IEC Service and Load Calculations 4 — Questions and Answers
Question 1: What is the maximum unbalanced load used to size the neutral conductor for a 120/240V single-phase service with a 100-ampere load on each hot leg and 80 amperes on the neutral?
- 100 amperes
- 80 amperes (Correct answer)
- 20 amperes
- 60 amperes
Correct answer: 80 amperes
The neutral conductor must be sized for the maximum unbalanced load, which is 80 amperes in this scenario.
Question 2: Per NEC 220.61, a demand factor of 70% may be applied to the neutral load for which type of load above 200 amperes?
- Electric heating loads
- Electric cooking equipment and dryers (Correct answer)
- Motor loads
- Lighting loads
Correct answer: Electric cooking equipment and dryers
NEC 220.61(B)(1) allows a 70% demand factor on the neutral load for household ranges, wall-mounted ovens, counter-mounted cooking units, and clothes dryers above 200 amperes.
Question 3: A motor with a full-load current of 28 amperes is the largest motor in a panel. Per NEC 220.50, what additional load is added for motor calculations?
- 25% of 28 A = 7 A (Correct answer)
- 125% of 28 A = 35 A
- 100% of 28 A = 28 A
- 80% of 28 A = 22.4 A
Correct answer: 25% of 28 A = 7 A
NEC 220.50 refers to Article 430, which requires adding 25% of the largest motor's FLA to the total load for feeder and service sizing.
Question 4: When sizing the service for a commercial occupancy with 50,000 VA of connected lighting, what demand factor applies to the first 20,000 VA per NEC Table 220.42?
- 50%
- 75%
- 100% (Correct answer)
- 80%
Correct answer: 100%
NEC Table 220.42 applies 100% to the first 20,000 VA of lighting load for commercial occupancies.
Question 5: A residential air conditioning unit draws 24 amperes at 240 volts and the electric heat load is 10 kW. Which load is omitted from the service calculation per NEC 220.60?
- The air conditioning load is omitted
- The smaller of the two loads is omitted (Correct answer)
- Both are included at 100%
- The heating load is omitted and AC used at 125%
Correct answer: The smaller of the two loads is omitted
NEC 220.60 permits omitting the smaller of the non-coincident heating or cooling loads since both are unlikely to operate simultaneously.
Question 6: For service entrance conductors, NEC 230.42 requires that the conductor ampacity must not be less than what percentage of the calculated load?
- 80%
- 100% (Correct answer)
- 115%
- 125%
Correct answer: 100%
NEC 230.42(A) requires service entrance conductors to have an ampacity not less than the calculated load (100%).
Question 7: A 400-ampere service has a calculated load of 320 amperes. The neutral calculated load is 210 amperes. What is the neutral ampacity after applying the NEC 220.61 reduction?
- 210 amperes
- 200 amperes + 7 amperes = 207 amperes (Correct answer)
- 147 amperes
- 168 amperes
Correct answer: 200 amperes + 7 amperes = 207 amperes
200 A at 100% = 200 A; (210 A − 200 A) × 70% = 7 A; total neutral = 207 amperes per NEC 220.61.
IEC Service and Load Calculations 4
What is the maximum unbalanced load used to size the neutral conductor for a 120/240V single-phase service with a 100-ampere load on each hot leg and 80 amperes on the neutral?