Power spec for the PCI-E 8 pin power plug

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John_VanKirk

John_VanKirk

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Hi everyone,

Question regarding the PCI-E graphics card accessory power cables. The 6 pin PCI-E cable has 3 12V lines (yellow) and 3 ground return lines (black) and is rated for an additional 75 watts of power for the card.

However, the 8 pin PCI-E cable has 3 12V lines and 5 ground return lines (black). No more 12V lines, just 2 additional black gnd lines.

I was under the impression adding the additional 2 pins was supposed to deliver an additional 75 watts of power to the card. (75 watts from the motherboard, and an additional 150 watts from the 8 pin accessory plug.

Why didn't they add 1 12V line and 1 black gnd line, rather than 2 additional ground lines?
Doesn't make electronic sense!

The #18 leads can carry a max of 10 amps each, so 3 leads @ 12V could theorically 30 Amps or 360 watts. Same for the black ground leads. So why just add ground leads instead of 1 and 1?

Thanks,
John Van Kirk
 
Solution
T
Something that I think hasn't been addressed in any of these posts is that the issue is not only about the PSU connector or wiring power handling capacity.

The power handling ability of the circuit board must also be considered. There can be even greater restrictions on the power handling ability of the "wiring" (aka "lands") in the respective motherboard or adapter card. The power-handling ability of a land generally depends on the copper alloy used, the thickness and width of the land, and the material of the circuit board (e.g. ability to handle localized circuit heat). The circuit-board developer faces quite a few physical and electrical limitations in physically implementing a given circuit design.

For example, a specific PSU to...
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The original 6 pin, 75 watt specification was for a minimum current of 2.08 amps for each +12 volt line, hence 75 watts.

3 +12 volt lines x 2.08 amps each = 75 watts.

However, The 6 pin PCI-e power connector is actually capable of handling more than 75 watts.

3 +12 volt wires x 4.16 amps each = 150 watts maximum wattage.

The two black ground wires were added to handle the increased current. 😀
 
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There must be an electronic reason for adding 2 black ground return leads to the standard, and not adding any more hot 12 V leads.

The 12V, 5V, & 3.3V leads are spec'd at #18 gauge, which can carry a max of 10 Amps per #18 wire. The 3 black return leads could therefore carry a total of 30 Amps, from the 12V yellow positive wires, which is a capacity of 12*3*10 = 360 Watts. So the capacity to go from 150 W to 300 Watts is already built into the design, without any additional black ground leads.

If you look at the ATX 24 pin power connector, there are 2 12 V yellow, 5 5 V red, and 3 3.3 V orange positive leads (10), but only 8 black return ground leads. Could the additional black return leads make up for fewer return leads on this power connector. However many PSU units keep the ATX12V and EPS12V accessory power cables on a separate rale from the main power connector, so that probably is not the answer.

Could possibly the black return leads be #20 gauge, rather than #18 Gauge, which would have less current carrying capacity?
The other item I thought about is that possibly the Molex tin male/female connectors might be rated at a lesser amperage, and be a limiting factor rather than the wire gauge. I search the Molex website for current ratings, but didn't find any listed.

Lastly, could the 1 or 2 of the additioinal black leads be used for sensing an 8 plug EPS12V plug, rather than a 6 pin one.

Maybe someone with in depth knowledge of the ATX12 V2.2 PSU specs might have the answer from thorougly reading the white page.

Any other ideas why the engineers would add 2 black return leads to the connector, and say it will now handle an additional 150 Watts?
 
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John - Remember, I said the original 6 pin PCI-e connection was capable of handling more than 75 watts. The reason two additional ground wires instead of a ground and a lead were added in the PCI-e version 2.0 specifications is that resistance is reduced with an additional ground more than it is with an additional hot lead. One of the two additional ground wires also functions as a sensor. This all happened in 2007 when Nvidia and ATI were about to introduce really power hungry video cards.

 
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Johnny,
Thanks for the information, sounds solid. Do you by chance have access to the PCI-SIG forum thru work or an association membership? They are the group I think with all the PCI Express specifications, but their membership must be at the corporation or engineer level since yearly membership is $3000, which I don't have access to. I tried to dig deeper into the ATX 2.2 specs, but the info doesn't get into lead carrying capacity or card contact current capacity, or wire gauge specifications.

A point of note for all is that the #18 gauge wire used in these power connectors theoritically can carry a max of 10 Amps of current. (Referenced in Scott Muellers book and the ARRL handbook.) Reviewing the older PC, AT, and ATX power connectors, the engineers have seemed to limit the current per lead to 5-6 Amps.

