In general, ampacities of most common types of flexible cord can be found in Table 400-5(A) You can usually use Column B, but check out the small print at the bottom of the table.

Here's an example, 2 burner hot plate draws 20 amps, needs to be on a 30 amp breaker since it will be on all day. Branch circuit conductors are #10, but the cord only needs to be a maximum of #12.

Few things are harder to put up with than the annoyance of a good example.