Ryan you lost me on that one, I do not see how an IG and 250.30(A)(2) would come up in the same conversation.

I have always used Table 250.122 to size IG conductors.

Typically that would mean a 12 AWG all the way back to the transformer XO terminal would be adequate as the OCP that IG will need to open will be a 20 amp. Very few IG circuits are larger than 20 amps.

Generally I have used a 6 AWG back to XO from a IG bar in the panel just because a 12 landing on XO of a 75 KVA transformer looks bizarre.

Thinking on this some more and it seems a strict reading of 250.122(B) would require the IG for a 20 amp circuit run back with the feeder to be the same size as the feeders ungrounded conductors. [Linked Image]

Bob

Bob Badger
Construction & Maintenance Electrician
Massachusetts