National Electrical Code 2008
Article 210 Branch Circuits
II. Branch-Circuit Ratings
210.19 Conductors — Minimum Ampacity and Size.
(A) Branch Circuits Not More Than 600 Volts.
(2) Multioutlet Branch Circuits. Conductors of branch circuits supplying more than one receptacle for cord-and-plug connected portable loads shall have an ampacity of not less than the rating of the branch circuit.
O que significa que, no seu caso, você terá que usar condutores 12 AWG para conectar o receptáculo.
note: Um receptáculo duplex na verdade conta como dois receptáculos de acordo com o NEC
No entanto, existem situações em que 14 condutores AWG podem ser usados em um circuito de 20 amperes.
(4) Other Loads. Branch-circuit conductors that supply loads other than those specified in 210.2 and other than cooking appliances as covered in 210.19(A)(3) shall have an ampacity sufficient for the loads served and shall not be smaller than 14 AWG.
Exception No. 1: Tap conductors shall have an ampacity sufficient for the load served. In addition, they shall have an ampacity of not less than 15 for circuits rated less than 40 amperes and not less than 20 for circuits rated at 40 or 50 amperes and only where these tap conductors supply any of the following loads:
(a) Individual lampholders or luminaires with taps extending not longer than 450 mm (18 in.) beyond any portion of the lampholder or luminaire.
(b) A luminaire having tap conductors as provided in 410.117.
(c) Individual outlets, other than receptacle outlets, with taps not over 450 mm (18 in.) long.
(d) Infrared lamp industrial heating appliances.
(e) Nonheating leads of deicing and snow-melting cables and mats.
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Push in #14 CU solid wire for 15A branch circuit only.
Installation Screw Terminal > #14 - #12 AWG CU Wire Only.
Observe na parte de trás deste receptáculo duplex de 125V 15A, ele afirma que você pode usar fio de cobre de 14 AWG ou 12 AWG ao terminar nos terminais de parafuso, mas que cobre 14 AWG deve ser usado ao terminar nos terminais de pressão. Ele também diz que se você estiver usando o terminal push in e o cobre 14 AWG, ele só pode ser usado em um circuito de 15 amperes. Se você estiver instalando isso em um circuito de 20 amperes, com fio de 12 AWG, você terá que usar os terminais de parafuso.