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Can this be right?

post #1 of 20
Thread Starter 
According to newegg, this antec 900w psu has 160 amps on the 12v rails:

http://www.newegg.com/Product/Produc...82E16817371050

+12V1@40A, +12V2@40A, +12V3@40A, +12V4@40A

How is that even possible on a 900w psu? Or am I missing something? By contrast, Corsair's 950 watter only has 78a, which seems a lot more realistic:

http://www.newegg.com/Product/Produc...82E16817139013
    
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post #2 of 20
The total +12V capability is not necessarily (and is apparently not in this case) the sum of the many +12V rails.
post #3 of 20
When figuring out the total +12V amperage capacity, the total wattage capacity is always divided by 12. The rails are never added together. There are no exceptions.

So 850 divided by 12 results in about 71A. It's not often I don't get a perfect calculation, but still: it has a +12V capacity of 850W. 160A is 1,920W.


Edit:
I'd guess that the 900W High Current Gamer's actual +12V wattage capacity is 840W because that results in a perfect 70A. 850W results in 70.83333333333333A, which is silly.

Oh, and 78A multiplied by 12 is 936W.
Edited by TwoCables - 8/5/11 at 2:15am
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post #4 of 20
Thread Starter 
thnx for the info. So that antec's 40amps/rail ... whats that mean then? That each rail can handle a total of 40amps, but the total 12v capacity over all 4 rails can never exceed 70amps? What if you have a psu with a higher wattage than amps on the 12v rail? For instance my corsair 650tx is 650 watts but it's single 12v rail is 52amps (52 x 12 = 624)
    
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post #5 of 20
Quote:
Originally Posted by TwoCables View Post
When figuring out the total +12V amperage capacity, the total wattage capacity is always divided by 12. The rails are never added together. There are no exceptions.

So 850 divided by 12 results in about 71A. It's not often I don't get a perfect calculation, but still: it has a +12V capacity of 850W. 160A is 1,920W.


Edit:
I'd guess that the 900W High Current Gamer's actual +12V wattage capacity is 840W because that results in a perfect 70A. 850W results in 70.83333333333333A, which is silly.

Oh, and 78A multiplied by 12 is 936W.
Are you sure about that math? Aren't you totally disregarding the wattage on the 3.3 V and 5v rails? According to my own calculations a quality PSU will come within 10w of its rated power. For example a 950W PSU will generate a total of 944W off real power.
    
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post #6 of 20
Quote:
Originally Posted by Steven-1979 View Post
thnx for the info. So that antec's 40amps/rail ... whats that mean then? That each rail can handle a total of 40amps, but the total 12v capacity over all 4 rails can never exceed 70amps? What if you have a psu with a higher wattage than amps on the 12v rail? For instance my corsair 650tx is 650 watts but it's single 12v rail is 52amps (52 x 12 = 624)
Higher wattage than amps? What do you mean? I can't make sense of what you're asking because to me, you're talking about the same thing. 624W is 52A, and 52A is 624W. It's the same thing.

Anyway, the 40A means that's the trip-point. I don't know how to explain it better than that because my understanding of it is currently limited. However, I am absolutely 100.00% certain that the +12V wattage capacity is divided by 12 to figure out the amperage, and the +12V amperage capacity is multiplied by 12 to figure out the wattage on it. So again, it's the same thing. I don't know of any other way to answer "what if you hav a psu with a higher wattage than amps on the +12V".

That's why I said 78 multiplied by 12 is 936. Check the sticker! It says 936W!

Check the sticker of every recommended power supply and you will see how this math is consistent.


Quote:
Originally Posted by Bit_reaper View Post
Are you sure about that math? Aren't you totally disregarding the wattage on the 3.3 V and 5v rails? According to my own calculations a quality PSU will come within 10w of its rated power. For example a 950W PSU will generate a total of 944W off real power.
Yes I'm sure.

Ask Phaedrus2129 the next time you see him in a thread (don't PM him - for some reason, he hates that and I will never understand why).

