How many watts an 8pin can support?

[keretzK]

Im trying to connect 3 GPUs with a 125W power limit to the same PSU. The PSU only have one 8-pin connector for what i can see. 
I used a splitter to get to the three GPUs and im aware that the maximum power recommended its arround 290W.
How likely is that the cable wont melt? what other option there are without buying another PSU?

[Skiiwee29]

a single 8 pin PCIE power connector is only rated for 150 watts... you will most definitely risk melting and fire and component damage if you want to adapt it to allow 3 GPUs off it. You absolutely need to upgrade your PSU to properly support 3 GPUs. 

[keretzK]

@Skiiwee29 Thank you!
I was using this image as reference, probably wrong for what you say... although i been using 2 GPUs with the same cable.... 
Anyway, i'll have to look for another PSU.... any idea of how much time do i have before it melt? i assume every second its a risk but i was wondering if it would last 24h so i can get the replacement without turning off the PC.

[Eigenvektor]

Not sure where those numbers are from, but a 6-pin is rated for 75w and an 8-pin is rated for 150w.

 

Going from 6-pin to dual 8-pin as pictured and drawing 200w+ is just asking for trouble.

 

There's no minimum safe time. As long as the GPUs are idle, it's potentially fine, but I definitely wouldn't want to risk starting a fire that way.

[Somerandomtechyboi]

If it doesnt catch on fire then id consider it fine but a setup like this is def risky, if your adapter or psu has thin wires then thats when you should consider swapping out the psu or just getting power from other stuff like molex to 8 pin adapters

 

i can run a 150w gts 250 just fine with molex - sata - 6+2 pin adapter, you can say its gonna catch on fire all you want but unless the cables get really hot and actually pose a risk of catching fire you arent gonne convince me, atm even if i stress it the cables seem to only get warm, from what i can tell its around low 30c in temp

 

Again im a cheapskate so im only gonna swap cables under the condition of the current ones get really hot and pose a real risk of fire, and i mean burning hot not lukewarm "hot"

 

What id suggest here is that if your cables dont get too hot to touch its fine although if you arent a cheapskate like me then just get a better psu to be safe

[mariushm]

PCI-e organization imposed the limits of 75 watts for the 6 pin connector, and 150 watts for the 8 pin connector. 

They're imposed for safety and for other reasons.

 

The wires can handle higher currents, up to 10-13A per wire, with reasonable heating (for example at 10A, the individual wires may heat from 25c ambient to 40c if spread out, maybe 50c in a bundle (ex under a sleeve wrapping the whole 8 wires) . They can do more if you don't mind bigger voltage drops and higher temperatures (the insulation is typically rated for at least 85c)

 

The metal pins inside the connectors are rated for at least 9A of current.  Depending on the construction of the pins both in the connector AND on the video card or motherboard, the limit may be higher. For example, the EPS connector (cpu 8 pin) may have pins that are made by bending the metal into a square tube and then placing a cap at the top to create the pin OR the pins may actually be made from solid thick metal polished at the top - the first kind of pins hollow on the inside are only rated for 9-10A, while the solid core ones are rated for 13A+ .

 

As a note: EPS connector that uses the same series of connectors rates the 4 pin version to 9A per pin, but derates the 8 pin version to 7A per pin for safety, because when you have 4 pairs of pins so close together all four producing some heat due to contact resistance there's higher risk of melting the housing of the connector with so much current through the pins and therefore they lower the maximum safe limits to 7A ... so the 4 pin EPS is rated for 216w (12v x 2 x 9A) for 4 pin, and only 336w for 4 pin (12v x 4 x 7A) 

 

So in theory, an pci-e 8 pin connector that has 3 pairs of wires carrying 12v to the video card could deliver safely around 9A through each pair. This means you could do up to 12v x 3 pairs x 9A =  324 watts. 

