2 molexes on each cable. So each cable powers 8 drives.

I am now looking for a way to power the 5th backplane. What would you advise?

1) splitting the molex on the 4th backplane. So that cable would then power 12 drives.
2) put down a seperate cable just for that last backplane (the 2nd molex of that 3rd cable would also be used to power 2 fans). That seperate cable would be equiped with sata to molex converters because I do not have enough molex only cables.





 
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The number of drives is not as important as the power they'll draw, so what are the drives ? 12 7200rpm drives without staggered spin-up, all drawing on a single wire, I would be a little concerned.
 
I would do 6 to 8 max. That is if the cables or short and have heavy gauge wire. You may want to get a multimeter reading on the furthest end connector to see if there is a significant voltage drop.
 
So the 1st and 2nd picture is the wire I'm working with: PSU cables that came with my Silverstone PSU.
(3rd picture is a SATA cable)
Instead of plugging the 4rd molex (bottom molex in the first picture) directly into the backplane, I'm going to use a molex splitter to power the 2 bottom backplanes (one of them is powerless at the moment, as you can see).

So the 2nd PSU cable, which has 2 molexes (before splitting) already powers 8 drives (as does the 1st cable). I'm trying to figure out whether I can go to 12 drives (after splitting).
I'm using 2 TiB Hitachi deskstars. Using something like a kill-a-watt-meter, I've been able to calculate that that each drive draws about 20W at startup. This is consistent with the Hitachi spec sheet.

And staggered spin up will not work. It's probably because of the HP sas expander.

edit: wires are 18 AWG. Does this help lolz?
 
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Typical Molex 4-pin power connectors (Molex 8981) are rated at up to 11 Amperes per pin. That comes to 55W on the 5V pin and 132W on the 12V pin. So you should be fine on the Molex connector itself.

As for the the PSU and the wires from the PSU to the connectors, that is more difficult. You should start by looking up the specs on your power supply to see how much 5V and 12V current it can supply. If it is not single-rail, then see how much it can supply to each of your modular cables.

For the cables themselves, if it uses the same wire as a typical Molex connector, that would be 18 AWG, which is typically used up to 16 Amperes for chassis wiring. So that is 80W on the 5V wire and 192W on the 12V wire. If you are lucky, they might have used 14 AWG wire on the cables, since they have two Molex connectors on each cable, and 14 AWG wire is typically used up to 32 Amperes. That would give you 160W on the 5V wire and 384W on the 12V wire.

You said your drives can draw 20W of power. If that is worst case, during spin-up, then you can probably use the approximation that the 20W is all on the 12V wiring during spin-up, since HDD motors typically use 12V.

Have you tried contacting Silverstone? Maybe they will send you another cable with two Molex connectors? Or at least sell you one.
 
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john4200 said:
Typical Molex 4-pin power connectors (Molex 8981) are rated at up to 11 Amperes per pin. That comes to 55W on the 5V pin and 132W on the 12V pin. So you should be fine on the Molex connector itself.
Click to expand...
If I were to split a molex, that one would have to provide power for 8 drives, right? that is 8 * 20 = 160W. I'll probaby have to substract the 5V draw from this.
All things considered, it might just be safe.

john4200 said:
As for the the PSU and the wires from the PSU to the connectors, that is more difficult. You should start by looking up the specs on your power supply to see how much 5V and 12V current it can supply. If it is not single-rail, then see how much it can supply to each of your modular cables.
Click to expand...
The PSU can take it: 42A for 12v (single rail) and 24A for 5v

john4200 said:
For the cables themselves, if it uses the same wire as a typical Molex connector, that would be 18 AWG, which is typically used up to 16 Amperes for chassis wiring. So that is 80W on the 5V wire and 192W on the 12V wire. If you are lucky, they might have used 14 AWG wire on the cables, since they have two Molex connectors on each cable, and 14 AWG wire is typically used up to 32 Amperes. That would give you 160W on the 5V wire and 384W on the 12V wire.
Click to expand...
I'm 100% sure it is only 18 AWG. So one wire path holds up to 12 drives. 12 * 20 = 240. Again, substracting the 5v stuff may get me close to what is still safe.

