Using the optional calculation method, four or more separately-controlled electric space-heating units are calculated at what percentage of their nameplate ratings?

When using the optional calculation method for calculating the load of electric space-heating units, the percentage applied to the nameplate ratings of four or more separately-controlled units is established by the National Electrical Code (NEC). Specifically, when there are four or more separately-controlled electric space-heating units, the NEC allows for the first three units to be considered at 100% of their nameplate rating, but the load for the additional units can be calculated at a lower percentage to account for diversity in usage.

The correct percentage specified for these additional units is 40%. This means that while the first three units are fully accounted for, only 40% of the nameplate ratings of each additional unit is added to the total load calculation. This method recognizes that not all units will operate simultaneously at their maximum capacity, thereby leading to a more efficient and practical load assessment.

While one might consider other options, this method is specifically designed to provide an accurate representation of expected load in real-world conditions, balancing safety with efficiency in electrical load calculations. Understanding this context is critical for electrical contractors when determining load requirements for electrical installations.