odd power draw with 7950s. 2 cards is 460, 3 cards is 800

[Philj]

I'm wondering if anyone has noticed a similar effect when going from two to three 7950 cards. Running two 7950s (I've tried several models from saphire, MSI, powercolor) the total system draw is between 450 and 470 watts at the wall (this includes mobo and 55watt Intel CPU). Using the same MB/CPU and the same type of card for the third card bumps the power draw up to 790-815 watts at the wall. If we assume the power draw when running two cards in 225 watts per card, then adding the third should only put the total around 680-700. I've tried 3 different gold rated power supplies: 750w, 850w and 1000w, and its the same on each. My initial thought was that I was hitting the max of the 750w PSU and losing a lot of efficiency, but the 1000w PSU has essentially identical results.

Just wondering if anyone has seen this, or knows why this would happen

[ssateneth]

Not every card is equal. I own many radeon 7970. I usually test them with occt:gpu:3d prior to mining to make sure they work as advertised. My kill-a-watt reading shows anywhere from 330 watts to 380 watts after temperature levels out with stock volts 925/1375 +20% powertune. It's not uncommon for certain cards to pull more power than another card at the same clocks and volts.

If you are using non-reference cards, it's likely you have a power hungry voltage locked card.

[PeeJWeeJ]

What kind of power connecters do the cards have? 3-pin + 4-pin or two 3-pin?

Also, what CPU and MB are you using?

[rethaw]

Can you supply the changes in temperature for the cards/system? Your PSU efficiency decreases as wattage increases past a certain point. Unlabeled image to give you an idea:

[ScaryHash]

Check the number of watts each one of those power supplies is rated for on the 12 V rail.

Does the 1000 W have one or two 12 V rails?

You might be hitting the brick wall in efficiency on the 12 V.

What brands are they and are they all one 12V rail?