Firingsquad recently posted a how-to to help determine your system's power requirements when shopping for a new power supply - see http://firingsquad.gamers.com/guides/power_supply/.

Using their figures (some of which come from the AMD guidelines), a 2.8G P4 with 2 HDD, sound, DVD and CD-RW, + 2X512M ram requires 17.3A on the +12v rail at max draw for all devices. They suggest adding 20% for expansion and to cover PSU fudging in manufacturer ratings, giving a need for 20.8A. Very few PSUs offer this much juice on that rail - the only quiet one is the Quietpc 460 Watt.

I wrote to ExoticPC.com, expressing interest in their new "ultraquiet" Silenx(14 and 18 db) modded Fortron 400W PSU models (rated at 15A for 12v), laying the above out, and here was their response:

"Those values from Anandtech are very exaggerated numbers far beyond worst case scenarios. There is no possible way a P4 2.8 processor can actually draw 10.1 amps of power on the 12v line. The P4 2.8 processor has a maximum power draw of 68.4W not all of which is used up by the 12v line, so more realisitically, it will only consume approximately 3-4 amps max on the 12v line. As for the hard drives, your typical 7200 rpm IDE hard drive consumes 500mA max, so two of those would be 1 amp (even two top-of-the-line 15000 rpm drives will not consume 4 amps, the fastest Seagate 15K drive uses 1 amp max). That's 5 amps so far, add CD, DVD, motherboard and fans to top out around 8-9 amps max, which is well within the maximum steady current the 400 watt SilenX PSU can handle. The 30% peak power delivery [you asked aobut] is a buffer on top of the 15 amps the [Exotic] PSU can provide for a total of 19.5 amps. It is capable of delivering this amount of power for 30 minutes, this is just a margin of safety you have in addition to the 15 amps to make sure your power supply doesn't die out just because your PC is exceeding the 15 amp max. "

I would be appreciative if anyone with knowledge in this area can comment on this power requirement discrepancy here. Firingsquad's figures do seem "out there", but on the other hand Exotic is looking to sell their units....I am aware of MikeC's power draw tests on his own systems on this site, but by today's standards those systems are only midrange or lower so their applicability to high-end, overclocked setups is very questionable.

German magazine C't tests motherboards for AMDs.
24/02 S.178 and 02/03 S.144

Xp2400+, 256 MB RAM, Geforce 3 TI500

the power on the 12/5/3,3 rails was: 15/55/30%

Only Abit AT7 MAX2 and NForce2 from Leadtek and MSI are different: ~70/~10/~20%

Whole power of system idle at desktop: 120-120W.
Under load 150-160W. Only Gigabyte KT400 draws 150/180W.

Former Tests of AMD MBs showd up to 85% load at the 5V rail.

-> 300W PSU are okay for non-9700pro users.

power of graphic cards: http://www.tecchannel.de/hardware/492/37.html

Well, I'm running my PIV 3.0Ghz with two HDDs, sound, CDrw, DVDrom, 2 x 512MB RAM on an Antec True480 PSU. I don't know how many amps are available on the 12V rail but I've run many, many 24hr test sessions of Prime95 +3Dmark 2001 with absolutely zero issues. So I'd say the TruePower 480 at least is up to the task.

On the German test, knowing the % on each rail is just not too useful - gives no indication of how much A is needed for each rail for a given system.

Ralf, no doubt the Antec 480 with its 22A on the 12v rail has no problems with this type of setup, it's just that by SPCR standards it is not a very quiet PSU unfortunately (assuming it's not much different in noise output from the Antec 550 reviewed here, which was 48db at 90 watts). Right now the only quiet solution looks to be the QuietPC 460 watt with a hefty 28A output at +12v - but boy the thing is pricey ($225 CDN, even with an institutional discount).

You should do the math youself. All numbers are there.

And you should do a spellcheck.... data on Athlon boards isn't directly relevant for P4 systems.

New data on Geforce FX - boardmakers are saying 350 watt minumum PSUs are required. These cards have a second power connection a la Radeon 9700.

Maybe maybe not. Exoticpc's answer is reasonably thorough. Why do you doubt it?

Look at the power draw on Leo's dual AMD-MP, dual Maxtor system: It hardly hits 175W PEAK!

I've mentioned a P4-2.8G I've been using on the test bench in various rigs -- again no matter what I do (dual drives, all kinds of benchmarking, artificial stresses) it won't even pull 150W in any system. I've even run fine it on an old generic 250W PSU modded with low airflow fan & a 12V adapter just for kicks (to see if I could blow that PSU with this system ). The only thing mid-rangy about this system is the VGA card, the hottest & fastest of which still can't pull more than maybe another 20W more than my Matrox G400 Max. That still keeps the total under 200W.

