The VA rating of the current transformer is determined through two straightforward equations.

Power = Current x Voltage e.g. VA = Is • V
Voltage = Current x Resistance e.g. V= Is • R

VA = Volt-Amp Rating (VA)
Is = Secondary Current (Amps)
R = Burden (ohms)

Combining these equations results in:
VA = (Is)2 • R

In some cases, required VA ratings are listed on the device the current transformer will connect with, ie. a meter, relay, etc. Typical internal meter burdens are in the milliohm range.