This was covered prior, I think Steve Terry provided this response, I copied for future reference, forgot the CB link.
The minimum
gauge of an extension
cord (
jumper) in article 520 venues is actually set by the following
NEC sections:
400.13 Overcurrent Protection.
Flexible cords not smaller than 18
AWG, and tinsel cords or cords having equivalent characteristics of smaller size approved for use with specific appliances, shall be considered as protected against overcurrent in accordance with 240.5.
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).
(A) Ampacities. Flexible
cord and flexible cable shall be protected by an overcurrent device in accordance with their
ampacity as specified in Table 400.5(A)(1) and Table 400.5(A)(2).
Fixture wire shall be protected against overcurrent in accordance with its
ampacity as specified in Table 402.5. Supplementary overcurrent protection, as covered in 240.10, shall be permitted to be an acceptable means for providing this protection.
(B) Branch-Circuit Overcurrent Device. Flexible
cord shall be protected, where supplied by a branch
circuit, in accordance with one of the methods described in 240.5(B)(1), (B)(3), or (B)(4).
Fixture wire shall be protected, where supplied by a branch
circuit, in accordance with 240.5(B)(2).
Note: [240.5(B) (1) and (2) omitted for clarity since they don’t relate to this issue on
portable cables
(3) Extension
Cord Sets. Flexible
cord used in listed extension
cord sets shall be considered to be protected when applied within the extension
cord listing requirements.
(4) Field Assembled Extension
Cord Sets. Flexible
cord used in extension cords made with separately listed and installed components shall be permitted to be supplied by a branch
circuit in accordance with the following:
20-ampere circuits — 16
AWG and larger
Above material (copyright
NFPA)
So, you can see that a cable must be sized for the load it is feeding, per the
ampacity tables of article 400. If it is a listed assembly, it must be used within its marked listing requirements. If it is a field assembled
cord, not listed as a complete assembly, it must be a minimum 16
AWG if used on a 20A
circuit. Of course, if used in a
theatre, it must be type SO or another extra-hard-usage
cord, unless otherwise allowed by section 520.68.
Note that a "field assembled" extension
cord may actually be provided by a manufacturer who has not bothered to have it listed as a complete assembly but who has assembled it from listed parts.
Also note that a common mistake is to misapply
ampacity table 520.44 and its maximum
breaker size column to single
circuit jumpers.