A master electrician is calculating the service feeder demand load for a single-family dwelling. The dwelling has a 3-ton (36,000 BTU/hr) central air conditioning unit, a 10 kW electric furnace, and a 4.5 kW electric water heater. What is the demand load for the largest motor-operated appliance or combination of motor loads, as per NEC 220.60?
- A10,000 VA
- B4,500 VA
- C5,625 VA
- D36,000 VA
Show answer & explanationAnswer & explanation
Correct answer: C. 5,625 VA
The largest motor-operated appliance is the 3-ton AC unit. According to NEC 220.60, the demand load for the largest motor is 125% of its full-load current rating. A 3-ton AC unit typically has a nameplate rating in VA or Amps, but for load calculations, we consider its equivalent VA. We need to find the largest single motor or a group of motors that are likely to operate simultaneously. Here, the AC unit is the primary motor load.
Why the other options are wrong
- A. This is the rating of the electric furnace, which is a resistance load, not a motor load.
- B. This is the rating of the water heater, which is a resistance load, not a motor load.
- D. This is the nominal rating of the AC unit in BTU/hr, not its electrical VA demand load.
Largest Motor Load Demand
The demand load for the largest motor-operated appliance or combination of motor loads is calculated at 125% of the full-load current rating of the largest motor or the sum of the full-load current ratings of the motors in the group, as per NEC 220.60.
- Applies to motors, not resistance heaters.
- Calculated at 125% of the largest motor's rating.
- Ensures adequate circuit capacity for motor starting and continuous operation.
Memory trick: Motor Magic: Multiply for More Power!