Several of the more detailed reviews of AMD's Radeon RX 480 reveal the card definitely needed an eight-pin PCIe power connector as its power consumption isn't what AMD claimed it would be. It seems this is one of the areas where AMD cut corners to make the card cheaper, not just in price but also in terms of quality.
Tom's Hardware investigated the card's power draw in great detail and learned that despite the 150W TDP, the card has an average gaming load power consumption of 164W!
Due to the load distribution, this works out to 86W drawn through the motherboard's PCIe slot, despite the latter being designed to handle a maximum of just 75W. And it gets worse, the site found that the maximum power draw includes spikes of up to 155W - more than twice as much as the PCIe slot is designed to handle!
With peaks of up to 155W, we have to be thankful they're brief, and not putting the motherboard in any immediate danger. However, the audio subsystems on cheaper platforms will have a hard time dealing with them. This means that the "you can hear what you see" effect will be in full force during load changes; activities like scrolling may very well result in audible artifacts.
As Tom's Hardware points out, this doesn't bode well for a Radeon RX 480 in CrossFire.
We’re also left to wonder what we'd see from a CrossFire configuration. Two graphics cards would draw 160W via the motherboard’s 24-pin connector; that's a tall order. Switching from the bars back to a more detailed curve makes this even more evident.
Hopefully AMD can resolve this via a software-based update but any fix for this may hurt performance. How this will play out over time will be interesting, to say the least. As it stands right now, the RX 480 reference design poses a risk to the lifetime of your computer, especially in combination with a budget motherboard and budget power supply (and that's the intended target market of this card).
Once you start overclocking things get pretty dangerous, Tom's Hardware explicitly mentions they cancelled their long-term overclocking and overvolting tests as they didn't want to torture their test platform. The brief data they gathered showed that the overclocked RX 480 pulled a whopping 100W through the PCIe slot, with massive 200W peaks! Best to wait a bit longer for the custom design cards as it looks like AMD messed up pretty big.