John: Since the raceway and the EGC will be bonded (directly or indirectly) at each end, you basically have the pipe and the EGC in parallel with each other, creater a lower impedance EGC.
For example: You could install 3" IMC with 350's and a 1/0 for 540 feet and still get a 4800 amp fault.
If you had 3" PVC with 350's and a 3/0 EGC and were trying to get a 4800 amp fault, you could only go 131 feet.
These numbers are assuming a fault of 4 times the rating of the OCPD to initiate the OCPD under ground fault.
I think that is why there is a note to table 250.122 that tells you that following the table values doesn't allways achieve compliance with 250.4(A)(5).
A also don't think this type of allowance is really stretching the code, since 250.4 is main requirement of article 250, and 250.4 is in fact a performance based requirement, not a prescriptive one. That tells me that the AHJ should be on his/her "A" game when sizing EGC's.
[This message has been edited by Ryan_J (edited 07-06-2004).]
