Tom,
Table 210-24 only applies to circuits with 2 or more outlets or receptacles. A single receptacle is covered only by 2109-21(b)(1). This section clearly requires that the receptacle have a rating at least equal to that of the branch circuit, but permits any single receptacle with a rating greater than the circuit rating to be installed. Now that I have said that the code would permit the installation of a 30 amp single receptacle on a 20 amp circuit, I will also tell you that the code says you can't really use it. 210-23 limits the load of any single cord connected equipment to 80% of the branch circuit rating. This would limit it to 16 amps in this case and I doubt if there appliances with 16 amp loads and 30 amp cord caps.
Don(resqcapt19)
