I must not have the latest standard, since I don't find annexs in my copy (NFPA70E-2000), but I did find the formula you mentioned, along with a statement that theseequations were derived in the IEEE paper by R.L. Doughty, T.E. Neal, and H.L. Floyd, II, "Predicting Incident Energy to Better Manage teh Electric Arc Hazard on 600V Power Distribution Systems", Record of Conference Papers IEEE IAS 45th Annual Petroleum and Chemical Industry Conference, September 28-30, 1998.
Perhaps you could find a copy of this to see if they adress the out of bounds amounts you refer to.