IEC - Independent Electrical Contractors Service and Load Calculations Questions and Answers — Questions and Answers
Question 1: When calculating the service load for a 2,500 sq. ft. single-family dwelling using the standard method, what is the total VA that must be allocated for the required small-appliance and laundry branch circuits before any demand factors are applied?
- 3,000 VA
- 1,500 VA
- 6,000 VA
- 4,500 VA (Correct answer)
Correct answer: 4,500 VA
According to NEC 220.52, a dwelling unit requires at least two 20-ampere small-appliance branch circuits and one 20-ampere laundry branch circuit. Each of these required circuits is calculated at 1,500 VA. Therefore, the total is (2 x 1,500 VA) + (1 x 1,500 VA) = 4,500 VA.
Question 2: An electrician is calculating the service for a single-family dwelling using the optional method per NEC Article 220, Part IV. The total calculated general load is 32,000 VA. How is the demand factor applied to this load?
- The first 3,000 VA at 100%, and the remainder at 35%
- The entire 32,000 VA is taken at 65%
- The first 10,000 VA at 100%, and the remainder at 40% (Correct answer)
- The entire 32,000 VA is taken at 100%
Correct answer: The first 10,000 VA at 100%, and the remainder at 40%
NEC 220.82(B) specifies the demand factor for the optional method calculation for a dwelling unit. The first 10,000 VA of the general load is calculated at 100%, and the remainder of the load is calculated at 40%. So, (10,000 VA * 1.0) + ((32,000 VA - 10,000 VA) * 0.40) = 10,000 VA + 8,800 VA = 18,800 VA.
Question 3: Which of the following loads in a dwelling unit is subject to a 75% demand factor when four or more are fastened in place and served by the same feeder or service?
- Electric clothes dryers
- Electric ranges
- Air-conditioning units
- Fixed appliances like a water heater, dishwasher, garbage disposal, and trash compactor (Correct answer)
Correct answer: Fixed appliances like a water heater, dishwasher, garbage disposal, and trash compactor
NEC 220.53 permits a 75% demand factor to be applied to the nameplate ratings of four or more fastened-in-place appliances served by the same feeder or service. This rule specifically excludes electric ranges, clothes dryers, space-heating equipment, and air-conditioning equipment, which have their own specific calculation rules.
Question 4: You are calculating the receptacle load for a 15,000 sq. ft. office building. The plans show 120 individual 125V receptacles. According to NEC 220.14(I) and Table 220.44, what is the calculated load after applying the appropriate demand factor?
- 21,600 VA
- 15,800 VA (Correct answer)
- 15,000 VA
- 10,800 VA
Correct answer: 15,800 VA
First, calculate the total connected receptacle load: 120 receptacles × 180 VA per receptacle = 21,600 VA. Then, apply the demand factor from NEC Table 220.44, which allows the first 10 kVA (10,000 VA) at 100% and the remainder at 50%. Calculation: 10,000 VA + ((21,600 VA - 10,000 VA) × 0.50) = 10,000 VA + 5,800 VA = 15,800 VA.
Question 5: For a commercial kitchen with six pieces of equipment, NEC 220.56 states that the feeder or service calculated load shall not be less than the sum of the ________.
- ratings of all equipment
- three smallest equipment loads
- largest two kitchen equipment loads (Correct answer)
- largest equipment load plus 50% of the others
Correct answer: largest two kitchen equipment loads
NEC 220.56 contains a provision that, regardless of the demand factors applied from Table 220.56, the final calculated load for commercial kitchen equipment cannot be less than the sum of the largest two kitchen equipment loads. This ensures the service is adequately sized for a worst-case scenario where the two biggest pieces of equipment run simultaneously.
Question 6: What is the minimum calculated load for a show-window lighting circuit that runs along a 40-foot base in a retail store, before accounting for it as a continuous load?
- 3,000 VA
- 1,200 VA
- 8,000 VA (Correct answer)
- 6,000 VA
Correct answer: 8,000 VA
According to NEC 220.46(A), the load for show-window lighting shall be calculated at not less than 200 VA per linear foot. Therefore, the calculation is 40 feet × 200 VA/foot = 8,000 VA. The 125% multiplier for continuous loads would be applied later when sizing the overcurrent protection and conductors, not for this initial load calculation.
