I have a 500 MCM parallel feeder cable type TC-ER. The EGC in each cable is a #2. The 800 amp breaker requires a 1/0 EGC based on Table 250.122. When I double the circular mil area of the #2's I'm just shy of the circular mil area of 1/0. I'm sure I need to run another EGC external to the two feeder cables but am unsure what size. The area of two #2's is 132738. The 1/0 is 133072, for a difference of 334 circular mils. What size additional EGC do I need?