Take the 2-minute tour ×
Super User is a question and answer site for computer enthusiasts and power users. It's 100% free, no registration required.

Can anyone please explain the math behind power supply wattage calculations?

I have a Thermaltake 700W Cable management PSU. It has 4 independent +12V rails max 18A each.

I'm trying to calculate, how much amps will GPU consume. I have an old NVidia 9600GT video card. In it's specification it's written, that it consumes maximum of 96W. Do I understand correctly, that I = W / V = 96 / 12 = 8A?

The problem is, that my PSU has two 6-pin PCI-E power cables. And my 9600GT doesn't want to start up when it's powered by one. I can start it up only when I get power from two independent rails it seems. And GTX570 I purchased some time ago doesn't startup at all whatever I do.

I just want to understand the correct numbers so that I wouldn't do same mistake now when I need to buy new PSU.

It looks that despite my box is 700W, because of independent rails current limit it just cannot handle really consuming video cards. I purchased it like 6 years ago, though...

share|improve this question
    
You probably don't have enough watts from the power supply. What other peripherals do you have running? (i.e. how many hard drives, cd-roms, etc,.) –  KronoS May 26 '11 at 18:16
    
Well, there are two SATA hdds, 1 cdrom, 6 full usb ports (some devices bus-powered, some are not). But what I'm trying to understand is to how do I calculate amps requirement for card? And it seems, I need to purchase only PSUs with combined +12V rail, right? –  FractalizeR May 26 '11 at 18:25

1 Answer 1

up vote 0 down vote accepted

Actually I feel that your 700W PSU is fairly oversized. Although without knowing the concrete brand and type as well as technical specifications it's hard to say. I've had some no-name/cheap PSU with > 500W in my lab which was unable to deliver more than 200W on 12V; on the other hand I've had many 300W well-known brand (Silverstone, Seasonic,...) PSU which were perfectly able to power even much more demanding hardware fairly above 300W in total.

It happens to me as well once that a graphics card won't work when connected to two PCIe 6-Pin connectors if they are connected to the same rail. The solution was to use one PCIe 6-Pin connector from the PSU and use an adapter from 6-Pin PCIe to 2xMolex 4-Pin which was included in the graphics card bundle.

In general you're right with your calculation: 96W max power consumption at 12V it's a maximum of 8A current. However this is split on several sources. PCIe cards are rated as follows: - PCIe board connector: Max. 75W - PCIe 6-Pin connector: Max. 75W - PCIe 8-Pin connector: Max. 150W

So for example a card with just one 6-Pin connector is allowed to take a maximum of 150W (75W from the board connector and 75W from 6-Pin PCIe power connector). A card with 2x 8-Pin connector is allowed to take a maximum of 375W.

Even a card with two PCIe 6-Pin connectors (max. 225W) should work with a power supply rated 700W. Remember that most modern power supplies deliver quite low power on 5V rails so all power is usually delivered via 12V connectors.

You should probably check all your power connections. Modern borads usually use the following connectors:

  • ATX 24-Pin power connector (make sure not to use a 20-Pin connector)
  • EATX12V 8-Pin connector (sometimes just a 4-Pin 12V connector on the mainboard, often referred as the "P4-connector".

The EATX12V connector is basically there to power the CPU. Some mainboards might refuse to start if you connect it to a PSU using a 20-Pin ATX connector instead of 24-Pin.

So in your case I recommend to try some PCIe 6-Pin adapters and hooking up the graphics card to two Molex 4-Pin connectors from different cable rails (yes, you should use two different cables, not connecting both Molex-connectors to the same cable). If you don't have such a power adapter you can get one cheap from your trusted hardware vendor. I personally really cannot imagine that your 700W PSU cannot power your setup. Here I am running an nVidita 8800GTS-512, Intel Q6600@3.1GHz, Maximus Formula, 4x 1TB HDD, TV-Card, PhysX P100 card and Asus Xonar soundcard on a Silverstone ST50+ PSU (500W).

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.