Using the optional calculation method, four or more separately-controlled electric space-heating units in a single family dwelling are calculated at ___% of their nameplate ratings.

Using the optional calculation method, four or more separately-controlled electric space-heating units in a single family dwelling are calculated at ___% of their nameplate ratings.

• A. 125%
• B. 40%
• C. 65%
• D. 100%

Answer: (B)

The main idea is applying a demand factor when sizing electric space-heating loads using the optional calculation method. When there are four or more separately-controlled electric space-heating units in a single-family dwelling, you don’t sum their full nameplate ratings. Instead, you calculate the load at 40% of the combined nameplate ratings. This reflects diversity of operation—not all heaters run at full power at the same time—and allows for a realistic, often smaller, load to size the service or feeders. For example, four space heaters each rated at 5 kW add up to 20 kW in nameplate terms, but with the optional method you’d use 0.40 × 20 kW = 8 kW for the load calculation. The other options would imply higher or unchanged loads, which isn’t how this method is intended to be used.

The main idea is applying a demand factor when sizing electric space-heating loads using the optional calculation method. When there are four or more separately-controlled electric space-heating units in a single-family dwelling, you don’t sum their full nameplate ratings. Instead, you calculate the load at 40% of the combined nameplate ratings. This reflects diversity of operation—not all heaters run at full power at the same time—and allows for a realistic, often smaller, load to size the service or feeders.

For example, four space heaters each rated at 5 kW add up to 20 kW in nameplate terms, but with the optional method you’d use 0.40 × 20 kW = 8 kW for the load calculation. The other options would imply higher or unchanged loads, which isn’t how this method is intended to be used.