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Remember that ampacity is defined in Article 100 as "the maximum current in amperes that a conductor can carry continuously under the conditions of use without exceeding its temperature rating."
Section 210.19's requirement to increase a conductor size for a continuous load is based on the fact that the conductor attached to an overcurrent device also conducts heat from the overcurrent device. It comes from the testing methods for overcurrent devices. That test method considers the conductor at and near the connection to the overcurrent device and does not consider the ambient temperature or number of adjacent conductors that may affect the conductor elsewhere. The concern is about a conductor being compatible with the overcurrent device, so the overcurrent device will operate as intended.
The fact that this minimum size is really based on the overcurrent device becomes clear from Section 210.19(A)(1), Exception. If the overcurrent device and its assembly are both rated for 100% operation, then it is not necessary to increase the size of the conductor.
© Hughes Associates,Inc. 2012