I really don?t understand your question. So let me present two of my own:
(1) What information do you already have?
(2) What information are you looking for?
You talk about wanting to know ?ampacity,? and you talk about measuring amps. Is this thing already installed, so that it is possible to measure amps? If so, then the size of the secondary conductors must already be determined.
As to this rule of thumb you were given, it is nonsense. You can divide amps by volts, and get resistance, using Ohm?s Law. But that is seldom applied in our daily jobs. There is no other useful formula in the electrical industry that starts with amps and divides that value by anything else. Also, if the loads are reasonably balanced, as they should be, then the amps on one phase should be nearly equal to the amps on the other phase, and the amps on the neutral should be nearly zero.