Master Electrician Exam (NEC)Load CalculationsMedium
A master electrician is calculating the feeder and service load for a multi-family dwelling with ten units. Each unit has a 12 kW electric range. The building also has a 20 kW central electric space heating system and a 15 kW central air-conditioning system. What is the demand load for the larger of the space heating or air-conditioning loads for the entire building, according to NEC 220.60?
- A20 kW
- B15 kW
- C35 kW
- D10 kW
Show answer & explanationAnswer & explanation
Correct answer: A. 20 kW
According to NEC 220.60, when calculating feeder or service loads for a dwelling unit or a feeder to a group of dwelling units, the 'larger of' the air-conditioning or space heating load is used, but not both. In this case, 20 kW (heating) is larger than 15 kW (A/C).
Why the other options are wrong
- B. This is incorrect; 15 kW is the air-conditioning load, which is smaller than the heating load.
- C. This is incorrect; it sums both heating and air-conditioning loads, which is prohibited by NEC 220.60.
- D. This is incorrect; this value does not represent either the heating or air-conditioning load.
HVAC Demand (NEC 220.60)
For dwelling units or groups of dwelling units, only the larger of the air-conditioning or space heating load is included in the feeder or service load calculation, per NEC 220.60.
- Applies to dwelling units and groups of dwelling units.
- Only one of the two loads (A/C or heating) is selected.
- The larger of the two loads is always chosen.
- This rule prevents over-sizing due to non-simultaneous operation.
Memory trick: HVAC: Hot or Cold, pick the bigger story told.