CAUTION: Molex Y-cable to pci-e - burnt cables

[dani]

Hi everyone,

just thought I'd share my experience I had with my setup. Running 6 Powercolor 270X PCS+ on two PSUs (700W coolermaster each, single rail) I was running out of pci-e plugs. I power 6 of them through the h61 pro btc board, no need for poweerd risers here. 2 cards + mainboard + cpu on psu1 (roughly 5xx watt) and 4 cards on the 2nd psu (<600W).

here is my rig (I added the brown cable after this for less load on the molex string)



I used one molex-y-cable to split one molex so i can have 4 molex-to-pci-e adapters for my two leftover cards. Turned out this y-cable is that poor that it melted. I was lucky to find it before it burnt further. This is how I found it while running:





It literally melted onto my bed.

This setup was for testing purpose - I don't run my rigs usually in my bed. I like to sleep there  

PS: how can I rescale that imagesize? [ img=200x200 ] didn't work

This cable has a thinner 12V diameter than my psu has (like half of it), so it runs hotter on with same Ampere. Anyone can advise me some good molex-y-cables?
I found these: http://www.amazon.de/NX4PY2EB-Nanoxia-4-Pin-Molex-Y-Kabel/dp/B008TSJIU8/ref=sr_1_49?ie=UTF8&qid=1392456139&sr=8-49&keywords=molex+y not sure if they are thicker, though.

[ANJULE]

Don't know if this helps but the standard 4-pin molex(/ide) power connector is specified at maximum of 11A and 12V.
So the maximum power-output per connector is 132W. This is why you have to use so many molex-connectors per pci-e
power-adapter (usually 4/GPU).

All this means that you can't take one output from a line and divide it as much as you want.
One connector and 132W, two connectors/GPU.
The "main-line" from your power-supply has thicker wires to provide more current and power, but theres a limit too.
Usually you can find some documentation about the wire-size printed on the yellow +12v wire.
If the wires are covered in a textile-like mesh, then you will have hard time finding the text.
Thickness is as xxAWG, "xx" being two digit value. AWG is definition such as square millimeter in Europe.
But in AWG the rule of thumb is: "The bigger the number the smaller the wire".

AWG to mm² spreadsheet:
http://www.engineeringtoolbox.com/awg-wire-gauge-d_731.html

Here are some guidelines for maximum current for given amperage:
http://www.rbeelectronics.com/wtable.htm

It seems that 18AWG-wire that is 3feet (about 1m long) can carry 40A (12v) an 240W.
Remember that when counting the length of a wire you must multiply it by 2, because
the same current flows at the "-"wire and thus the length doubles.

[dani]

Thank you for your post, very informative! I'll keep that in mind next time I mess around with this stuff.

Mine was 22, the bigger ones on my psu are 18. So 0.33mm² vs. 0.82mm², that's hell of a difference. Next PSU will have just as many PCI-E cables as I need, just to be safe