My cable is gonna melt :( plz help

[Dumb guy]

Hello guys, right now Im having a problem with my GPU ( GTX 960 2gb) power usage. The problem is that is not getting 75 watts from the pcie express on the motherboard, instead is getting more power from the 6 pin. This is a problem for me because im using a MOLEX to 6 pin power adapter and i dont want the thing to melt. 

 

The TDP of the GTX 960 is 120 Watts, but in my testing never got over 110 Watts.

 

[kelvinhall05]

The only solution you should even think about here is to upgrade your PSU.

[Dumb guy]

1 minute ago, kelvinhall05 said:

The only solution you should even think about here is to upgrade your PSU.

Yeah i know, i will upgrade the PSU in a few months but i dont have the money right now ( I just bought the GPU). and i was wondering why is the pcie express not proving 75 watts. Do you know why?

Thanks

[kelvinhall05]

Just now, Dumb guy said:

Yeah i know, i will upgrade the PSU in a few months but i dont have the money right now ( I just bought the GPU). and i was wondering why is the pcie express not proving 75 watts. Do you know why?

Thanks

The card will only draw all of its power from the PCIe slot if it has no external power connectors. It doesn't prioritize the slot's power.

[Kilrah]

3 minutes ago, Dumb guy said:

and i was wondering why is the pcie express not proving 75 watts. Do you know why?

Becasue there is no reason for a card to max out the slot power when it has a perfectly good separate supply. 

[jaslion]

6 minutes ago, Dumb guy said:

Yeah i know, i will upgrade the PSU in a few months but i dont have the money right now ( I just bought the GPU). and i was wondering why is the pcie express not proving 75 watts. Do you know why?

Thanks

It will draw from the pcie power connectors first and slot second

[Dumb guy]

6 minutes ago, kelvinhall05 said:

The card will only draw all of its power from the PCIe slot if it has no external power connectors. It doesn't prioritize the slot's power.

Oh Thank you

[Dumb guy]

5 minutes ago, Kilrah said:

Becasue there is no reason for a card to max out the slot power when it has a perfectly good separate supply. 

And is there a way to make the card prioritize the pcie slot instead?

[jaslion]

Just now, Dumb guy said:

And is there a way to make the card prioritize the pcie slot instead?

No

[kelvinhall05]

Just now, Dumb guy said:

And is there a way to make the card prioritize the pcie slot instead?

No, and you should upgrade your PSU regardless. This is literally a fire hazard. You said so in your OP.

[mariushm]

The video card may or may not pool the power together.

The card has two main dc-dc converters, one that powers the gpu chip (let's say 100 watts) and one that powers the ram on the video card (let's say 10-20 watts).

Each dc-dc converter  can have one or more phases - think of it like cylinders on a car, and each phase can be powered from various sources .. think of power like squirts of gasoline being injected into the cylinders.

 

So in the case of your video card, the gpu dc-dc converter may have 5 phases, and 2 out of 5 phases may be connected to the pci-e slot, and the other 3 are connected to the pci-e 6 pin connector.

So those 100 watts the gpu need are split into 40w to slot, and 60w to pci-e 6pin...

 

Anyway ... each molex connector is rated for up to 5A of current .. or 5x12v = 60 watts. So if you have a 2 x molex -> 1 x 6-8 pin pci-e adapter, it should be fairly safe to use, as those 65w are split between 2 molex connectors.

 

[Dumb guy]

Thank you all of you guys, I guess i will upgrade my PSU asap, or at lest try to use 2 Molex instead of one for now... 

[Dumb guy]

5 minutes ago, mariushm said:

The video card may or may not pool the power together.

The card has two main dc-dc converters, one that powers the gpu chip (let's say 100 watts) and one that powers the ram on the video card (let's say 10-20 watts).

Each dc-dc converter  can have one or more phases - think of it like cylinders on a car, and each phase can be powered from various sources .. think of power like squirts of gasoline being injected into the cylinders.

 

So in the case of your video card, the gpu dc-dc converter may have 5 phases, and 2 out of 5 phases may be connected to the pci-e slot, and the other 3 are connected to the pci-e 6 pin connector.

So those 100 watts the gpu need are split into 40w to slot, and 60w to pci-e 6pin...

 

Anyway ... each molex connector is rated for up to 5A of current .. or 5x12v = 60 watts. So if you have a 2 x molex -> 1 x 6-8 pin pci-e adapter, it should be fairly safe to use, as those 65w are split between 2 molex connectors.

