Augie is correct - I'd just add that technically you do also need to account for the power factor (i.e. go from KW to KVA): I = ( P(KW)/P.F. ) / E * sq. root(3). Assuming a PF of 0.8 or so should be a safe bet since I'm guessing that you don't know the power factor. But the best way, and one I use everytime possible, is give the mfg. a call and ask the nameplate rating!