Stupid Question: Why does a CPU 8-pin EPS provide 336W of power but a PCIe cable provides only 150W??

[CommanderAlex]

Ok folks, so I see from a pinout diagram that the CPU 8-pin EPS power cable can provide 336W and has 4x 12V pins, yet a PCIe 8-pin can only provide 150W with only 3x 12V pins, so why do they do that??? Why didn't they make it so a PCIe pin has an additional 12V pin??? Is it just due to the ATX spec or they didn't want to interchange 8-pin EPS with 8-pin PCIe?

 

Just a curious question as to why 8-pin PCIe cables have one less 12V compared to 8-pin EPS.

 

Thanks!

[RONOTHAN##]

Technically speaking, going by ATX spec the 8 pin EPS cable is rated for 225W, so right in line with the extra 12V pin. It's just that the ATX spec has a ton of safety headroom. 

 

2 minutes ago, CommanderAlex said:

Why didn't they make it so a PCIe pin has an additional 12V pin???

They wanted to give PCIe cable detection, so they need 2 pins dedicated to that, 1 for the 6 pin part and 1 for the +2 part. 

[CommanderAlex]

2 minutes ago, RONOTHAN## said:

Technically speaking, going by ATX spec the 8 pin EPS cable is rated for 225W, so right in line with the extra 12V pin. It's just that the ATX spec has a ton of safety headroom. 

That's pretty interesting that they have more safety on ATX than PCIe. 

 

 

5 minutes ago, RONOTHAN## said:

They wanted to give PCIe cable detection, so they need 2 pins dedicated to that, 1 for the 6 pin part and 1 for the +2 part. 

Ah, yeah that would make sense in a way for PCIe. So they why doesn't the CPU EPS have a sense pin? The CPU is going to pull whatever wattage it needs via that 8-pin EPS, just the same as say a graphics card with 8-pin PCIe. 

[RONOTHAN##]

1 minute ago, CommanderAlex said:

That's pretty interesting that they have more safety on ATX than PCIe. 

 

No, they have about the same amount of safety headroom. Practically speaking the PCIe connector can handle about 300W, and the ATX connector can handle a bit under 400W, so they both have about 50% power headroom. 

 

2 minutes ago, CommanderAlex said:

Ah, yeah that would make sense in a way for PCIe. So they why doesn't the CPU EPS have a sense pin? The CPU is going to pull whatever wattage it needs via that 8-pin EPS, just the same as say a graphics card with 8-pin PCIe. 

I've heard a couple theories about this, so take these for what you will. One I've heard of is for PCIe hot plug so that the GPUs know when they can turn on. The other is that because GPUs have split 12V power planes, so they need to know that all connectors are plugged in for the VRM to not attempt to start up with only have the VRM plugged in, while motherboard don't have split power planes and so can work on single 8 pin connectors. 

[CommanderAlex]

1 minute ago, RONOTHAN## said:

No, they have about the same amount of safety headroom. Practically speaking the PCIe connector can handle about 300W, and the ATX connector can handle a bit under 400W, so they both have about 50% power headroom. 

 

I've heard a couple theories about this, so take these for what you will. One I've heard of is for PCIe hot plug so that the GPUs know when they can turn on. The other is that because GPUs have split 12V power planes, so they need to know that all connectors are plugged in for the VRM to not attempt to start up with only have the VRM plugged in, while motherboard don't have split power planes and so can work on single 8 pin connectors. 

Well yeah, I can see that ATX has a bit more safety headroom (as previously stated in your post) so it's just about the same when you look at how many watts/pin they can provide. 

 

Those are quite some theories that's pretty interesting.

 

Thanks!!!

 

I really thought about this the other day when I was on here and following through on some posts about the additional EPS cable and then in my mind about PCIe wattage. I couldn't find an answer and glad I posted my question here.