So far as I know, this hasn't changed. I suspect the receptacles are safe up to 40 A if wired properly. Even with a 20 A receptacle, you can't stop someone from using two 18 A devices in the same duplex at once.
To answer the why part ... a 15 amp duplex receptacle is 2 places to plug in. Therefore you could put 15 amps load on each and very happily trip the 20 amp OCPD; but it would happen safely. A single plug cannot be rated less than the OCPD because to trip the OCPD you would have to overload the device. In short, circuits usually need distribution, not capacity in one central place; hence the allowance. As for the 2002 code ... no udea, haven't seen it yet.
Virgil, Check out 210-21(b)(1)"A single receptacle installed on an individual branch circuit shall have an ampere rating of not less than that of the branch circuit." (with exceptions pertaining to motors and welders). A duplex isn't a single recep.
But why can we put a 50 Amp Recep on a 40Amp circuit??? (210-21(B)(3).
Maybe some of our Canadian counterparts will comment? I just got a job hookin' up a manufactured home from up there, all 15A devices on !%a circuits, except for those that we'd normally supply with 20A
Sparky, Up here in the Great White North, we in the inspection department most definitely do not accept overcurrent protection rated or set at more than the ampere rating of the receptacle...except under the following condition...
The Canadian Electrical Code states:
Protection and Control of Miscellaneous Apparatus
Rule 14-600 Protection of Receptacles
Receptacles shall not be connected to a branch circuit having overcurrent protection rated or set at more than the ampere rating of the receptacle except as permitted by other Sections of this Code.
Rationale for Rule 14-600.
Receptacle configurations are standardized according to voltage and ampere ratings as shown in Diagrams 1 and 2 of the CE Code, Part I. The configurations for either locking or non-locking types are arranged to prevent connection of a load greater than the rating of the receptacle.
Intent for Rule 14-600.
We intend that branch circuit overcurrent protective devices supplying receptacles be rated not larger than the rating of the receptacles, except in specific circumstances as allowed in other Sections (eg, Rule 42-004).
Rule 42-004 states that:
Rule 42-004 Receptacles and Attachment Plugs
Where a welder is cord connected, the rating of the receptacle and attachment plug shall be permitted to be less than the rating of the overcurrent devices protecting them, but not less than the ampacity of the supply conductors required for the welder.
Rationale for Rule 42-004. To allow for portability, certain welders need to be connected by a receptacle and attachment plug cap. This Rule provides an exception to the normal requirement of receptacle sizing, as specified in Rule 14-600, because of the nature of the connected load.
Intent for Rule 42-004. We have determined that the ampacity of the branch circuit conductors of the welder should be the determining factor in selecting the current-carrying capacity of the receptacle. Ampacity of the branch circuit conductors has been chosen rather than the rating of the overcurrent device, because the latter is sized to accommodate short-duration inrush currents, which occur during start-up, as well as the continuous current of the welding operation.