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Power supply basics


PEG power connectors

Alright, let's continue with the PCI express graphics power connectors. These are often called PEG power connectors so I'll refer to them by this abbreviation from here on down. As was explained in the previous post there is a current limit per wire for any leads running from the power supply to the mainboard or other devices. This is the reason that some graphics cards have a single 6-pin power connection, some have dual 6 pin power connections, a few have one 6 pin and one 8 pin and extreme cards like the GTX 590 and HD6990 have dual 8 pin connectors. There are even cards that have a combination of 3 of these connectors.

The difference between the 6 pin and 8 pin power connectors is easy but also important:

  • A 6 pin PEG connector can deliver 75W of 12V power to the graphics card.
  • An 8 pin PEG connector can deliver 150W of 12V power to the graphics card.

PEG 6 pin connector. PEG 8 pin connector.

There is a 6+2 pin PEG hybrid connector as well. Since this allows you to plug it into an 8-pin socket that cable should be able to deliver 150W, otherwise the power supply would (should) not come with this cable!

PEG 6+2 pin connector.
Additionally, the PCI express slot itself can deliver 75W of power to the graphics card.

6+2 pin fairytales

Remember, ratings ultimately determine how much current can safely flow through a particular electrical connection. How much (Amperage) it can take is not the same as how much current will actually flow through any connection. Therefore with the 6+2 wire connecting it to a 6-pin socket won't overload anything. The socket will simply pull a theoretical maximum of 75W from this plug while the plug can provide a theoretical maximum of 150W.

Knowing what power connectors are on your graphics card makes it easy to determine what kind of Wattage your PSU must be able to deliver to the graphics card alone:

  • A GTX 560 with dual 6 pin connectors is allowed to draw a theoretical maximum of 75W (slot) + 75W (6-pin) + 75W (6-pin) = 225W from the power supply. This means you have to allocate 225W for the graphics card alone, on top of all the other stuff that is in your system. For dual cards you thus have to have 450W of power allotted for graphics. It's important however to also look up the specifications of your card. According to NVidia specifications the GTX 560 draws a maximum of 150W, not 225W. This makes sense because maxing out the power lines running to your card (although theoretically allowed) is generally not considered to be good practice from an engineering point of view. The slightly more powerful GTX 560 Ti draws another 20 Watts (170W) so it also totals well within the theoretical 225W the cables and slot can carry.
  • A GTX 580 which has one 6 pin and one 8 pin PEG connector can thus draw a theoretical maximum of 75W (slot) + 75W (6-pin) + 150W (8-pin) = 300W from the power supply. A quick look at the GTX 580 specifications shows that in reality the GTX 580 draws a maximum of 244W total so this is again well within the allotted 300W.

Custom cards vs. reference cards

Various graphics card designers have boards of their own design as well as factory overclocked GPUs on these boards. The power your graphics card consumes can therefore differ from the reference power requirements that are mentioned in the examples. The specificied Wattage of the slot, 6-pin and 8-pin power connections does not change however so the theoretical maximum for a card is always easy to determine by which power connectors are on the card.

Native PEG connectors

If a power supply natively supplies the correct power connections for a graphics card, connecting it is obviously a no-brainer. Remember however that a power supply also has to be able to power the rest of your system as well. I was able to put my system together on experience and took 300W for mainboard, CPU and other components (Phenom II X6 3.7GHz, some harddisks, fans, lighting, etc.), added the theoretical maximum of 300W for my dual GTX 560s adding up to a total of 600W.

Reality check

My power supply is a Corsair HX650 which is rated to deliver 650W continuously up to an ambient temperature of 50C. Measurements during gaming showed it pulled 522W peak from the wall socket. Taking into account the 80% efficiency of my PSU that means the components were actually drawing just 522W x 80% = 418W. All in all my system is balanced: it's well within the theoretical maximum power draw and gaming usage still leaves a margin of a pretty massive 650 - 418 = 232W!

Don't gamble!

Taking a 300W baseline usage is probably enough for most systems out there although for an overclocked 8-core Bulldozer system you'll probably want to take a considerably higher margin. Be aware though that even these power-hungry systems are only power-hungry if you stress them to full load. Normal webbrowsing, gaming or watching a video will most likely keep them under the 300W baseline as well. You do need to account for the headroom however, you wouldn't want your PSU to shut down if you encode a video ;-)

Check against multiple sources

Don't start calculating just yet though, there are more fool-proof ways than taking a default baseline of 300W and even if you do, you should check your outcomes against a PSU calculator. There are more and more of these out there and once we've discussed actual knowledge I'll point you to a few!

Rails, limitations and more

Talk about your PSU shutting your system down brings us to a topic that probably a lot of you have been waiting for: rails, overcurrent and overcurrent protection. I mention these in one sentence because they are intimately tied together. All of them have to do with the maximum current (Amperes) that your components are allowed to pull from your PSU or part of your PSU (rails). This subject will also bring us to the molex-to-PEG converter cables and what to watch out for when using those.

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