Molex to PCIe ...is it REALLY ok to do?

[niic]

Hi all,

I have been reading conflicting posts from people that say it is fine and people that say it is not.

I am setting up a 6 gpu (MSI 7960s) rig powered by a 1600W Lepa PSU on a MSI Z77A-GD65 motherboard. So I am short 2x PCIe cables.

I was going to get a couple of these... Athena CABLE-MPCIE4628 2 x 4Pin to 2 PCI-Express
http://www.ebay.com/itm/Athena-CABLE-MPCIE4628-2-x-4Pin-to-2-PCI-Express-/280686303841?pt=US_Power_Cables_Connectors&hash=item415a351a61

But then I started reading posts about this can cause all sorts of problems because the cards a too powerful and draw too much W. Even reviews that advise against it.
http://www.guru3d.com/articles_pages/msi_radeon_hd_7950_twin_frozer_iii_review,8.html
"...converting them from a Molex Peripheral connector anno 2012 we feel is a no-go"

So is there a definitive answer here or is it always going to be subjective? Really don't want to destroy my hardware or worse burn down my house while at work

Thanks for any advice!

Nick.

[Colin Miner]

So is there a definitive answer here or is it always going to be subjective?

It depends on the current carrying capacity of the cables and the quality of the connections in those plugs and sockets you linked to.

They come in cheap and nasty and higher quality, depending on who makes them. The cheap and nasty ones will have cables that can only handle lower currents than you are trying to pass through them and get hot. Same for the way the cable is connected to the pins.

You could always just take it one step at a time and monitor the temperature rise of the cables, but going for top quality stuff is going to help avoid problems.

Double molex to single PCI-e will carry the current if the quality is up to it.

(2x molex to 1x PCI-e) like these (but I couldn't vouch for the quality) - you will need 2 of them and not use one each of the spare PCI-e connectors.
http://rover.ebay.com/rover/1/711-53200-19255-0/1?icep_ff3=2&pub=5575000824&toolid=10001&campid=5337427080&customid=&icep_item=231067121288

[niic]

hey mate... thanks for the reply, much appreciated.

So here is perhaps a stoopid question ...is there any way I can gauge the quality? Are there any questions I can ask the seller that will give me some idea on really how good the materials are?

Also, when you say "'you will need 2 of them and not use one each of the spare PCI-e connectors."  ...I didn't realise your link was converting two 4x molex pin plug to (6+2) PCIe ...so even though my card only needs 6+2 you are recommending plugging both cables in?

Thanks for the info. Again much appreciated.

Cheers

[ssateneth]

No. This is why.





This was with only 2 radeon 5850. 7950 is more power hungry and will melt/cause a fire in no time. Use only PCI-E plugs, or PCI-E power splitters. Theres only 1 12v wire for molex. PCI-E has 3, and usually is thicker gauge wire.

[aznatama]

If you're mining LTC, you will be undervolting your cards

Calculate the wattage draw yourself

Each 18ga wire's max amp for power transmission is 2.3A, 20A for chassis wiring as per AWG  (chassis wiring = single wire, better cooling; power transmission = bundled wiring, less cooling)

PCI standard for 6-pin is 75w w/ ~2.05A per 18ga lead wire, and 3 GND wires
PCI standard for 8-pin is 150w w/ ~4.1A per 18ga lead wire, and 5 GND wires

Using molex Y-splitters is possible, but be aware that the fewer wires you use, the more resistance and hotter the wires will get.  Also, shit molex connectors only use 20ga wiring.  If you're going to use molex, you need to ensure that the power draw from your PSU is from MULTIPLE LEADS, and not all coming from one lead (as in the picture above)

[niic]

Hey... thanks for the info. Very much appreciated ...some good digits there I wasn't aware of. In light of more recent research I've decided given how much cash I've sunk into this thing, I don't want to cut any corners and for the most part this molex to pcie just seems a little to hacky to me. So I have decided to go the tether option and join 3 PSUs together ...with the bonus of loads more cable options across the rig. I should only have to worry about load balancing now ...I think

From what I can tell, 7x MSI 7950s undervolted will run off 2x Coursair 850AX PSUs with the smaller third one taking care of the motherboard, ssd hdd, and a couple fans. I think this is the safer option with some allowance for a bit of extra headroom in the watt consumption.

Thanks for the infos again. It is a great community this ...regardless of the mining aspect ...i've learnt allot of geeky gpu/power stuff I didn't know before.

Cheers

[Colin Miner]

Also, when you say "'you will need 2 of them and not use one each of the spare PCI-e connectors."  ...I didn't realise your link was converting two 4x molex pin plug to (6+2) PCIe ...so even though my card only needs 6+2 you are recommending plugging both cables in?

Sorry, The only ones I could see one ebay were the ones with double PCI-e, (which didn't add the clarity) 

Plug in all 4 molex connectors, and plug in one PCI-e from each of the new cables.

That way 2 molex plugs power one PCI-e, thats much more up to the job.

As for quality, I had some where the cable was very thin and some had poor crimping, others felt more substantial. Fatter cables carry more current.

I would imagine a respected brand name could be better than unbranded? You may just have to grab a couple and use the best.

[HellDiverUK]

No. This is why.

This is why you shouldn't use modular PSUs.  That melting has nothing to do with running a GPU off a molex, it has to do with a bad connection in the modular cable connection.  If there was a good connection, the WIRE would melt, not the connector.

Most molex to PCI-E cables come with two molex plugs - use two different cables from the PSU so you're splitting the load.  Less chance of melting the stupid connectors.