A multi-family dwelling unit has a 12 kW electric range, a 4.5 kW electric dryer, and two 1.5 kW 120V electric water heaters. All other general loads for the unit are 6000 VA. Using the optional method for dwelling units (NEC Article 220.84), what is the calculated demand load for this unit?
- A21,000 VA
- B11,800 VA
- C16,200 VA
- D18,000 VA
Show answer & explanationAnswer & explanation
Correct answer: C. 16,200 VA
The optional method (NEC 220.84) for dwelling units calculates the total load by summing all loads and applying demand factors. Total connected load = 6000 VA (general) + 12000 VA (range) + 4500 VA (dryer) + (2 * 1500 VA) (water heaters) = 6000 + 12000 + 4500 + 3000 = 25,500 VA. According to Table 220.84, for a dwelling unit, the first 10,000 VA is at 100% demand. The remainder is subject to a 40% demand factor. So, 10,000 VA (at 100%) + (25,500 VA - 10,000 VA) * 0.40 = 10,000 VA + 15,500 VA * 0.40 = 10,000 VA + 6,200 VA = 16,200 VA.
Why the other options are wrong
- A. Incorrect. This calculation sums the full connected load without applying the optional method demand factors.
- B. Incorrect. This calculation likely misapplies the demand factors or omits certain loads.
- D. Incorrect. This calculation does not correctly apply the optional method demand factors.
Optional Method (Dwelling Units)
A simplified method for calculating the service or feeder load for a dwelling unit, often resulting in a smaller service size, by applying specific demand factors to the total connected load as outlined in NEC Article 220.84.
- Used for single-family or individual dwelling units.
- Requires summing all connected loads (general lighting, appliances, HVAC).
- Table 220.84 applies demand factors: first 10 kVA at 100%, remainder at 40% or 25%.
Memory trick: Optional Dwelling Load: Sum Everything, then Demand Factors.