There is no formula for this. Ask the manufacturer. In the USA the equipment should have a lable stating its KAIC rating at applicable voltages. But in general KAIC rating at 480V should be applicable to 208V rating, as it should result in lesser fault MVA (kV*kAIC*1.732). The reverse is not true though.
Suggestion to PhilDevine (Electrical) May 16, 2004 marked ///\\A 480Y/277V, 3P4W, 35KAIC SC fully rated panelboard applied at 208/120V has a KAIC rating of? Thanks in advance for the input.
///Normally, 480V, 35kAIC SC panel may be requested to be replaced by 208V panel with required kAIC rating for the particular power distribution location having determined kAIC by software calculations, for example. However, the 480V panel manufacturer may provide its 208V short circuit rating. The figure may be the same, e.g. 35kAIC, since buses do not have stronger bracing for higher kAIC than 35kAIC.\\\
