If it's AC then it becomes a little easier (maybe)
Use a conventional CT to convert your 1500A via a 5000:1 transformer to 0.3A. Drive this current through a 10ohm burden resistor to generate 0-3V AC. From there use a precision rectifier to produce a 0-3volt DC voltage. After smoothing you'll get near enough a 0-5V DC signal for your Arduino.
Alternatively measuring the mains input current (circa 15A) might be a lot easier using readily available hall sensor devices.