mxslick,
It depends on the work type, fault current and clearing time for the upstream protective devices.
The information is available in IEEE 1584 (detailed calculations) and NFPA 70E (chart and detailed calculations).
NFPA 70E can be found at http://www.nfpa.org/aboutthecodes/AboutTheCodes.asp?DocNum=70E click on preview this document.

Ron