30W may be spec, but I can tell you cards pull more than spec often, especially when overclocked. I'll be happy to send you a couple motherboards to prove it.
Well 75W is the spec but how do you know it pulled more than 30W? 5 card @ 30W = 150W. If the MB wasn't designed to handle more than 120W you could easily kill the MB without any card pulling more than 30W. I am not saying people shouldn't use powered extenders I was just trying to clear up some of the completely false claims.
Per the post I responded to 4x5870 @ 30W ea = no need for powered extenders. A single 5970 @ 30W = MUST HAVE powered extender.
Wrong info is wrong info.
There is no hard and fast rule and people need to decide what they are comfortable with. The reality doesn't fit into a sound bite but here are some ideas:
1) High end cards tend to draw less from the bus and more from the dedicated connectors because they are more reliable. So the idea that a 5770 might be fine but a 5970 must be dangerous is actually backwards.
2) The more CARDS (not GPU) you have installed the more you need to consider powered extenders. 6x5870s is going to draw more from the bus than 4x5970s despite having less GPUs.
3) The harder you overclock the more wattage you are going to pull and some share of that is going to come from the bus.
4) The more PCIe 1x slots you use the more you should consider powered extenders. A card in PCIe x16 slots is going to pull the same from the bus extender or direct thus hopefully the designer planned on that wattage being used. However a PCIe x1 slot normally doesn't pull more than 10W or less. Using a board w/ lots of PCIe 1x slots may indicate the MB can't handle 150W+
5) Downclocking memory helps a lot and since 5000 series can go as low as 150Mhz (model dependent) they are going to pull less wattage than a similar caliber 6000 series card.