What is the limiting factor to limiting this cable to 20 amps.
The cable is 10 AWG. I would suspect the cable to be rated for at least 30 amps because it is inside of a jacket.
The manufacturers follow guidelines for flexible cord as set forth by UL and the National Electrical Code based on the insulation, temperature rating and number of conductors within the cord.
More specifically, NEC Article 400 covers flexible cords and cables. Table 400.5(a) column B limits this cable type to a maximum amperage of 30 amps on a 10 awg cord with a maximum of 2 current carrying conductors. Column A limits a 3 conductor cable to a maximum of 25 amps.
This cable has 5 conductors, so by the NEC Table 400.5 adjustment factors, I would be further derated by 80% using the column A rating aforementioned, thus giving this cable a 20 amp rating.
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