john4200 said:
You said your drives can draw 20W of power. If that is worst case, during spin-up, then you can probably use the approximation that the 20W is all on the 12V wiring during spin-up, since HDD motors typically use 12V.
Click to expand...
If the 20W/drive is entirely on the 12V line, would you agree I'm in trouble with my plan to use a splitter? (solution 1 in my first post)

I'll have to rethink a bit then. I need molexes not SATA's (I'm powering backplane not drives directly).

edit: sorry I misread. You got it right. The thing is, even if I do get an extra cable, I've got a small problem connecting that cable to the PSU. The number of "holes" for SATA/Molex cables are limited... BTW, where did you find the number regarding how many amps the connector and wire can hold?
 
Jeroen1000 said:
If the 20W/drive is entirely on the 12V line, would you agree I'm in trouble with my plan to use a splitter? (solution 1 in my first post)
Click to expand...

Not exactly in trouble, but probably over spec and/or typical usage. You might get away with it since spin-up is only for a few seconds. You'd certainly not want to run over spec continuously.
 
Spin up time is so short that you can ignore the power requirements for that time period.

10w is a lot for a modern hard drive.

I have 8 hard drives on one molex conductor (one power supply cable). I never considered that was close to a limit.
 
I'll have a looksy what kind of power the drives draw after spinup.
Just out of fear, is there a way to find out when I'm close to the limits? And, if I push my luck, will the PSU safe the day? Or should I keep the fire dept. on stand by hehe.

John, are these numbers part of the ATA specification? I'm trying to google them :)). Thanks for running them down though. I think I might just be okay.
 
Jeroen1000 said:
Just out of fear, is there a way to find out when I'm close to the limits? And, if I push my luck, will the PSU safe the day? Or should I keep the fire dept. on stand by hehe.

John, are these numbers part of the ATA specification? I'm trying to google them :)). Thanks for running them down though. I think I might just be okay.
Click to expand...

As someone already mentioned, you can measure the voltage drop. In other words, measure the voltage between 12V and ground at the PSU end of the cable, and then measure it at the backplane end during spin-up. If the voltage drops too far below 12V, then you might have a problem. But it is a little tricky to do, since you need to find probe points to make the measurement -- hopefully there is something exposed on the backplane you can get your probes onto.

No, not the ATA specification.

The spec for Molex connectors can be found by a web search, or wikipedia.

The current capacity for various gauge wires comes from something I copied down years ago. I don't remember the source. But I guess you should be able to find something on a web search.
 
just check for hot wires and hot connectors. If they're all reasonably cool and the PSU doesn't shut itself off then you're probably fine. Make sure your molex connectors are secure though, they can be picky about that and partial insertion seems to drastically reduce contact area on the pins.
 
john4200 said:
Typical Molex 4-pin power connectors (Molex 8981) are rated at up to 11 Amperes per pin.
Click to expand...

I saw this on wikipedia. But that's with a 30 °C rise ! I wouldn't be happy with anything close to that.
 
Aesma said:
I saw this on wikipedia. But that's with a 30 °C rise ! I wouldn't be happy with anything close to that.
Click to expand...

Well, the Molex 8981 has an operating temperature range of -40C to +105C. So the ambient would have to be 75C before a 30C rise takes it out of spec. Not likely.
 
I also asked Silverstone what they think. I'm going to play it safe and just pull the extra cable. Although I'd only be out of spec during start up, it is hard to tell how big a deal this is.
 
john4200 said:
Well, the Molex 8981 has an operating temperature range of -40C to +105C. So the ambient would have to be 75C before a 30C rise takes it out of spec. Not likely.
Click to expand...

I'm just saying I don't want to have cables/connectors at 50C or even 60C now that it's summer time.
 
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