Question 5V and 3.3V power draw in modern PC`s?

[naftaliin]

Hello.
I want to use a slim server PSU to power a PC with an mATX motherboard in a small case. Server PSU pretty much only has reasonable 12V rail (26A) and I need to create 5V and 3.3V outputs using DC-DC converters. However I do not know what power draw to expect from 5V and 3.3V. In the past those voltage rails were specked very high. Many websites claim that today most power is taken from 12V but I cant find any actual data.

Has anybody actually measured the power draw from 5V and 3.3V rails in modern systems?

I think PCI-E 16x card can take 3A from 3.3V pins. Do modern graphics cards even fully use that 3.3V?

 

[thx1138v2]

A fairly good review is here: PSU Rails

 

[naftaliin]

A fairly good review is here: PSU Rails

Yes I have read that more than 10 year old piece before. The 2013 ATX12V 2.0 spec had 5V + 3.3V load of 120W. However the about 5 year old Corsair VS350 has max combined load on the lower voltage rails of 90W which likely is designed with some over-provisioning. Was hoping someone has done actual measurements on newer platforms, AM4 perhaps.

Assuming lower voltages are taken directly from PSU and not regulated down from 12V on the motherboard:

5V is used by:
2 1/2 in SSDs
USB
chipset? something else like on-board Wi-Fi?

3.3V:
PCI-E (also M.2 slot)
RAM
chipset?

So realistically couple of SSDs 5W max each would take 2A from 5V, if chipset uses 5V then about 1A there. Leave a few A for USB and I guess 6A would be realistic for 5V. For 3.3 we would have 3A from PCI-E 16x, about 2A or less from M.2 slot for an NVMe drive and up to 3A for DDR4. If chipset uses 3.3V then about 1.5A additionally. So about 10A total for 3.3V.

12V:
CPU
GPU
fans

Actual measurements would be better of course.