A master electrician is performing an optional method load calculation (NEC 220.82) for a single-family dwelling unit. The dwelling has a 16 kW electric furnace and a 5 kW central air-conditioning unit. What is the demand load for these heating/cooling loads?
- A5 kW
- B10 kW
- C16 kW
- D11 kW
Show answer & explanationAnswer & explanation
Correct answer: C. 16 kW
According to NEC 220.82(C)(5), for the optional calculation method, if there are four or more fixed appliances other than ranges, clothes dryers, space heating equipment, or air-conditioning equipment, the demand factor is 75% of the total nameplate rating. However, for heating and A/C, the rule is to use the larger of the heating or A/C load. In this case, the electric furnace is 16 kW and the A/C is 5 kW. The larger value is 16 kW. This is the same principle as the standard method under NEC 220.60, which applies here implicitly.
Why the other options are wrong
- A. This is the smaller of the two loads, which is incorrect.
- B. This is an arbitrary value and not derived from NEC rules.
- D. This would be the sum of heating and A/C, which is incorrect.
Optional Method Heating/A/C Load
When using the optional calculation method (NEC 220.82), the demand load for fixed electric space heating and air-conditioning equipment is generally determined by using only the larger of the two connected loads.
- NEC 220.82 is the optional calculation method.
- The principle of using the larger of heating or A/C load applies.
- This avoids overestimating the demand since both systems are not typically used simultaneously.
Memory trick: Optional calculation, but still pick the biggest climate control.