The NFPA 70E will give you those values, but they are sometimes overly restricive. I work at a small power generator. One of the plants that our company owns went ahead and did the calculations (actually brought in a contractor) and significantly lowered some of these values.
We are preparing to bring in someone to do the calculations as well. When you really get into the text, start figuring "flash in a box" and other modifiers to the standard values, you find that you need the "moon suit" an awfull lot of the time.
