240.5 Protection of Flexible Cords, Flexible Cables, and Fixture Wires.
Flexible cord and flexible cable, including tinsel cord and extension cords, and fixture wires shall be protected against overcurrent by either 240.5(A) or (B).
(B)(3) Extension Cord Sets.
Flexible cord used in listed extension cord sets, or in extension cords made with separately listed and installed components, shall be permitted to be supplied by a branch circuit of Article 210 in accordance with the following:
20-ampere circuits ? 16 AWG and larger