If you are derating wire due to conduit fill(example - 4/0 derated to 3/0 ampacity) should the circuit breaker be sized to protect the new derated amapacity of the 4/0?

No, stay with circuit breaker size and just increase the awg of the wire when you have to derate.




[This message has been edited by RobbieD (edited 02-17-2007).]

The derated ampacity cannot exceed the overcurrent protection. So, you either have to downsize the breaker or upsize the cable. Normally, it's the latter since the original breaker size is chosen for a reason, but using a smaller breaker is OK if circumstances allow.

Huh?
Are you saying that I can't feed a 15 amp load with a 20 amp breaker using #8 wire???
...or did I read your post wrong?