What is the difference between a calculation done for NFPA 70 E to determine PPE requirements and NEC 110.24 Available Fault Current (added in 2011 NEC)?
The number used for the NEC is often the worst case and is only used for equipment selection. Using this number for a 70e calculation may give you a lower incident energy than a lower available fault current. Since the electric utility system is a dynamic system many engineers that do 70e calculations will do them using a range of the fault currents that may reasonably be available.
Arc flash calculation have a basis in short circuit calculations, but as mentioned by Don, the NEC 110.24 Available Fault Current (added in 2011 NEC) are generally high and will not be good to use for the 70E calculation basis to get the incident energy.