Okay, lets take the case of a central heating furnace. NEC 422.12 requires that the furnace be supplied by an individual branch circuit. The NEC does not consider a furnace to be a continuous load. If it did then we would all be in trouble. If the furnace were marked at a rating corresponding to a standard OCPD rating as given in NEC 240.6 then we would use that OCPD rating. If it didn't correspond then we would have to use an inverse time circuit breaker with a long time setting and adjust the trip to the marked rating on the furnace.
Eric Kench, P.E.
If it's not broken don't fix it