Q I have a Code question that has been bothering me. This question is not the result of a particular project - just general Code book understanding. Article 220, paragraph 10 (b) is reasonably clear as to the definition of continuous and non-continuous loads and how they affect the size of feeders and services. My question is, "How does the NEC expect the designer/engineer to treat the loads that have been decreased due to a Demand Factor elsewhere in the Code?" A very clear example of my question comes from Table 220-20, Kitchen Equipment. After a demand factor of 65% is applied to six pieces of kitchen equipment, is this demand load considered continuous or non-continuous? I will admit that the equipment is probably non-continuous, but the load is computed with the idea that it could be in continuous use only with different pieces of equipment operating.
A In the 1996 NEC, Section 220-10(b) did require that conductors and overcurrent devices be increased in size when subjected to continuous loads. This requirement is intended to address the overcurrent devices rather than the conductors, as ampacity is a continuous value by definition. However, overcurrent devices are tested under specific conditions that include connection to a wire with an ampacity essentially equal to the rating of the overcurrent device. Therefore, minimum sizes are imposed on conductors because of the overcurrent device they connect to rather than because of any risk of overheating the conductor itself. Now, in the 1999 NEC, the issue of continuous and non-continuous loads has been removed from Article 220 to clarify that it's not part of any load calculation, but is only a minumum sizing requirement for conductors and overcurrent devices. The relocated requirements are found in Sections 210-19 and 210-20 for branch circuits, in Sections 215-2 and 215-3 for feeders, and in Section 230-42 for services.
A load that is continuous is not permitted to be reduced by a demand factor. Even where demand factors are permitted for a specific type of load, such as lighting loads as covered in Section 220-11, the demand factors do not apply to loads "where the entire lighting is likely to be used at one time." In the example you gave, the load is reduced by a demand factor because of intermittent operation. Because of intermittent operation, perhaps due to thermostatic controls, none of the individual kitchen loads would be continuous. The resulting demand load is generally not considered as continuous under the Code either. However, depending on the actual use of the equipment, the calculated load may be imposed on the feeder for three hours or more, and this condition would require an increase in the size of feeder conductors and overcurrent devices. In general, this condition would be unlikely, as the actual demand is likely to be even less than the calculated demand load, but it certainly is possible. Outside of the references given above, I see no specific language in the Code to cover this situation. However, the language of Section 220-35 requires that a measured demand load be increased by 25% and is thus treated as continuous in determining whether an existing feeder has spare capacity.
Section 220-20 is permissive. Higher demand factors could be used, and should be used if we know enough about the load to expect the scenario you describe. For example, motor loads are handled on a case-by-case basis under Section 430-26, and require substantial information about the nature of the load in order to apply a demand factor. Note that the continuous load requires a minimum conductor size selected independently of the actual load calculation under Article 220, and this minimum size is independent of the conditions of use of the conductor. Conditions of use might require ampacity corrections or adjustments due to ambient temperatures or more than three current-carrying conductors run together.