You can size you tap condcutors for 400 amps if they're 25' or less since 400 amps is 1/3 of the 1200 amps ampacity ahead of the tap. If you size them at 1200 amps you won't have a tap at all, you will have a splice so the tap rules don't apply. For a single set of condcutors one example of what you'll need is 600 kcmil THHN cu tap conductors.