Cord-and-Plug Built-ins
Built-in dishwashers may be cord-and-plug-connected or connected to their own dedicated branch circuit. Cord-and-plug dishwashers may be connected to any general purpose receptacle circuit, but, as outlined in NEC 210.23 Permissible Loads, the dishwasher load shall not exceed 50 percent of the branch circuit's ampere capacity rating. When connecting a cord-and-plug dishwasher to a 20 ampere branch circuit, its current draw shall not exceed 10 amperes. The average built-in dishwasher draws 15 amperes when operating, which violates NEC 210.23, which in turn requires it to be connected to its own dedicated branch circuit. Under no circumstances are dishwashers permitted on one of the two 20 ampere small appliance circuits.
Cord-and-Plug Portable Dishwashers
NEC section 210.23 does not apply to portable dishwashers since they do not qualify as a fixed-in-place appliance. They can be plugged into any general purpose receptacle circuit. However, you should avoid plugging them into receptacle circuits with other motor loads, such as refrigerators, trash compactors or garbage disposal units, if at all possible. Because of their heavy current draw, they also should not be plugged into a countertop small appliance receptacle if another receptacle is available for use.
Permanently Connected Built-ins
NEC Section 210.23 does apply to fixed-in-place, permanently connected appliances. Because their current requirements exceed the 50 percent rule, they must be connected to their own dedicated 20 ampere branch circuit with its own 20 ampere circuit breaker installed. All local building codes have this requirement as well. The electrical code also requires that these new circuits be AFCI, or arc fault circuit interrupter, protected. An AFCI is a new type of circuit breaker that replaces the standard circuit breaker in new construction.
Not Good Practice
Many do-it-yourself people will plug a cord-and-plug dishwasher into a duplex receptacle supplying a garbage disposal unit or trash compactor, but this is not a good idea. Since garbage disposals and trash compactors are also motor loads, this violates NEC 210.23. The best practice, which satisfies the requirements of the NEC and the local codes, is to install separate dedicated branch circuits for each appliance -- garbage disposal, trash compactor and dishwasher.