Initial reports from reviews indicate that RX 480 draw considerable power from the PCI-E slot
(I'm talking about +75w, with short-term peaks of +100w)
Until we have some solid measurements under mining conditions, I would caution everyone to avoid using unpowered ribbon risers, since this is anywhere from 2-5x the wattage typical mining cards pull from PCI-E slot, more than double that drawn by a MSI 290X Lightning.
Hell, I would even caution against direct-mount triple card setups at this point.
RX480 82w average from PCI-E slothttp://www.tomshardware.com/reviews/amd-radeon-rx-480-polaris-10,4616-9.html
MSI R9 290x Lightning 32w average from PCI-E slothttp://www.tomshardware.com/reviews/r9-290x-lightning-performance-review,3782-5.html
Sapphire R 380X Nitro 48w average from PCI-E slothttp://www.tomshardware.com/reviews/amd-radeon-r9-380x-nitro,4361-6.html