To do this you must have at least two pieces of information:
kVA = (V * I * 1.732) / 1000
where 1.732 is the simple numerical value for the square root of 3 (1.7320508...)
Then round up to the next standard 3ph kVA rating as found in Section 14 of SquareD/Schneider Electric Catalog, Digest 177.
Example: V = 208, I = 175; Therefore: kVA = (208 x 175 x 1.732) / 1000 = 63.05kVA
This calculates to 63+ kVA, thus we round up to 75kVA. From the Digest, a EX75T3H will satisfactorily serve this load.
Note: This is a simple kVA calculation and does not take into account possible special load requirements, such as found with motors or some medical equipment. For assistance, contact the Schneider Electric US Customer Care Center.