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I'm not sure if this question belongs here, so please be kind if it doesn't. I just happen to have two question that are related to the same project, that's why I combined them. I'm building a custom PC and I need to know this.

1)When they say

PCI Express 3.0 x16
    2 (single @ x16 or dual @ x8)

PCI Express 2.0 x16
    1 @ x4

In the specs why does it say PCI Express 2.0 x16 then 1 @ x4. Why does this say 1 @ x4 under x16 (isn't this contradictory)? What does 1 @ 4x mean. Also what does 2 (single @ x16 or dual @ x8) mean. Why is there a difference between single and double. Is 16 and 8 the pci express speed.

2)How do you know how many watts you need for a power supply. Is it mostly intuition or do you go by a certain rating.

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migrated from Feb 25 '12 at 19:38

This question came from our site for electronics and electrical engineering professionals, students, and enthusiasts.

This is off topic here but is on topic at superuser. – Kellenjb Feb 25 '12 at 18:54
up vote 2 down vote accepted


From the description, I assume this is a motherboard with three physical PCI-E 16x slots.
In this case it means that the PCI-E lanes are distributed in a certain fashion:

  • Option A:
    1. PCI-E 16 Lanes
    2. No PCI-E Lanes
    3. PCI-E 4 Lanes
  • Option B:

    1. PCI-E 8 Lanes
    2. PCI-E 8 Lanes
    3. PCI-E 4 Lanes

It's important to note that the physical connector for all three slots is a PCI-E 16X connector, so you can plug a PCI-E 16X device into any/all of them. However, the available bus bandwidth (e.g. the amount of data that can be transfered over the PCI-E bus in a specific period of time) varies, depending on the number of lanes.

PCI-E devices automatically adjust to the number of available lanes. Plugging your main video-card into the 4-lane slot will probably affect the graphics performance.

Here is an article from tom's hardware looking at how PCI-E bandwidth (which is a direct function of number of lanes) affects performance.

Different processing loads effect how much bandwidth a graphics card uses. Here is a discussion on how a GPU configured as a dedicated PhysX processor is effected by available bandwidth. PhysX works far better under low bandwidth situations then normal graphics-card loads.
However, it's probably safe to assume that the only time someone would want to place a GPU in the third slot is after they already have GPUs in the first two, and therefore, using the third GPU as a dedicated physics processor is a fairly reasonable configuration, which is probably why the motherboard is laid-out in the manner it is.


With power supplies, you need to ensure that the wattage of every component in your computer added together is less then the power supplies ratings.

Therefore, you need to look-up the power rating for your CPU and graphics cards (which are the primary power consumers in a modern computer), and add something like ~10W (It should be specced on the drives, but they're generally pretty power-efficent) per CD Drive and hard drive. Finally, add 100W or so for headroom, and there you go.

It's generally a good idea to have a bit of headroom, as running a power supply right at it's wattage ratings will stress it more then running it below it's rated power. If you have a 500W power supply, and you draw 500W from it, it's lifetime will be shorter then if you only draw 400W from it.

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Let me check that I get this, the physical size is 16 lanes, but the number of available lanes is 4 for the first, etc... So basically there is a difference between the size of the slots and physical lanes, and the literal lanes, and the card will adjust? – rubixibuc Feb 25 '12 at 3:54
@rubixibuc - Yes. The first slot has 16 lanes if there is no card in the second, and 8 if there is a card in the second. The third always has 4 lanes. Basically, look at it this way. There is a total of 20 lanes. 4 always run to the third slot. There are 8 that always run to the first slot, and an additional 8 that are switched between the first and second slot, depending on if there is a card in the second slot. – Fake Name Feb 25 '12 at 4:23
However, all the connectors are physically capable of supporting a 16 lane card. The cards auto-negotiate the number of required lanes. If you follow the link above (Specifically here, to toms's hardware), you will see that the way they restrict the number of lanes to a card is to tape over some of them. – Fake Name Feb 25 '12 at 4:25

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