Normal
Tomshardware's test of the 980ti (among other) measures the power draw from the motherboard as about 55W on average, peaking to 70W, so even cards with external power still draw power from the motherboard. I suppose it's possible that PCIe card manufacturers would add in logic that if power from the motherboard is cut that the PCIe power connectors will 'take up the slack', but as that's well outside what the PCIe standard says should happen it's more likely that a failure to detect motherboard power will just result in the card not powering up at all.There's also the nonzero 3.3V power draw, so even if the 12V traces from the motherboard were cut and a supplemental 12V supply connected, you would either need to keep the 3.3V traces connected, or supplement those as well.
Tomshardware's test of the 980ti (among other) measures the power draw from the motherboard as about 55W on average, peaking to 70W, so even cards with external power still draw power from the motherboard. I suppose it's possible that PCIe card manufacturers would add in logic that if power from the motherboard is cut that the PCIe power connectors will 'take up the slack', but as that's well outside what the PCIe standard says should happen it's more likely that a failure to detect motherboard power will just result in the card not powering up at all.
There's also the nonzero 3.3V power draw, so even if the 12V traces from the motherboard were cut and a supplemental 12V supply connected, you would either need to keep the 3.3V traces connected, or supplement those as well.
