NEC 2008
210.21 Outlet Devices. Outlet devices shall have an ampere rating that is not less than the load to be served and shall comply with 210.21(A) and (B).
(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.
Exception No. 1: A receptacle installed in accordance with 430.81(B).
Exception No. 2: A receptacle installed exclusively for the use of a cord-and-plug-connected arc welder shall be permitted to have an ampere rating not less than the minimum branch-circuit conductor ampacity determined by 630.11(A) for arc welders.
(2) Total Cord-and-Plug-Connected Load. Where connected to a branch circuit supplying two or more receptacles or outlets, a receptacle shall not supply a total cord and-plug-connected load in excess of the maximum specified in Table 210.21(B)(2).
(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.
Exception No. 1: Receptacles for one or more cord-andplug-connected arc welders shall be permitted to have ampere ratings not less than the minimum branch-circuit conductor ampacity permitted by 630.11(A) or (B) as applicable for arc welders.
Exception No. 2: The ampere rating of a receptacle installed for electric discharge lighting shall be permitted to be based on 410.62(C)
(4) Range Receptacle Rating. The ampere rating of a range receptacle shall be permitted to be based on a single range demand load as specified in Table 220.55.
Se for um único receptáculo em um circuito de 20 Amp, ele deve ser um receptáculo de 20 Amp (além das 2 exceções). Se houver múltiplos receptáculos em um circuito de 20 Amp, os receptáculos podem ser 15 ou 20 Amp (exceto as exceções).
Isso permite que você tenha vários dispositivos conectados a menos de 15 Amps cada, mas o consumo total no circuito pode ser maior que 15 Amps.
NOTA:
Tenha em mente que a NEC define um receptáculo da seguinte forma.
Receptacle. A receptacle is a contact device installed at the outlet for the connection of an attachment plug. A single receptacle is a single contact device with no other contact device on the same yoke. A multiple receptacle is two or more contact devices on the same yoke.
Assim, um receptáculo dúplex seria considerado dois receptáculos, de acordo com essa definição. O que significa que você pode instalar um único receptáculo duplex de 15 Amperes em um circuito de 20 amperes, sem violar este código.