The PCI-E 6 connector is shown with 3 12V yellow leads, carrying 2.048 Amps * 3 supplying the additional 75 Watts for the graphics card.
I had read somewhere that the PCI-E 8 pin connector carries 4.096 Amps on each lead, for the 150 watts additional power. Still 4 amps is very conservative and not anywhere near a point requiring adding additional wires.

All of this may be moot in the overall scheme of knowing that a PCI-E 6 or 8 pin connector should supply adequate power to the newer graphics cards, but it is interesting to try to understand the engineer's reasoning for making changes.
 
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I do not have access to the PCI-SIG forum.

I've never found complete, detailed atx specifications on the web. The information is in bits and pieces in the form of excerpts or discussions.

Nvidia and ATI pushed for the increased current because they were getting ready to introduce power hungry video cards. If memory serves, Nvidia came up with the two additional ground wires..

It's not much different from Intel's push for the original atx standard. The Intel cpu's were running hotter and hotter. Back then the cpu's did not have heatsinks and cases did not have much in the way of fans. Intel specified the power supply should be located near the cpu because the psu fan could help cool the cpu. When the horizonal desktop pc's were converted to vertical towers the power supplies wound up in the top rear of the cases. There have been a lot of changes since then.

The current movement is heading in the opposite direction. The push for energy efficiency as a way to reduce costs in the corporate arena has resulted in components that use less energy. The movement also resulted in benefits for gamers and enthusiasts. We now have more efficient power supplies and cpu's and gpu's that use less power.
 
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Johnny & the group,

Have learned several interesting facts about the PCI-E plugs and power after researching it this past week, that is worth sharing

The PCI-E connecter has 6 pins. They are not all used for power. For the 6 pin connector, Pins 1 & 3 are 12V & each can carry 8 Amps. Pin 2 by spec is not connected, although some PSU manufacturers do add a 12V line there. Pins 4 & 6 are Com return lines. Pin5 is Com for sensing. Using 2 lines, you get 12V*8A*2= 192Watts, much over the required 75Watts. With an 8 pin PCI-E connector, 2 Com lines are added (4&8) not a 12V & Com. There, Pins 1,2,3 are 12V, Pin 4 is a Com for the 8 pin connector sensing, Pin 5,7,8 are Com return lines, & pin 6 is for the 6 pin connector sensing. Using that config, 12V*8Amp*3=288 Watts, much over the required 150Watts.

Since Mar 2005, the molex pins are required to be "HCS" rather than "Std", which each carry a max of 11Amps. So properly made, an 8 pin PCI-e can supply 12V*11Amps*3lines=396Watts of power for the graphic cards.

Ther aren't 4 12V pins on the 8 pin connector, only 3, and 2 leads are used for sensing the connector type.

The other limiting factor of course is the width of the PCB lines on the graphics card, each typically carrying 1-2 Amps. That would also determine how much power it could carry to the graphics electronics.
Also worth noting Scott Meuller in his latest 19th Ed of "Upgrading and Repairing PCs" has this information incorrectly listed.

Hope that answers most of the questions regarding this unusually engineered connector.

John VK
 
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Thanks for posting the information and clearing up the purpose of the two extra ground wires. I learned something new.

When I get home I am going to check the pinout on my video card. I have no idea whether the #2 pin contains a lead or not. I'm betting it does. The better power supplies usually include a few extra features to cover several possible configurations.

I always knew the PCI-e power connectors were capable of delivering more power than necessary. I assumed the manufacturers did it in case power requirements for cpu's and gpu's continued to increase.
 
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I checked the 6 + 2 pin PCI-e power cable connectors that came with my Seasonic X650 Gold psu. The 6 pin connector has 3 yellow cables and 3 black cables. The additional 2 pin connector has 2 black cables. Now I'm wondering what the 6 pin PCI-e header on the video card has.
 
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Hmm, the only thing I remember about my new PSU's connectors (Corsair) is that only six wires came out of the PSU for PCI-E, then the get the 8 pin connector, they simply plugged two wires into the back of the card side of the plug and added a little two pin connector on to the end.
 
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I just powered down completely and pulled my XFX Radeon HD 4770 video card so I could get a really good look at the 6 pin header. It definitely has 6 pins. The #2 pin is not blank.

EXT64 - I have a Corsair HX620 in my emergency backup pc. I decided to take a look just for the heck of it. The 6 pin power cable connector also has six wires.
 
