Vann is correct and I have few on powered pc risers as I type
But i bet you have a newer MOBO with PCI-e 2.0 or 3.0 and i know these work just fine, but i have PCI-e 1.0 and it worries me. Guess nobody uses these anymore and i will just have to risk it and test for myself.
The newer PCI-E specifications can deliver more power through the slot, but the speed of the slot is not an issue. My old Dell XPS 700 had PCI-E 1.0 slots and I had no problem mining with three cards using powered risers. The problem I had was I could not access the Bios when a newer PCI-E 2.0 card was installed in the primary PCI-E 16x slot, but it still mined.