Power supply - PEG-Slot

The target of this article is the measurement of power consumed by a graphics card based on PCI Express. But where does a PCIe-graphics card get its electricity? A Question we need to solve before we go into details with our measuring method.

The Peripheral Component Interconnect Special Interest Group is responsible for an overall guideline in regard to PCI-Express. Industry-giants like AMD, IBM, Intel and Microsoft belong to this organization - which is perhaps better known under the abbreviation PCI-SIG. Across a bunch of not public accessible papers, things like data communication, clock and - for this article very important - power supply of PCI Express are specified.


Let´s first take a look at the power supply directly over the mainboard through the PCIe-slot. Base-specifaction 1.0a from March 2003 specifies the following maximal values for the different slot-types:

Source: PCI-SIG

Over the PCIe-x1-, x4-, and x8-slots cards can consume up to 25 Watt. Since the 1.0a update of the Base-specfication maximum power consumption of a x16-slot is up to 75 Watt. The following table shows the maximum values for the different power-rails.

Source: PCI-SIG

The calculated electric power maximums are illustrated in the next table. 3.3VAux is used for wake up from sleep and isn´t used by any active card.

Rail	Maximum current	Maximum power
3.3 V	3 A	9.9 W
12 V	5.5 A	66.0 W
		------------------
	Combined (PEG-Slot)	75.9 W

This ends up in a theoretically possible power supply of 75.9 Watt. However total power amount over the PEG-slot is limited by specification to exactly 75 Watt. Nonetheless a huge improvement at this time, if compared to AGP which offers only 20 Watt.