The Stuff store has 400A and 800A CTs that work with IoTaWatt. Up to 1600A is available with special order CTs.
If the service uses multiple parallel cables, it’s generally better to place a CT on each cable and add them together in the IoTaWatt. Divide and conquer. There is a three-phase 600A 277V/480V industrial install with 400A CTs on each of the six mains cables (two per phase). It has been working fine for months and matches the electric meter within 1%.