This biz of adding the rated current or wattage of every component for max power required is nonsense. You simply cannot find an app that can make every component pull full power simultaneously. Think about it. It is so unlikely as to be moot.

I wasn't commenting on the noise level of the True480, I was showing that it runs a "more badass" system than the Firingsquad site says it should on "only" 22A. The point I was trying to make was the same point MikeC made in his post right above this one:

MikeC wrote:

Because as I mentioned they are hardly a disinterested party in this, with their own PSU to sell. I checked IBM's specs on the 180GXP - it does indeed need 2A at +12v for spin-up, so Exotic's answer with respect to HDDs is incorrect.



Very useful, thanks for the link. However anyone planning for a system with a Geforce FX or equivalent, with its 75 or so watts, has to keep in mind that his video card probably only draws 1/2 of that.



I suspect the difference between a FX and G400 is more like 30-40 watts - I have been unable to find any official specs however for the FX. Anyone know what rail that wattage would be drawn from? That would be useful info in determining the suitability of the Nexus for my system build.



Granted it is extremely unlikely, but all you might need is one instance to cause you some major headaches; although the momentary peak power boost most PSUs can provide might cover this. Anyway, assigning a more-realistic 6A (72 watts) to the P4 2.8G lowers my system's 100% level on the +12v rail to 11 Amps, with 21A on the +5 and 10A on the +3.3. I have included 6A on the 3.3 rail for the video card here, but I know that a FX or 9700 Pro would draw more than that from some rail (which one)? Given those specs would you feel safe going with a Nexus if you wanted headroom for an FX class card?

is not thinking about a quiet system.

Not necessarily true - Gainward is promising a low-noise FX, perhaps with some passive/active combination cooling. It's been done with the 9700 now by Sapphire and Zalman; the FX is just the next step.

Actually, I found out that my G550 only draws 9 watts of heat max. So if I added a GFFX to my system, the power draw would increase to the 240 watt range, but who knows from which voltage rails?

EDIT: make that 20 watts when doing 3d benchmarking. Click for link.

Ralf,
Have you modded your True 480 - installed 2 new fans, or just one etc...? If so could you possibly elaborate...

Thank you.

I'm sorry for pasting this all over, but I've round the following PSU calculator (I'd say it's estimator) very useful:

http://takaman.jp/psu_calc.html?english

Take those figures as worst case "everything running at max at the same time" theoretical figures.

Then consider that Kill-A-Watt readings give usually c. 20-25% lower figures under normal "full load" situation.

That's one way of looking at the problem and although far from perfect, it's much better than most ones I've come across.

regards,
Halcyon

Why don´t you check out the "how much will a 300W PSU run?"-thread? There you will find plenty of evidence that the firingsquads numbers are off. I´ll let you decide for yourself if you think my system is "midrange" when it comes to powerdraw...

CPU: XP2200+ 63W on default, tested at 71W
GPU: have run both FX5600 and R9600pro
RAM: 2*256MB
Sound: Two different 5.1-cards
Fans: two 120mm Alu-fans
HD: two HD:s Samsung p80 and Hitachi 180GXP
misc: DVD-burner, TV-tuner.

Never a hickup from neither my 350W PSU nor my my 300WPSU.

Clever little calculator, a nice find, halcyon.

I just tried plugging in the closest equivalent components from the ARM StealthPC P4-3.2 System I just reviewed. The calculator came up with 292.8W max. That is DC power.

The high AC power draw of the ARM StealthPC was 235W momentary peak. OK. What does that translate to in DC power? Well the Zalman PSU is rated for 75% efficiency, let's take them at face value: 75% of 235W is is 176.25W -- the power delivered in DC. About 60% of the recommended value the calculator gives.

When you come right down to it, a PSU that is fairly and honestly rated at 300W will power just about any PC and still have lots of headroom. For most silence enthusiasts, if we're staying with stock PSUs, the big question is whether higher power also buys us quieter fan behavior. If not, there's little point in more power, I'd take a lower power unit and mod fan/cooling system in it.

For giggles, I just looked at the PSU in one of our HP Pavilion T260 test machines here at work.

The unit is rated at 250W, with 14a on 12v, 25a on 5v and 18a on the 3.3v.

Specs are:

P4 3.06
1GB PC2700 (2x512)
250GB Maxtor drive
16x DVD reader
4x DVD+R
Cardreader
SB Audigy 2
Asus TV-tuner
ATI Radeon 9800
92mm case fan

It'll take a hammering all day long...

That post is over a year old. I haven't used that PSU for a long time. It was too noisy, even for a gaming rig that I wasn't too obsessed (at the time) with silencing. There have been people that have modded their TruePower PSUs and posted about it here. Use the search and you should be able to find them.