NEC Load Calculations 3 — Questions and Answers
Question 1: According to NEC 220.18, what is the minimum load in VA that must be used for a household electric dryer when the nameplate rating is less than 5,000 W?
- 3,500 VA
- 4,000 VA
- 5,000 VA (Correct answer)
- 6,000 VA
Correct answer: 5,000 VA
NEC 220.54 (formerly 220.18) requires a minimum load of 5,000 VA (5 kW) for each electric clothes dryer, regardless of the nameplate rating.
Question 2: What demand factor does NEC Table 220.44 allow for the portion of a non-dwelling receptacle load that exceeds 60,000 VA?
- 100%
- 75%
- 50% (Correct answer)
- 25%
Correct answer: 50%
NEC Table 220.44 allows a 50% demand factor on the receptacle load that exceeds 60,000 VA for non-dwelling occupancies.
Question 3: When sizing a feeder for six household electric ranges each rated at 12 kW, what is the demand load per NEC Table 220.55?
- 72,000 VA
- 43,000 VA (Correct answer)
- 36,000 VA
- 21,000 VA
Correct answer: 43,000 VA
Table 220.55 Column C lists 43 kW demand for six ranges rated 8¾–12 kW each (maximum demand factor applied).
Question 4: A 480V, 3-phase, 4-wire system supplies a 150 kVA transformer. What is the full-load primary current?
- 180A
- 203A
- 210A
- 180.4A (Correct answer)
Correct answer: 180.4A
I = kVA × 1000 ÷ (√3 × V) = 150,000 ÷ (1.732 × 480) = 150,000 ÷ 831.36 ≈ 180.4A.
Question 5: Under NEC 220.60, when diverse loads such as heating and cooling cannot operate simultaneously, what is the rule for including both in the feeder calculation?
- Add both loads at 100%
- Add only the larger load (Correct answer)
- Add 75% of both loads
- Add 50% of both loads
Correct answer: Add only the larger load
NEC 220.60 (noncoincident loads) permits omitting the smaller of two loads that will not operate simultaneously from the feeder calculation.
Question 6: What is the calculated load in VA for general lighting in a 2,400 sq ft dwelling using the NEC unit load of 3 VA per sq ft?
- 4,800 VA
- 6,000 VA
- 7,200 VA (Correct answer)
- 9,600 VA
Correct answer: 7,200 VA
2,400 sq ft × 3 VA/sq ft = 7,200 VA per NEC Table 220.12 for dwelling occupancies.
Question 7: Per NEC 430.24, when two or more motors share a feeder, conductor ampacity must be based on the sum of all motor FLCs plus what percentage of the largest motor FLC?
- 15%
- 20%
- 25% (Correct answer)
- 125%
Correct answer: 25%
NEC 430.24 requires feeder conductors sized at 125% of the largest motor FLC plus the FLC of all other motors on the feeder.
NEC Load Calculations 3
According to NEC 220.18, what is the minimum load in VA that must be used for a household electric dryer when the nameplate rating is less than 5,000 W?