The reason that manufacturers get away with smaller wire and plugs is because they get the whole assembly tested as a unit, and different standards are used for those evaluations. The NEC was written for general purpose applications where the whole assembly is not evaluated by an independant agency. Because there's less certainty of the load, the NEC is more conservative.
That's the reason Delta can get away with shipping their 5hp Unisaw (20A motor) with a NEMA 6-20 (20A) plug and 10ga flexible cord, although telling you to hook it to 40A protection which means you have to run 8ga from the breaker to the outlet to meet the NEC code requirements.
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