In a one-family, two-family, or multifamily dwelling, it is permissible to apply a demand factor of 75 percent of the nameplate rating load of which number of appliances?

The correct answer is that it is permissible to apply a demand factor of 75 percent of the nameplate rating load for four appliances. This is based on established standards in residential electrical load calculation, which recognize that not all electrical appliances will operate at full load simultaneously.

The reasoning behind the specified number of appliances is based on the likelihood of simultaneous usage. In residential settings, it's reasonable to assume that of multiple appliances, only a specific number would be in use at any given time. When considering the demand factor, the calculation takes into account the average use patterns, leading to a more efficient and realistic assessment of load requirements.

Utilizing this demand factor helps to prevent over-sizing of electrical service and ensures that electrical systems are both safe and efficient. For instance, in a single-family home, you might have a variety of appliances such as a refrigerator, washer, dryer, and air conditioning unit. Applying the demand factor to four appliances allows for a more practical approach to capacity planning, recognizing that not all appliances will run at their maximum capacity simultaneously.