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Hi Johnny,

I just bought a Silverstone 1000W modular PSU, and the 6 pin PCI-E connectors like yours also have a 12V line in the #2 pin. Scott Meuller in his 19th edition of PC Upgrading and Repair says the ATX 2.2 standard has no lead in position #2, but that many PSU manufacturers add the additional 12V lead for compatiilty and upgrading.

When you think about it, the 2 12V leads in pins #1 and 3 match the Com return leads in pins #4 and 6, with #5 used for sensing the 6 pin PCI-E connector. In the 8 pin PCI-E connector 1 additional 12V lead is added in pin #2, and that matches the additional Com lead now in pos #8. Pin #4 now is the sensing lead to identify the 8 pin connector as compared to the 6 pin connector.

My PCI-E cables are the 6 + 2 kind, where you add on a 2 pin connector with both black Com leads, no 12V leads. If the manufacturers were to use both the 6 pin and separately an 8 pin PCI-E connector, both could be spec'd correctly. However, when they use the 6 pin PCI-E with the add on 2 pin adapter to make the 8 pin PCI-E, they have to add that 12V lead at position #2 to the 6 pin one, or when you add the 2 pin to adapt it for the 8 pin PCI-E adapter, it would'nt be there and wouldn't meet the power specifications.

So they are most probably doing that for flexibility and the increased power requirements of the newer graphics cards. One thing for sure, the power requirements of these mamouth Graphics cards isn't going to go down.
 
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Hi,

I hope this is not overlapping...but I need two cables like this but I can't find anywhere. I mean anywhere that sells them. I don't want to use two 6 pin to 8 pin PCIe. I want 8 pin female to female PCI e connector cabling.

I really do not understand why they are not out there except for some 6+2 to 8 for example. I don't want that. I mean this is surely the most rudimentary of cable requirements. I could knock one up, because I can remove pins and re-arrange cables, but to add to this dis-pleasure is the confusi-ness around why some ground cable are not running straight pin 4 to pin 4 or pin 5 to pin 5. I have a ground cable that is pin 4 one end and pin 5 on the other. Very confusing.
 
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Hi Ace,

What PSU are you using and are the cables modular? I would guess you don't find the 8 pin PCI-e plugs because the 12V + leads are opposite to the standard 12V ATX accessory CPU plug shown below. Checked my 8 pin PSU female plug on a Silverstone 1000 Watt PSU and it is pin for pin the same as the PCI-E 6+2 Video card end.

If you wanted to, you can easily extract the pins from the PCI-e card end using the standard female pin extractor, and then just push them into the position you want on an 8 pin female plug with the original pins removed. No soldering necessary.

If you ever plugged that connector into your 12V ATX accessory MB connector, you would have a direct short and probably blow out your PSU.



 
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Something that I think hasn't been addressed in any of these posts is that the issue is not only about the PSU connector or wiring power handling capacity.

The power handling ability of the circuit board must also be considered. There can be even greater restrictions on the power handling ability of the "wiring" (aka "lands") in the respective motherboard or adapter card. The power-handling ability of a land generally depends on the copper alloy used, the thickness and width of the land, and the material of the circuit board (e.g. ability to handle localized circuit heat). The circuit-board developer faces quite a few physical and electrical limitations in physically implementing a given circuit design.

For example, a specific PSU to connector wire may be able to handle X watts, but for design/space limitations on a given circuit-board the land(s) associated with that specific connector pin may be unable to handle the full X watts, so the PSU spec provides additional wires such that a card can distribute the power amongst other lands as needed. As alluded to earlier, there can also be RF and electrical noise design issues that can be mitigated by providing more connectivity options.
 
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the increase current is comming from the backplate on the motherbord. Remember motherbord desigen dosent use a single layer of circutry, on many mobos their are at least 3 layers, the 2nd is commonly referd to as the backplate, and the 3rd is what you see on the back of a mobo, the 1st is what you see on the front were the cpu is located.

All the power coming of the leads on the mobo power connecter, ushuall
y connect to a series of power regulater chips on the backplate, these are full of transistors, resistors, diodes and capacitors. In fact these PRIC's (Power Regulator IC's) have more nano capacitors in them then the cpu. 2 of the 3 12v pins going to the PCIE card weather its graphics or a diffrent type of PCIE card, draw power directly from the backplate along the PCIe bus.

Because the tottal cappacitance of the PRIC's is equal to the voltage coming out of the wall, when the computer is working harder is draws more power, more watts are stored in the PRIC's cappacitors, and thus you have a higher V reading then that wich is coming out of the wall.
 
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