According to the National Electrical Code (NEC), the rating of each receptacle on a 20-ampere branch circuit must not be less than 15 amperes.

Here's why:

* NEC Article 210.21(B)(1): This section of the NEC states that receptacles on a branch circuit must be rated no less than the ampacity of the branch circuit.

* Exceptions: There are some exceptions to this rule, but they generally apply to specific types of receptacles or circuits.

Important Note: While the NEC allows for 15-ampere receptacles on a 20-ampere circuit, it's crucial to remember that the total load connected to the circuit should never exceed the circuit's ampacity (20 amps in this case).