 

The PCI-e limitation of 150 watts is chosen to allow 2 8 pin connectors on a single cable coming from power supply - keep in mind that there's ONE connector plugged into the power supply, usually with 3 or 4 pairs of 12v wires which are then shared between those 2 connectors. So,  2 x 150w max = 300w max, which is within the reasonably safety limits of the connectors on the power supply end (if the power supply is modular)

 

The limitation is also chosen so that if for some reason the cable is damaged or one wire is broken or one pin is loose in the connector and doesn't make good contact, the other 2 out of 3 pairs of wires in a 8 pin connector can still safely carry that 150w  (12v x 2 pairs x 9A = 216 watts).

 

It's also chosen so you don't actually get close to the individual pin current limits, or the wire limits, keeping the cables cooler and with less voltage drop on the wires. Less current through each wire, means less voltage drop on the wires, means cooler wires. 

[Guest]

1 hour ago, keretzK said:

How likely is that the cable wont melt? what other option there are without buying another PSU?

The cable won't melt.  The terminal will.

 

So, if the the wires are 16g, that means that HCS or comaptible terminals are used. And if the connectors are UL 94 rated, they won't melt.

 

[mariushm]

1 hour ago, keretzK said:

Im trying to connect 3 GPUs with a 125W power limit to the same PSU. The PSU only have one 8-pin connector for what i can see. 
I used a splitter to get to the three GPUs and im aware that the maximum power recommended its arround 290W.
How likely is that the cable wont melt? what other option there are without buying another PSU?

 

Keep in mind a few things: 

 

1. video cards don't take all power from the 8 pin connector. They can also take up to around 65w from the SLOT.  (officially it's 10w on 3.3v and 65w on 12v but recently I think they loosened the standards to allow up to 75w on 12v)

The pci-e slots are powered by the only 2 (if my memory is correct)  12v wires in the 24 pin connector. So, assume around 200 watts can go into the motherboard through the 24 pin connector, which are then split between fan headers, rgb headers, and possibly  12v to 5v / 3.3v  dc-dc converters on the motherboard that may power things like ram, m.2 connectors (m.2 has only 3.3v but because a m.2 SSD may go up to 3A when writing, some motherboards don't use the 3.3v from PSU but use a dc-dc converter to produce clean 3.3v from 12v)

 

2. While the video cards may have a 8 pin connector, that doesn't mean they "pull" 150 watts from that connector. For example, they may pull 80-90w from the 8 pin connector, and 30-50w from the slot, for a total of around 125w. They had to go with pci-e 8pin because they pull more than 75w, and maybe to allow overclocking without overloading the slot or a 6 pin connector.

 

3. IF your power supply has only one 8 pin connector, the first thing you should do is to look at the label of the power supply and see how much current the psu can actually supply on 12v  (doesn't matter how many adapters you have, if the power supply can only produce 250-300w on 12v)  and second ask yourself if you trust the power supply to not lie about its capabilities on the label. If it's from a brand with decent reputation, chances are it's not lying. If it's some cheap model or some chinese psu that came with the case, most likely the psu is inflating the numbers. 

 

You can have pci-e 8 pin to 2 x pci-e 8 pin adapters - these would be your best bet.  You know the input can take nearly 300w and then you have 2 8 pin connectors, so lots of wires between the input and output.

 

You can have 2 x molex -> pci-e 8pin ... each molex has only one 12v wire, and each molex is rated for 5A of current, so in theory your adapter would be good for 2 wires x 5A x 12v = 120 watts. 

 

The 2 x SATA -> pci-e 8 pin I don't recommend. SATA is more loose, and it's only rated for 1.5A per contact -there's 3 12v contacts in the connector so you have max 4.5A of current per sata connector. In practice I wouldn't use them for more than 3A per connector. 

So in theory your adapter would do  2 sata x 4.5A x 12v = ~ 110w  but like I said, I wouldn't use them for more than let's say a pci-e 6 pin and only if have no other option.