See NEC Article 620 and the Annex D Example D9 Feeder Ampacity Determination for Generator Field Control, and Example D10 Feeder Ampacity Determination for Adjustable Speed Drive Control
See 620.13 and 620.61 in the 2002 NEC.
NEC Article 100 defines a continuous load as one that operates for 3 hours or more.
Duty, Continuous. Operation at a substantially constant load for an indefinitely long time.
Duty, Intermittent. Operation for alternate intervals of (1) load and no load; or (2) load and rest; or (3) load, no load, and rest.
Duty, Periodic. Intermittent operation in which the load conditions are regularly recurrent.
Duty, Short-Time. Operation at a substantially constant load for a short and definite, specified time.
Duty, Varying. Operation at loads, and for intervals of time, both of which may be subject to wide variation.
Joe Tedesco, NEC Consultant