Chozart

How much current do you really get from the 12V rails of your PSU?

When buying a power supply, the first thing we immediately look at is the total number of Watts. However, this number does not always give a good indication of how much useful power you really get. For modern high-end PCs, it is recommendable to have preferably >35A of total current, on either one or multiple 12V rails. Of course, the amount of current needed depends on what you hook up to the PSU.

When shopping for a PSU, it is not always clear how to determine the available current on the 12V rails. The current is listed in the specifications of the various PSUs, and on the label of the PSU itself. For single rail PSUs this is not a problem, since the current listed for the 12V rail is the total available current on the 12V rail. For multiple rail PSUs, things get a bit trickier, since the current listed for each 12V rails cannot be added up to get the total available current on the 12V rails. The total current on the 12V rails is limited to the available power (Watts) that the PSU can deliver on the 12V rails. Sometimes, this number is listed in the specifications, but in other cases we need derive the maximum power on the 12V rails from the total maximum power of the PSU. To confuse matters even more, manufacturers use different methods to determine how the power of the PSU is distributed along all rails.

Below is a listing of the available current on the 12V rail(s) of several 500W power supplies. All information is obtained from the websites of the manufacturers. Also, this is in no means an indication of quality of the power supplies, nor an endorsement of specific brands. The power supplies are divided in price categories (prices per April 19, 2006 on Newegg.com) with several representative samples in each price category. Mentioned are the normally available currents (not peak) on the 12V rails. This could be less, if there is such a heavy load on the other (3.3V and 5V) rails that the rated power of the PSU becomes the limit. This is, however, very unlikely.

Information is listed in the following format:
BRAND, MODEL, RATED POWER (WATTS)
Number of rails: 12V1 current, 12V2 current, maximum current on the combined 12V rails (maximum power on those rails)

$0 - $50

HIPRO HP-P500W/TOP-500P5 ATX V2.01 500W
Dual 12V rail: 12V1 = 20A, 12V2 = 20A, maximum current on 12V rails = 29A (348W)

Rosewill RE502-SLV ATX12V 2.01 500W
Single 12V rail: 12V = 34A (408W)


$50 - $100

COOLMAX CP-500T EPS12V 500W
Dual 12V rail: 12V1 = 18A, 12V2 =18A, maximum current on 12V rails = 35A (420W)

FSP Group (Fortron Source) AX500-A, 2.0 version
Dual 12V rail: 12V1 = 15A, 12V2 = 15A, maximum current on 12V rails = 27A (est.)
Combined 12V not reported. Estimation: total power on 12V+3.3V+5V = 445W. Assuming 3.3V and 5V are taxed for 50%, they draw 119.5W combined, leaving 325.5W for the dual 12V, or 27A combined.

$100 - $200

ENERMAX Liberty ELT500AWT ATX12V 500W
Dual 12V rail: 12V1 = 22A, 12V2 = 22A, maximum current on 12V rails = 32A (384W)

Antec Phantom 500 ATX12V 500W
Dual 12V rail: 12V1 = 17A, 12V2 = 18A, maximum current on 12V rails = 32.5A (390W)

>$200 (note: no 500W power supply available in this price range)

PC Power & Cooling Turbo-Cool 1KW EPS12V SLI
Triple 12V rail: 12V1 = 16A, 12V2 = 16A, 12V3 = 36A, maximum current on 12V rails = 66A (792W)

Conclusion: the power on the individual 12V rails can be somewhat misleading. The best example is in the $100 - $200 price category, where the Enermax Liberty seems significantly more powerful at first glance (when looking at the individual 12V rails), but the total power on the 12V rails of the Enermax Liberty and the Antec Phantom is more or less the same.

Thanks to SpookedJunglist for his comments :)

CyberDruid

Helpful to those in the market for a PSU.

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SpookedJunglist

22+22=44
44x12v = 528watts

Is that a typo? If so it needs to be fixed :D

muffin

Thats not a typo Spooky. You look at 22A on each rail and think it means 44A total, but if you look at the combined wattage you see that they can't handle that. He is correct.

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Chozart

Thank you muffin :)

The point of the FAQ was to show that the total Amps on the 12V rails is NOT the sum. I'll revise it to make that more clear

SpookedJunglist

I understand that :) 22amps is peak at a given temperature. they deduct a certain percent to get a realistic wattage spec.

His way of wording it is confusing. Combining and adding two integers together will get the same result. 22 add 22 = 44. 22 combined with 22 = 44.I und

All manufacturers have different ways of calculating wattage/amperage. That should be pointed out . I came in here to rename the title to include the propper tag. I didn't want to edit myself because the FAQ is not mine. I have not researched the data on how some manufactureres rate there PSUs. It would be good information to add to this FAQ. Below is one of the many guidlines I have read about in the past.

Power Supply Derating Guidelines:
Operating Temperature range is normally 0 to 50oC, derate 20% for each additional 10oC increase above 50oC.
Operating Temperature range; 0oC to +70oC. Full power to +50oC then derate 2.5%/oC to +70oC. Or, derate linearly from 100% load @ 50oC to 75% @ 60, derated at 10% per oC to a Maximum of +65oC
Forced air cooling [20CFM nominal]: a power supply may require a forced air flow to operate at its rated maximum temperature rating. Derate 20% with out cooling
Open Frame supplies: may require an additional 15% power de-rating when covered by a frame.
About: The power supply specification should detail its operation, use the above as a rule of thumb. Normal operational temperature range is defined as ambient [Maximum].


Plus credit to the origin http://www.interfacebus.com/Power_Su...ufactures.html

Chozart

I rewrote the FAQ :) I hope it is more clear now.

Retrospekt

Omg, i did not understand anything. I have a lot of reading to do. But dont worry Chozart, its a good faq, i jsut have no clue what any of this is. Nice faq!

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cokker

Good FAQ, im a bit of a PSU newbie so this is very helpful, Rep for you ;)

CyberDruid

I just re-read it and it still makes sense...

Just wish I had NewEgg searched that 34A Rosewill RE502-SLV ATX12V 2.01 500W--sweet looking unit (off topic)