NEC Load Calculation Applications Questions and Answers 1 — Questions and Answers
Question 1: An electrician is performing a standard load calculation for a 2,200 sq. ft. single-family dwelling. What is the calculated load for the general lighting, small-appliance, and laundry circuits before any demand factors are applied?
- 11,100 VA (Correct answer)
- 6,600 VA
- 4,500 VA
- 10,500 VA
Correct answer: 11,100 VA
According to NEC Article 220, the calculation is as follows: General lighting is 2,200 sq. ft. x 3 VA/sq. ft. = 6,600 VA. Two small-appliance branch circuits are required at 1,500 VA each (2 x 1,500 VA = 3,000 VA) per 220.52(A). One laundry circuit is required at 1,500 VA per 220.52(B). The total connected load before demand factors is 6,600 VA + 3,000 VA + 1,500 VA = 11,100 VA.
Question 2: When calculating the feeder load for a group of continuous-duty motors, which of the following methods is required by NEC 430.24?
- 125% of the full-load current (FLC) of all motors combined.
- 100% of the FLC of all motors plus 25% of the largest motor's FLC.
- 125% of the FLC of the largest motor, plus the sum of the FLCs of all other motors. (Correct answer)
- 100% of the FLC of all motors on the feeder.
Correct answer: 125% of the FLC of the largest motor, plus the sum of the FLCs of all other motors.
NEC 430.24 requires conductors supplying several motors to have an ampacity equal to the sum of the full-load current ratings of all the motors plus 25% of the highest rated motor in the group. This is equivalent to taking 125% of the FLC of the largest motor and adding the sum of the FLCs of the remaining motors.
Question 3: A feeder in a multi-family dwelling supplies ten 12 kW household electric ranges. Using the standard method and Column C of NEC Table 220.55, what is the calculated demand load for these ranges?
- 120 kW
- 25 kW (Correct answer)
- 30 kW
- 40 kW
Correct answer: 25 kW
According to NEC Table 220.55, Column C is used for calculating the demand load for household ranges rated from 8.75 kW to 12 kW. For 10 ranges, the table specifies a maximum demand of 25 kW.
Question 4: For which of the following occupancies does NEC Table 220.42 allow a demand factor of 100% for the first 12,500 VA of general lighting load and 50% for the remainder?
- Dwelling Unit
- Hospital
- Office Building
- Warehouse (Correct answer)
Correct answer: Warehouse
NEC Table 220.42, "Lighting Load Demand Factors," specifies that for warehouses (storage), the first 12,500 VA of the general lighting load is calculated at 100%, and the remainder of the load over 12,500 VA is calculated at 50%.
Question 5: An electrician is using the optional method (NEC 220.82) to calculate the service load for a single-family dwelling. After summing the general loads and all fastened-in-place appliances, the total connected load is 34,000 VA (excluding HVAC). What is the demand load for this portion of the calculation?
- 19,600 VA (Correct answer)
- 13,600 VA
- 34,000 VA
- 24,000 VA
Correct answer: 19,600 VA
Per the optional calculation method in NEC 220.82(B), the first 10,000 VA is taken at 100%, and the remainder of the load is taken at 40%. Calculation: 10,000 VA (at 100%) + (34,000 VA - 10,000 VA) x 40% = 10,000 VA + (24,000 VA x 0.40) = 10,000 VA + 9,600 VA = 19,600 VA.
Question 6: When performing a standard load calculation for a dwelling unit, what is the minimum load in VA that must be used for a single household electric clothes dryer, unless the nameplate rating is larger?
- 1,500 VA
- 5,000 VA (Correct answer)
- 4,500 VA
- The nameplate rating only
Correct answer: 5,000 VA
NEC 220.54 specifies that the load for household electric clothes dryers in a dwelling unit shall be 5,000 VA (watts) or the nameplate rating, whichever is larger, for each dryer served.
NEC Load Calculation Applications Questions and Answers 1
An electrician is performing a standard load calculation for a 2,200 sq. ft. single-family dwelling.
What is the calculated load for the general lighting, small-appliance, and laundry circuits before any demand factors are applied?