Tap rule

[Big Guns]

If you have a 1200 amp fuseable safety switch feeding a 800 amp CT cabinet and a 400 amp Tap box and your 400 amp Tap box is outside of 10 feet but inside of 25 feet, do you size your conductors for the 400 amp at 400 amp or 1200 amp

 

[infinity]

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.

 

[KevinVost]

Check article 240.21 (2005 NEC). There may be additional requirements if you do make a tap, based on the length.