how to determine amp in psus

[jhyukkang]

i want to know how to findout amps!!!
can you guys give me some examples??

 

[Mondoman]

Almost all modern power supplies have a sticker on the side that lists the amperage ratings for each voltage. newegg.com normally has a photo of this sticker as one of the pictures for each power supply it sells.
Sometimes the information is available at the manufacturer's website. Sometimes it's also listed on the vendor's website in the product description, although these listings often have errors.
For example: http://www.newegg.com/Product/ShowImage.aspx?CurImage=17-104-935-03.jpg&Image=17-104-935-09.jpg%2c17-104-935-06.jpg%2c17-104-935-02.jpg%2c17-104-935-03.jpg%2c17-104-935-08.jpg%2c17-104-935-05.jpg&S7ImageFlag=0&Depa=1&Description=FSP+Group+FSP400-60THN-R+400W+Power+Supply

 

[jhyukkang]

i know that but i mean the one you guys talk about..
you know what im saying
lets say im buying graphics card and what amp do i need??

 

[lostandwandering]

you need to look at the +12v rails. You can't just add the amps that you see though.

For example: you have a 400watt unit that has 2 +12v rails with 18 amps each and 360watts are supplied to the +12v rails. Now, you would think "just add 18 and 18, 36 amps", that would be wrong. Each rail can support up to 18 amps independently, but they can't sustain that combined. What you need to do is take that "watts supplied to the +12v" number (the 360w) and divide that by 12. That number will be what the rails can support constant. So, in the case of our 400watt unit, you would have 30amps on the combined +12v rails (360/12=30).

edit: most tags on PSUs will either list the combined amps or they will give you the watts supplied to the +12v rails.

 

[lunyone]

i know that but i mean the one you guys talk about..
you know what im saying
lets say im buying graphics card and what amp do i need??

Well you can buy a calculator program to generate your total power requirements, or you can use this "lite" version and figure out the PSU that you need.
http://www.extreme.outervision.com/psucalculatorlite.jsp

Most manufacturer's will suggest certain power requirements to power their particular GPU. You can usually find them at their respective websites.

 

[Mondoman]

luny, that calculator doesn't help much because, for example, it assumes every PCIexpress graphics card draws the same amount of power.
jh, for a +12V multi-rail PS (which applies to most that you'll be looking at), +12V2 is the rail used by the CPU, so make sure that is at least 18A (20A is better). If there are only two +12V rails, +12V1 supplies the whole rest of the system including GPU; on PSs with 3 or more +12V rails, +12V3 should be supplying the GPU via a 6-pin connector. Make sure the rail supplying the GPU has at least 18A (or 20A if only 2 rails total).

 

[Zorg]

luny, that calculator doesn't help much because, for example, it assumes every PCIexpress graphics card draws the same amount of power.

I just checked the difference between an X1950pro and an 8800 ultra and I got a difference of 60 watts. I don't know how you came to that conclusion.

 

[systemlord]

I just checked my PSU needs for the past 5 months and my Antec 480W PSU I calculated that I was using 472 watts, thats cutting it close to 480W - 472W = 8 watts headroom! Good thing that my Antec PSU got a kicka$$ #10 award from Maximum PC mag. When I get my 8800 GTX I'll only need 54 more watts on top of 472W.

 

[lunyone]

luny, that calculator doesn't help much because, for example, it assumes every PCIexpress graphics card draws the same amount of power.
jh, for a +12V multi-rail PS (which applies to most that you'll be looking at), +12V2 is the rail used by the CPU, so make sure that is at least 18A (20A is better). If there are only two +12V rails, +12V1 supplies the whole rest of the system including GPU; on PSs with 3 or more +12V rails, +12V3 should be supplying the GPU via a 6-pin connector. Make sure the rail supplying the GPU has at least 18A (or 20A if only 2 rails total).

Funny, I just plugged in a x1300 PCI-e GPU and a 2900xt 1gig version and got a 127 watt difference, so I'm wondering what your talking about?

 

[Mondoman]

luny, sorry! In the calculator, I was just looking at the PCIexpress card section before and didn't notice the separate video card section listing individual video cards.

 

[lunyone]

luny, sorry! In the calculator, I was just looking at the PCIexpress card section before and didn't notice the separate video card section listing individual video cards.

Not a problem, just was trying not to confuse people. I even had to check for myself, becasue I didn't want to feel that I was giving out bad advice.