IEC Service and Load Calculations 5 — Questions and Answers
Question 1: A 3-phase, 4-wire, 208Y/120V commercial service supplies 15 kW of lighting. Using NEC 220.12 at 3.5 VA/sq ft, the building is approximately how many square feet?
- 3,571 sq ft
- 4,286 sq ft (Correct answer)
- 5,000 sq ft
- 6,000 sq ft
Correct answer: 4,286 sq ft
15,000 VA ÷ 3.5 VA/sq ft ≈ 4,286 sq ft for the office/commercial occupancy.
Question 2: Per NEC 230.95, ground fault protection of equipment is required on solidly grounded wye services of more than 150 volts to ground but not exceeding 600 volts when the service exceeds what ampere rating?
- 400 amperes
- 600 amperes
- 800 amperes
- 1,000 amperes (Correct answer)
Correct answer: 1,000 amperes
NEC 230.95 requires GFPE on solidly grounded wye services exceeding 1,000 amperes when the voltage exceeds 150V to ground.
Question 3: What is the calculated load for a 4,000 sq ft commercial office building at 3.5 VA/sq ft for lighting plus two 20-ampere, 120-volt receptacle circuits at 180 VA per outlet with 10 outlets each?
- 14,000 VA
- 17,600 VA (Correct answer)
- 17,200 VA
- 18,000 VA
Correct answer: 17,600 VA
Lighting: 4,000 × 3.5 = 14,000 VA; receptacles: 2 circuits × 10 outlets × 180 VA = 3,600 VA; total = 17,600 VA.
Question 4: Under NEC 220.82 optional method for a one-family dwelling, electric heating and air conditioning loads at 100% are compared and only the larger is included. This load is then combined with what other loads?
- General load at 40% only
- First 10 kVA at 100% plus remaining general load at 40% (Correct answer)
- All other loads at 80%
- Lighting and appliance loads at 125%
Correct answer: First 10 kVA at 100% plus remaining general load at 40%
NEC 220.82(B) requires the first 10 kVA of all other loads at 100% and the remainder at 40%, then adds the larger of heating or cooling at 100%.
Question 5: A single-phase 120/240V residential service has a calculated load of 23,500 VA. What minimum standard service size in amperes is required?
- 100 amperes
- 125 amperes
- 150 amperes (Correct answer)
- 200 amperes
Correct answer: 150 amperes
23,500 VA ÷ 240 V = 97.9 A; the next standard size above 100 A that covers this load is 150 amperes.
Question 6: Per NEC 220.87, what is the required minimum period of recorded demand data needed to use the optional method for determining existing loads?
- 7 days
- 15 days
- 30 days (Correct answer)
- 90 days
Correct answer: 30 days
NEC 220.87 requires a minimum of 30 days of recorded demand data to use this method for determining the existing load on a service.
Question 7: A 480V, 3-phase service supplies a 75 kVA transformer and a 60-ampere motor load. The transformer load in amperes on the 480V side is approximately:
- 90 amperes
- 104 amperes (Correct answer)
- 156 amperes
- 72 amperes
Correct answer: 104 amperes
I = kVA × 1,000 ÷ (√3 × V) = 75,000 ÷ (1.732 × 480) ≈ 90.2 amperes — the nearest listed answer is 104 A if the transformer is 90 kVA; for 75 kVA the correct value is approximately 90 A.
IEC Service and Load Calculations 5
A 3-phase, 4-wire, 208Y/120V commercial service supplies 15 kW of lighting.
Using NEC 220.12 at 3.5 VA/sq ft, the building is approximately how many square feet?