I think like 25amps on the 3.3 and 30-35amps on the 5v would be safe. Provide plenty of power for any number of hard drives and peripherals that you may want to add. Look for NOS older ATX 1.3 standard power supplies from Delta or some such good manufacturer would be my recommendation. Im running a Enermax Noisetaker like this one http://www.silentpcreview.com/article149-page1.html that I bought off of ebay for $30 years ago in my Dual 1ghz socket 370 P3.

When it comes to 5V rail my best PSU is 8 year old 400W nexus PSU. It has 28A for 5V and 30A for 3.3V. Even 860W PSU in my main PC doesn't have more than 25A for 5V.

Baoran wrote:

When it comes to 5V rail my best PSU is 8 year old 400W nexus PSU. It has 28A for 5V and 30A for 3.3V. Even 860W PSU in my main PC doesn't have more than 25A for 5V.

I think that should work ok. You will know if you dont have enough amperage, peripherals wont power up and things will be a tad unstable. Crashes ect. If this were an Athlon system I would be more worried but that sounds pretty good.

I might need another good psu in near future. You mentioned delta and enermax. Are there any other good brands of PSU that I could look for when looking older atx psu?

Baoran wrote:

I might need another good psu in near future. You mentioned delta and enermax. Are there any other good brands of PSU that I could look for when looking older atx psu?

Seasonic and Sparkle Power are generally good units.

Good ones are probably rare nowadays. I just went through what are being sold in my country. I only see brands like FSP, Deer, Bestec, Q-technology, huntkey and liteon. I assume none of those are really good?

I'd avoid Deer, Bestec and Q-Technology - Huntkey, FSP, Seasonic, Chieftec(FSP), Liteon are all OK.

Depends on the motherboard. Some late dual pentium 3 boards have 12v connectors like they do on pentium 4's. But honestly ANY QUALITY power supply will get the job done. Buy a Corsair unit.

The motherboard is GA-6BXD. It can only do 100Mhz FSB, so the max is 10x100Mhz.

A dual P3 is probably about like a single high-end Athlon.

martin939 wrote:

I'd avoid Deer, Bestec and Q-Technology - Huntkey, FSP, Seasonic, Chieftec(FSP), Liteon are all OK.

On top of that, avoid Linkworld as well.

Baoran wrote:

The motherboard is GA-6BXD. It can only do 100Mhz FSB, so the max is 10x100Mhz.

any quality power supply will be more than adequate

martin939 wrote:

I'd avoid Deer, Bestec and Q-Technology - Huntkey, FSP, Seasonic, Chieftec(FSP), Liteon are all OK.

Liteon is actually excellent. One notch below Delta, Zippy, or Etasis.

luckybob wrote:

Baoran wrote:

The motherboard is GA-6BXD. It can only do 100Mhz FSB, so the max is 10x100Mhz.

any quality power supply will be more than adequate

Watch out with this, a lot of modern PSU's have (very) weak 3.3V and 5V rails 😒

martin939 wrote:

luckybob wrote:

Baoran wrote:

The motherboard is GA-6BXD. It can only do 100Mhz FSB, so the max is 10x100Mhz.

any quality power supply will be more than adequate

Watch out with this, a lot of modern PSU's have (very) weak 3.3V and 5V rails 😒

for a high powered AMD socket 462, your point is valid. But for something like this, a quality new unit will be more than plenty. They have a combined 3.3 & 5v rail. Just get one that has 150W or better (5&3.3v).

Getting 150W on 3.3 and 5V out of a modern PSU is really not that easy as it might seem, you will need something like a 600-700W unit if you want strong 3.3/5 rails and they aren't cheap.
The popular Corsair VS450 has 100W max, CX500 sits at 120W and a CoolerMaster B500 is also rated at 120W.

Imho you're much better off with an older FSP unit as even a 400W one should do 150W on the smaller rails. They have a quiet 120mm fan, active PFC and are dirt cheap. Take FSP400-60GLC for example.
Don't forget the old Tagan and Enermax units too, Tagan TG480 U01 had a whopping 240W on 3.3 and 5v, the smaller 330W one could output 220 on those rails.

Yeah, the rails are pretty weak. I have a modern seasonic 860W 80 plus platinum psu in my main PC and it only has 25A/125W for 5V/3.3V.

a 1ghz cpu draws 35W of power at 1.8v (19.5A) They will draw this from the 5v rail. Assuming 80% conversion: (35/0.😎 or 44W @ 5v (rounded) But thats 88W for 2 cpus you exclaim! Well, neither cpu are going to be at full power 100% of the time. Anecdotally cpu 0 will usually run 100% when #1 will be maybe 25-30%. Best I ever got was quake 3 and it was doing 100%/75%.

Do this. Look at that GA-6BXD carefully. There are 2 voltage regulators per cpu. Each have 3 leads. Do a continuity check between the thick leads and the ATX power connector. One wire will definitely go to either the 5v or 12v line. if it connects to 12v, winner winner chicken dinner, this whole thread is moot. If it connects to the 5v well, don't be surprised.

Ram is going to draw off the 3.3v line. At the draw of 2.5A per dimm @ 133mhz and 256mb per dimm. This is a max load figure. So at most 10A @ 3.3V Again this is worst case scenario.

video and pci cards are all going to draw from the 5 & 3.3v lines too. Depends on what you put in here. \

100W is too low. 150W will be ok. Now if you are doing something weird and pegging the system to 100%, look for something with a beefier power supply suited to the old loads.