# Will someone please tell me?

Dec 24, 2007
1. the one thing i'm really curious to know is how to calculate the amperages offered on the 12v rail(s) of any given psu.

if you were given these numbers, what would you do with them?
output:
+3.3V@23A, +5V@17A, +12V1@17A, +12V2@17A, +12V3@17A, -12V@0.8A, +5VSB@2.5A

i'd like a step by step process if you would please...

i'd like to finally be able to calculate this for myself rather than just relying on the fact that it's a "good psu". any replies and explanations are very much appreciated.

2. ### Ph30nIXTS RookiePosts: 243

As far as I know the only way to determine it is from the ratings given by the maker. I.e That PSU can drive 17A on each of the 12V rails it has. The total wattage it made up of the 3.3v + 5v +12v ratings, and because they can vary and are not a set ratio (as far as I know), it would be difficult to work out. Though I may be wrong. Generally the more watts, the more power on 12v rails, but this isnt the only important factor. 5v and 3.3v are also important, its the overall specs that give you an indication of a quality psu, but this can easily be determined by good brands such as antec etc.

3. ### DaveskaterBannedPosts: 1,687

If you want to work out the combined Amperage of the 12V rails, what you do is this: 12V1 + 1/2 12V2 (+ 1/2 12V3) = total current

Don't worry, I will explain it (Me and SNGX1275 worked this out before )

Basically what that says is you get the Amperage of the 12V1 rail, then add to it half of the Amperage of the 12V2, and do the same for each other rail.

So in your example, it would be 17 + 8.5 + 8.5 = 34A

If there's 3 12V rails then it makes it easier becaues you can basically add the first two together and ignore the last one. However, if there's 2 or 4 then you obviously need to add half the value on for each rail.

4. ### kirockTS RookiePosts: 1,221

Hmm, I'm not sure that's correct or why you think that is Daveskater.
My understanding (and I'm an Electronics Engineering Technologist for 25 years)
is: each rail as list is the peak amps (more often then not). So most good quality PSU brands are rate themselves at around 80-85% efficiency.

So your best guess is to just take 12V1@17Amps and take 0.8 of that. So 17X0.8= 13.6Amps continuous. This is the same for all the other 12V rails.

To calculate the total (12Volt) WATTS it's just 12V X 17 X #of 12V rails=12X17X3=612Watts (now take 80% of that for continuous power).
The total watts of the whole PSU is just repeat for the 5 and 3.3V rails and their rated current and adds these to the results for 12V rails.

So 3.3 X 23A + 5 X 17 + (612watts from the 12V rails)= 723Watts (this is probably rated as a 700W or 725W ) PSU. Now take this number and multiply by 80% (for continuous power out).

5. ### DaveskaterBannedPosts: 1,687

That's fair enough, I see what you mean about the continuous power at 80% efficiency.

The thread where me and SNGX1275 worked out the "add half the amperage..." theory is here, posts 18 and 19. http://www.techspot.com/vb/topic88849.html

We didn't prove it by trying it with loads of different PSU ratings, but we tried it with a couple and it worked

6. ### Ph30nIXTS RookiePosts: 243

I think the first post he has only given an example. I think he is trying to work out the amperages of these 12v rails from only a wattage. Say a 400w PSU, and he wants the 12v amperage, However I am assuming he doesnt have this rating on the powersupply for some reson. And yes, I have seen some power supplies without these ratings, so perhaps this is the case.

Link590o, can you give us confirmation of the EXACT situation you are trying to work it out for, and then we can help further.

you're basically right. my situation is essentially this...if you were given this power supply, and looking at the ratings given, how would you determine the EXACT amperage offered by the 12v rails?? this isn't actually a 700 or 725 watt as kirock suggested. this is actually a 500 watt antec neo power psu.

daveskater and sngx's method seems to make sense, but kirock, if you're convinced the method is false, then perhaps you could clarify better now that you know exactly what power supply i mean. you too if you know how phoenix.

here's the link for the supply i meant:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817103940

8. ### Rage_3K_MoizSith LordPosts: 5,442   +36

I do it this way:
1) Calculate max power on the 5V rail by mutliplying the current on it by 5. This is because practically every PSU creates its 3.3V output by regulating down its 5V rail, so the 3.3V and 5V rails typically have a shared rating. We're looking for a situation for maximum power draw, which indicates that the 5V rail will almost always be the one with the most power available. Thus we're assuming all power is being drawn by the 5V rail and none by the 3.3V rail

2) Calculate max power on the +5VSB and -12V rail by doing the appropriate multiplication.

3)Add together both power values and subtract this subtotal from the total wattage offered by the PSU.

4)Divide the remainder by 12 to get the actual number of amps on the +12V rails combined.

Works every time.

that's not the same method you tried to teach me once before is it?

because if i remember right that method was flawed somehow.

but anyway. i work best by example. i sort of get the idea, but i'm not sure exactly which numbers i'd be using. could you show me the calculations you would do with my psu?

10. ### Rage_3K_MoizSith LordPosts: 5,442   +36

Nah this is a slightly different one. It gives accurate results every time. Here's a breakdown:
1)Power on +5V rail = 5x17=85W
2)Total power on +5V, +5VSB & -12V rails = 85+9.6+12.5=107.1W
3)Power on +12V rails = 550 - 107.1=442.9W
4)Total amperage on +12V rails combined = 442.9/12 = 36.9A approximately.

SWEET. alright. i'm going to look up some various ones...maybe post some examples i did and see if i did it correctly. thanks for the method rage

12. ### kirockTS RookiePosts: 1,221

I know this thread is a little old now: But I did misunderstand your question, simply because I thought you "couldn't" be asking for currents because these are always given. Or if they are not...run! Find a PSU that properly lists it's rated currents and wattage for each voltage rail.

Daveskater: I'm not saying your derivation MUST be wrong (and in those examples you two gave it looks legit), but the best way to determine that would be to multiply the VXI=Watts, add all watts and see if it equals the total rated power of the PSU (650 or 700 Watts.etc.)

some say combined some don't
and even when the specs are true who's to say the quality of components
the cheaper values can work for awhile then poof
ya I got 500 watt out of it for a whole 6 months
or a derated system that hopes you will not over load
I had a 3 ton crane put 4 tons on it (for awhile)

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