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The issue of ampacity only considers the temperature of the conductor itself, but other issues must also be considered when choosing a conductor. In addition to avoiding overheating in the conductor itself, the conductor's size must be adequate for compatibility with the overcurrent device and to avoid overheating the terminals.Section 210.19
Section 210.19 addresses the minimum size conductor for a branch circuit, and is based on whether or not the load is considered to be continuous or noncontinuous. A continuous load is defined in Article 100 as "a load that's likely to continue for three hours or more."
For some types of loads, the NEC simply states that they will be calculated at 125% or they will be considered continuous, and they are treated as continuous loads whether or not they are known to be continuous in a specific case.
For example, Section 422.13 says an electric storage type water heater will be considered to be a continuous load for the purpose of sizing a branch circuit. A similar rule applies to fixed electric space heating in Section 424.3.
For other loads, a judgment must be made about the nature of the load. A common example of a continuous load is the lighting load of a commercial building, where all the lights are likely to remain on for the entire working day. Convenience receptacles such as those calculated at 180 VA or those included in general lighting are generally not considered to be continuous loads.
© Hughes Associates,Inc. 2012