Check the sticker of every recommended power supply and you will see how this math is consistent.

A quality 950W power supply (like the TX950) is capable of delivering 950W continuously. It has a +12V capacity of 936W, or 70A. 936 divided by 12 is 70. 70 multiplied by 12 is 936.
Edited by TwoCables - 8/5/11 at 3:39am
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post #7 of 20
Ok, the power supply can do 70a or so on 12v, which is then fed into 4 rails, each of which can take a 40a load. The fact that if you put a 40a load on all the rails at once, it'd be 160a is unimportant.
The reason the OCP trip points on each rail are so much more than 1/4 of the 12v is to allow you to run a high power GPU on one rail, using anything up to half the power of the PSU, without affecting anything else.
High OCP trips just let you wire up your rig without worrying about what load is on which rail.
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post #8 of 20
V=IR (voltage=current x resistance)

P=VI (power= voltage x current)

With that said, just because a spec states that it has 4 12v rails with 20 amps on each rail doesn't mean that it can supply 20 amps to each rail simultaneously. There can be a limit as to total current available.
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post #9 of 20
Well, as far as i know, "Multiplying the amps on +12V Rail to get the wattage" formula works only on a single +12V Rail PSU.
In multiple +12v rail PSU's the amps given in specs only tell the "maximum amount of load a single rail can take, given that the total load on the remaining rails does not exceed the total 12V load limit of the PSU."
In other words, if a 900 W PSU boasts of 4 +12V Rails each with 40Amp, it doesnt mean that all the four rails can take full load of 40 Amps on each of them simultaneously.
Normally a 900 W PSU can take sumwhere around 800-860 W of load on the 12V rails, the rest being shared with 3.3V and 5 V rails ( use to power RAM sticks, HDDs and fan etc).

The bloated looking amps on the individual 12V rails are provided so that you dont have to mix and match different rails to power a Power hungry PSU (as pointed out already by allikat). Otherwise, a normal 900W PSU with around 71-72 amps for 12V rails, will have 18Amps per rail if there are 4 of them. So if you are using a GPU which requires more than 18 Amps (which is 18*12= 216 W, a fairly normal number for High End GPU's these days) you have to power it using two different rails, which can be cumbersome for new users.

I hope it^ made sense
post #10 of 20
Thread Starter 
Quote:
Originally Posted by TwoCables View Post
Higher wattage than amps? What do you mean? I can't make sense of what you're asking because to me, you're talking about the same thing. 624W is 52A, and 52A is 624W. It's the same thing.

Anyway, the 40A means that's the trip-point. I don't know how to explain it better than that because my understanding of it is currently limited. However, I am absolutely 100.00% certain that the +12V wattage capacity is divided by 12 to figure out the amperage, and the +12V amperage capacity is multiplied by 12 to figure out the wattage on it. So again, it's the same thing. I don't know of any other way to answer "what if you hav a psu with a higher wattage than amps on the +12V".

That's why I said 78 multiplied by 12 is 936. Check the sticker! It says 936W!

Check the sticker of every recommended power supply and you will see how this math is consistent.



Yes I'm sure.

Ask Phaedrus2129 the next time you see him in a thread (don't PM him - for some reason, he hates that and I will never understand why).

Check the sticker of every recommended power supply and you will see how this math is consistent.

A quality 950W power supply (like the TX950) is capable of delivering 950W continuously. It has a +12V capacity of 936W, or 70A. 936 divided by 12 is 70. 70 multiplied by 12 is 936.

lol, I think me and him misunderstood you when you said "When figuring out the total +12V amperage capacity, total wattage capacity is always divided by 12" ... we thought you were referring to the entire psu. But I see now you meant the 12v rails. That's why I asked about my psu being 650w but 52 x 12 being 624, ie- 650 =/= 624. But now I see you meant the total wattage capacity of the 12v rails. Makes sense ... I didn't even think to check for that stat since newegg doesn't report it on their details section. Had to look at the images to find it. That would have cleared everything up from the start, lol.
    
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