Q. What is the Code limitation for sizing circuits that supply cord- and plug-connected equipment?
A. Cord- and plug-connected equipment not fastened in place, such as a drill press or table saw, must not have an ampere rating more than 80% of the branch circuit rating [210.23(A)(1)].
However, UL and other testing laboratories list portable equipment (such as hair dryers) up to 100% of the circuit rating. The NEC is an installation standard, not a product standard, so it can't prohibit this practice. So there's really no way to limit the load to 80% of the branch-circuit rating if testing laboratories permit equipment to be listed for 100% of the circuit rating.
Equipment fastened in place (other than luminaires) must not be rated more than 50% of the branch-circuit ampere rating if this circuit supplies luminaires, receptacles, or both [210.23(A)(2)]. For example, can a whole house (central) vacuum motor rated 13A be installed on an existing 20A circuit that supplies more than one receptacle outlet? The answer is no. An individual 15A or 20A branch circuit will be required in this situation.