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The two rules in Sections 210.19 and 210.20 are based on the assumption that the overcurrent device is an ordinary device, meaning it is only intended and listed for continuous loading at 80% of its rating. It may be loaded to 100% of its rating, but only with a noncontinuous load.
If the overcurrent device and the assembly in which it is installed are listed for 100% loading, neither the overcurrent device nor the conductor is required to be increased in size from the calculated load.
Continuous load is defined in Article 100 as “a load where the maximum current is expected to continue for 3 hours or more.”
This is covered in more detail in the course module Conductor Selection & Application.
© Hughes Associates,Inc. 2012