 

Thank you for your explanation, really helpful

[Dumb guy]

12 minutes ago, mariushm said:

The video card may or may not pool the power together.

The card has two main dc-dc converters, one that powers the gpu chip (let's say 100 watts) and one that powers the ram on the video card (let's say 10-20 watts).

Each dc-dc converter  can have one or more phases - think of it like cylinders on a car, and each phase can be powered from various sources .. think of power like squirts of gasoline being injected into the cylinders.

 

So in the case of your video card, the gpu dc-dc converter may have 5 phases, and 2 out of 5 phases may be connected to the pci-e slot, and the other 3 are connected to the pci-e 6 pin connector.

So those 100 watts the gpu need are split into 40w to slot, and 60w to pci-e 6pin...

 

Anyway ... each molex connector is rated for up to 5A of current .. or 5x12v = 60 watts. So if you have a 2 x molex -> 1 x 6-8 pin pci-e adapter, it should be fairly safe to use, as those 65w are split between 2 molex connectors.

 

So the 2 MOLEX has to be in 2 different rails right?

[Kilrah]

Rails depend on the PSU, but you at least want to use 2 separate cable bundles coming out the PSU.

[Dumb guy]

1 minute ago, Kilrah said:

Rails depend on the PSU, but you at least want to use 2 separate cable bundles coming out the PSU.

yeah thats what I ment by rails ( the 2 separate bundle of cables) sorry...

[mariushm]

18 minutes ago, Dumb guy said:

So the 2 MOLEX has to be in 2 different rails right?

IDEALLY yes,  but in practice it would would still be safe with just one rail.

 

The actual wire going to the chain of molex and sata connectors is rated for at least 10-13A  (can handle a bit more but will be hotter than normal and that can damage the insulation on the wire, 10-13A is guaranteed to only raise the cable temperature by let's say 10 degrees celsius, still way below 85c...105c which is the usual rating)

IF your power supply is modular, the individual pins in the modular connectors are rated for at least 9A of current, so the whole chain of molex  or molex + sata connectors is rated for at least 9A of current.

 

IF you use two molex connectors on same chain you basically get this :

 

10A at modular connector on psu or inside psu ---  10A cable between psu/psu connector and first molex ----[ 1st connector] ----- 5A between 1st and 2nd connector ----- [ 2nd connector ]

 

5A is the rating of the molex connectors - it's just a made up limit, for safety reasons, the connectors can do more but they may overheat, the plastic housing could melt or stretch and contacts may become looser, they may go bad over time if this rating is exceeded. I'm saying 5A above because that's the rating.

 

If your video card takes only 75w (the maximum allowed for video card to take on 6pin pci-e), then that 75w will more or less be evenly split between the two molex connectors, so each molex connector will give the video card 75w / 2 = 36 w (rounding it down) or 3A of current.

So you would have ...  [psu output] ---- 6A on cable between psu and 1st connector ---- [ 1st molex 3A] ---- 3A between 1st and 2nd molex ---- [ 2nd molex 3A ]

 

It's a bit better to have two separate rails, or at least two separate chains of connectors, in order to reduce losses in the cable between psu and molex connector.

For example, let's say there's 1 meter of cable between psu and the first molex connector, so there's 2 meters in total between psu and connector (because electricity goes back through the negative/ground wire)

An AWG18 rated wire has a resistance of around 0.021 ohm per meter of cable.

 

So the voltage lost between the psu and connector will be Voltage = Current x Resistance  =  3A x 0.021 ohm x 2 meters = 0.126v 

Your power supply may output 12v, but your video card will "see" 11.874v , which is still fine, because the video card will convert this to voltages like 1v..1.4v

 

If you use two molex connectors on the same chain, you'll have a bit more losses on the cable between psu and connector, in the above example, you'll have 6A x 0.021 x 2 meters = 0.25v  so your video card will see 11.75v which is still fine.

 

This is why there's 3 pairs of wires giving power to a video card in a pci-e connector : you get 3 pairs of wires, so much lower losses.

A 8 pin pci-e cable is rated for up to 150 watts, or 150w / 12 = 12.5A  of current, but since you have 3 pairs of wires this means each pair of wires only has to carry 4A of current... which means you can give 150w to the video card with only around 0.1v of losses between psu and video card.