A master electrician is performing an optional calculation for a 200A, 120/240V single-family dwelling unit according to NEC Article 220.82. The dwelling has a 5 kW electric clothes dryer, a 3 kW water heater, and a 12 kW electric furnace. What is the demand load for these fixed appliances before applying the general optional method demand factors?
- A25 kW
- B20 kW
- C15 kW
- D10 kW
Show answer & explanationAnswer & explanation
Correct answer: B. 20 kW
According to NEC Article 220.82(B)(3), the demand load for four or more fixed appliances (other than ranges, clothes dryers, space heating, or air conditioning) is 75% of the total nameplate ratings. However, for the specified appliances (dryer, water heater, furnace), you sum their full rated loads if they are not specifically covered by another demand factor. For the optional method, we first sum the nameplate ratings for these fixed appliances. 5 kW (dryer) + 3 kW (water heater) + 12 kW (furnace) = 20 kW. This 20 kW is then subject to the optional method demand factors later.
Why the other options are wrong
- A. Incorrect. This value is too high for the sum of the given appliance ratings.
- C. Incorrect. This value is not the direct sum of the nameplate ratings.
- D. Incorrect. This value is too low; it might represent an incorrect application of a demand factor.
Optional Dwelling Unit Calculation (Fixed Appliances)
For the optional method (NEC 220.82), fixed appliance loads (excluding ranges, clothes dryers, space heating, or AC if already accounted for) are summed at their nameplate ratings before applying the overall demand factors.
- NEC 220.82 is the optional method for dwelling unit load calculations.
- Specific fixed appliances (dryer, water heater, furnace) are summed at full nameplate rating initially.
- The optional method then applies a demand factor to the total calculated load.
Memory trick: Just add up the appliance 'powers' first, like adding prices in a shopping cart before applying a coupon.