How watts do i need for 3 Radeon R9 290 cards attached to a single mobo?

samwisegamgie1

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Dec 24, 2011
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I'm following this guide* for a litecoin mining righ, but I'm using R9 290's instead of 7950's (on account of their basically sold out globally) and I'm baffled by how power supplies work, 500 watts for 1 card, 750 for 4? Any way, if theirs a formula please let me know,

How many watts do 3 Radeon R9 290's require?

*http://www.cryptobadger.com/2013/04/build-a-litecoin-mining-rig-hardware/