Yes it is sufficient.
A 15A receptacle is designed such that an appliance with a 20A plug should not be able to plug into it. This means that at most under normal operating conditions of a 15A appliance, that the receptacle itself will only draw 15A. If the circuit shorts then the breaker will kick at 20A however.
The tab of a 15A receptacle is designed to handle a 20A load.
EDIT for clarification above: A duplex 15A receptacle contains a tab that connects the hot lead to the hot load. This allows the receptacle to carry the load of another outlet by wiring the lead of Outlet(2) to the load of Receptacle(1). When you break off this hot tab then you are specifically wiring for a switch to control only of the receptacles of the outlet, or you are specifically wiring for each receptacle to have its own circuit on a shared neutral. For two circuits to be wired to a single receptacle on a shared neutral, then you need to ensure that the breakers are on different service legs or you risk overloading the neutral. If you also break the neutral tab then you are wiring the receptacle to have two separate circuits each with its own dedicated neutral. These should be considered fringe cases for residential so it is probably best not to break the tabs if not needed so as to avoid confusing future DIY'ers and licensed electricians. Every time a DIY'er or electrician gets confused then they are potentially risking themselves.
The NEC states that this is acceptable as long as:
The wire is 12 AWG sized for a 20A circuit
There is more than one receptacle on the circuit.
It is important to note that under no circumstances should you install a single 15A receptacle onto a dedicated 20A circuit. This is against code.
210.21(B) Receptacles (1) Single Receptacle on an Individual Branch Circuit. A single receptacle installed on an individual branch circuit shall have an ampere rating not less than that of the branch circuit.
(3) Receptacle Ratings. Where connected to a branch circuit supplying two or more receptacles or outlets, receptacle ratings shall conform to the values listed in Table 210.21(B)(3), or where larger than 50 amperes, the receptacle rating shall not be less than the branch-circuit rating.
Table:
Circuit Rating (Amperes)
Receptacle Rating
(Amperes)
15 Not over 15
20 15 or 20
30 30
40 40 or 50
50 50
In conclusion, a standard 15A receptacle that one would buy in your hardware store is quality tested and approved to comply with all NEC codes to be legally sold in the United States. If the tab couldn't handle a 20A load without overheating then they would call this out specifically.
Best Answer
No.
For a receptacle to be rated for 15A, it must be able to handle 15A at either outlet. For example. If you plug one item that draws 15A into a receptacle, the receptacle must be able to handle that current.
Receptacles are actually designed to handle 150% over their rated current (or something like that, I'd have to find the specification to be sure). A 15A receptacle should be able to handle 22.5A, so your breaker should trip before the receptacle fails.
Consider this image...
Assuming the tab between the hot terminals is removed, and the red and black wires are each connected to separate 15A breakers. This is a valid configuration, and would allow 15A to be drawn at each outlet.
However, It's usually not a good idea to tempt fate, or stress test electrical components. So in theory, it will not fail. In practice, it should not fail.