1. A 120V, single-phase circuit supplies a 1,500W resistive space heater. Using the NEC 125% continuous load rule (Article 210.19), what is the minimum ampacity required for the branch-circuit conductors?
- A. 12.5 A
- B. 15.625 A ✓
- C. 20 A
- D. 25 A
Per NEC 210.19(A)(1), branch-circuit conductors for continuous loads must have an ampacity not less than 125% of the continuous load. Load current = 1,500W ÷ 120V = 12.5 A. Minimum ampacity = 12.5 A × 1.25 = 15.625 A. The conductor must be rated at least 15.625 A before rounding up to the next standard size.
2. A 4-inch square metal box (1-1/2 inches deep) contains the following conductors: four 12 AWG circuit conductors, two 12 AWG equipment grounding conductors, and one duplex receptacle (yoke). Using NEC Table 314.16(A) and 314.16(B), what is the minimum required box volume in cubic inches?
- A. 13.5 cu in
- B. 18 cu in ✓
- C. 20.25 cu in
- D. 22.5 cu in
Under NEC 314.16(B), 12 AWG conductors use 2.25 cubic inches per conductor equivalent. The four insulated circuit conductors count as 4 conductor equivalents, the two equipment grounding conductors together count as 1 conductor equivalent, and the duplex receptacle yoke counts as 2 conductor equivalents. Total conductor equivalents = 4 + 1 + 2 = 7. Required volume = 7 x 2.25 = 15.75 cubic inches. No internal clamps, fixture studs, or other additional allowances are stated. Therefore the exact required minimum volume is 15.75 cubic inches, which is not listed; among the listed choices, 18 cubic inches is the smallest volume that exceeds the requirement, but the question/options are flawed if an exact NEC calculation answer is expected.
3. A 240V, single-phase, 10 AWG copper conductor circuit is 150 feet long (one-way) and carries 24 amperes. Using the standard voltage drop formula (VD = 2 × K × I × D ÷ CM), where K = 12.9 for copper and CM for 10 AWG = 10,380, what is the approximate voltage drop?
- A. 4.5 V
- B. 7.1 V
- C. 8.9 V ✓
- D. 10.6 V
VD = (2 × K × I × D) ÷ CM = (2 × 12.9 × 24 × 150) ÷ 10,380. Numerator: 2 × 12.9 = 25.8; × 24 = 619.2; × 150 = 92,880. VD = 92,880 ÷ 10,380 ≈ 8.95 V ≈ 8.9 V. NEC 210.19(A) Informational Note recommends limiting voltage drop to 3% for branch circuits, which for 240V = 7.2 V, so this circuit exceeds the recommended limit. The formula uses 2× for single-phase two-wire circuits (round-trip distance).
4. According to NEC 220.52, what is the calculated load for the small-appliance branch circuits in a dwelling unit that has a kitchen, pantry, dining room, and breakfast room?
- A. 1,500 VA
- B. 3,000 VA ✓
- C. 4,500 VA
- D. 6,000 VA
NEC 220.52(A) requires a minimum of two 20-ampere small-appliance branch circuits for the kitchen, pantry, dining room, and breakfast room of a dwelling unit. Each circuit is calculated at 1,500 VA per NEC 220.52(A). Total = 2 circuits × 1,500 VA = 3,000 VA. These circuits are in addition to the general lighting load and must be included in the service or feeder calculation.
5. A residential electric range is rated 12 kW. Using NEC Table 220.55 Column C, what demand load should be used when calculating the service for a single-family dwelling?
- A. 8,000 W ✓
- B. 8,400 W
- C. 10,000 W
- D. 12,000 W
NEC Table 220.55, Column C applies to ranges rated over 8¾ kW through 12 kW. For one range in this rating, the demand load is 8,000 W (8 kW). This demand factor accounts for the fact that the range is rarely operated at full capacity. A 12 kW range does not require the full 12 kW to be used for calculation; the table allows 8 kW demand.
6. Using NEC 220.42 and Table 220.42, what is the general lighting demand load for a 2,800 sq ft dwelling unit (floor area excluding unoccupied spaces)?
- A. 2,800 VA
- B. 5,600 VA
- C. 8,400 VA ✓
- D. 11,200 VA
NEC Table 220.42 sets the unit load for dwelling units at 3 VA per square foot for general lighting. Total connected load = 2,800 sq ft × 3 VA/sq ft = 8,400 VA. No demand factor reduction applies to this initial load from Table 220.42 at this square footage (demand factors under NEC 220.42 apply only when the general lighting load exceeds the threshold, and for this problem the full load is used as calculated per the table).
7. Four 10 AWG THHN copper conductors are installed in the same conduit. The ambient temperature is 86°F (30°C). Using NEC Table 310.16, what is the corrected ampacity of each 10 AWG THHN conductor after applying the conduit fill correction factor?
- A. 24 A
- B. 28 A
- C. 32 A ✓
- D. 40 A
Under NEC Table 310.16, 10 AWG copper THHN is taken from the 90°C column at 40 A. At 86°F (30°C), no ambient temperature correction is needed. With four current-carrying conductors in the same raceway, NEC 310.15(C)(1) requires an 80% adjustment factor for 4 through 6 conductors. Therefore, the adjusted ampacity is 40 A × 0.80 = 32 A. Terminal temperature limitations or the small-conductor rule may limit the allowable overcurrent protection in practice, but they do not change the adjusted conductor ampacity asked for here.
8. A 208V, 3-phase, 4-wire panel serves a balanced load of 30 kVA. What is the line current per phase?
- A. 48.1 A
- B. 72.2 A
- C. 83.3 A ✓
- D. 144.3 A
For a balanced 3-phase load: I = kVA × 1,000 ÷ (√3 × V_L-L) = 30,000 ÷ (1.732 × 208) = 30,000 ÷ 360.3 ≈ 83.3 A per phase. The √3 factor (1.732) is required for 3-phase calculations. Using single-phase formula (30,000 ÷ 208 = 144.2 A) or dividing by 3 instead of √3 are common errors.
9. According to NEC 430.22, a motor branch-circuit conductor must have an ampacity of at least what percentage of the motor's full-load current (FLC) for a continuous-duty motor?
- A. 100%
- B. 115%
- C. 125% ✓
- D. 150%
NEC 430.22(A) requires that branch-circuit conductors supplying a single motor used in a continuous duty application shall have an ampacity of not less than 125% of the motor's full-load current rating as determined by NEC 430.6(A)(1) (from NEC Tables 430.247–430.250). This is separate from the 125% continuous load rule of 210.19, but the result is the same multiplier for a different stated reason (motor starting and running characteristics).
10. A junction box contains the following 14 AWG conductors: 3 hot conductors entering, 3 neutral conductors entering, 2 equipment grounding conductors, and one single-pole switch (device). Per NEC 314.16(B), what is the minimum required box volume?
- A. 12.0 cu in
- B. 16.0 cu in
- C. 18.0 cu in ✓
- D. 20.0 cu in
Per NEC 314.16(B) and Table 314.16(B), each 14 AWG conductor allowance is 2.0 cubic inches. The 3 ungrounded conductors plus 3 grounded conductors count as 6 conductor equivalents. All equipment grounding conductors in the box together count as 1 conductor equivalent. One single-pole switch yoke counts as 2 conductor equivalents. Total conductor equivalents = 6 + 1 + 2 = 9; 9 × 2.0 cu in = 18.0 cu in.