[PSU] Can Someone Explain This?

[glo]

So that's what the back of my OCZ ModXstream 600w looks like.

Since wattage = amps * volts, there should be 600w among both rails, right? Why is this off? I looked at the 700w model of the same PSU, and it lists two 25a rails as well, but lists it as 550w~ combined.

I don't get it.

[haxartus]

Each rail could provide 300W but not both at the same time.

[glo]

Why not? That's what I've never really understood.

Also, is there any way to know whether or not a certain PSU has a rail combiner in it or not? AFAIK, non switching models can only supply a single rail's power to a component, like a GPU. So (in this case), I would only have 300w maximum available to my video card.

[haxartus]

Because there are a lot of components in the PSU itself and they might not be capable of providing 600W with the required efficiency and at the required temperature.
In your case the CPU and the GPU are on different rails.

[glo]

Yeah I think I'm wording it all poorly. I guess what I'm getting at is why are PSU makers able to essentially get away with misleading amperage labels.

[haxartus]

It would be misleading if they wrote 50A on the 12v since the PSU can't handle it.

[glo]

It would be misleading if they wrote 50A on the 12v since the PSU can't handle it.

Well, then that would indicate a single rail. :P

It would just make more sense if they labeled the amperage ratings @ 20/20 which is about what the stated max wattage on the 12v rails is labeled as.

[vesseblah]

This is great place to start for some single vs multiple rail PSU theory -> http://www.overclock.net/t/761202/si...rail-explained

I guess what I'm getting at is why are PSU makers able to essentially get away with misleading amperage labels.

Super-short version is: the ATX specs allows manufacturers to embellish the the total wattage because you can include in the numbers also +5v and +3.3v rails.

AFAIK, non switching models

All current computer PSUs are switching mode transformers because it offers much higher efficiency. Even the $30 chinese shit models. Linear transformers able to push the required amperage would be much bigger and heavier, and also probably more expensive.

can only supply a single rail's power to a component, like a GPU. So (in this case), I would only have 300w maximum available to my video card.

When it comes to your video card question... 6-pin PCI-express power connector according to specs must be able to give 150W of power, or 12.5A, no more, no less. 8-pin power cable must be able to give twice as much, or 25A. Connectors in PSUs should always adhere to this rule, and there's no risk of pulling too much power from properly functioning PSU unless you ignore the limitations and use some adapter cables to change 6 pin into 8 pin power. In addition motherboard can give a total of 75W (about 6A) to all PCI sockets, so the maximum possible power draw of graphics card with single 6-pin connector is 75+150 = 225W, usually much less because it's shared with other PCI devices such as sound and network cards. Most cards use 2 6-pin connectors instead of 8 pin connector because PSUs might not be able to handle full 25A on single rail and it can be divided in two cables. Only better PSUs with much higher wattage than 600 has 8-pin connectors for graphics cards.

edit:

It would just make more sense if they labeled the amperage ratings @ 20/20 which is about what the stated max wattage on the 12v rails is labeled as.

You should read it as "25A maximum per rail, 504W (42A) total for +12v output". When you look at the picture there's 25A under each rail, but both are in a bigger box that says 504W. Yeah, it's not entirely intuitive and bit misleading, but follows it's own twisted logic.

[Deleted]

great reply Vasseblah