Licensed Electrical Contractor Code, Theory, and Calculations 2 — Questions and Answers
Question 1: According to NEC Article 310, what is the ampacity of a 12 AWG copper conductor with THHN insulation installed in a conduit with an ambient temperature of 30°C?
- 20 A
- 25 A
- 30 A (Correct answer)
- 15 A
Correct answer: 30 A
A 12 AWG copper THHN conductor has a base ampacity of 30 A at 75°C column per NEC Table 310.15(B)(16), with no correction needed at 30°C ambient.
Question 2: When applying the NEC demand factor for household electric dryers in a multifamily building with 20 dryers, what demand factor applies per Table 220.54?
- 35% (Correct answer)
- 50%
- 70%
- 100%
Correct answer: 35%
NEC Table 220.54 assigns a 35% demand factor for 20 household electric dryers in a multifamily dwelling.
Question 3: Kirchhoff's Voltage Law (KVL) states that the algebraic sum of all voltages around any closed loop equals:
- The supply voltage
- Zero (Correct answer)
- The total current times resistance
- The maximum branch voltage
Correct answer: Zero
KVL states that the sum of all voltage rises and drops around any closed loop is zero, reflecting conservation of energy.
Question 4: A 240 V single-phase motor draws 18 A at a power factor of 0.85. What is the true (real) power consumed?
- 3,672 W (Correct answer)
- 4,320 W
- 2,040 W
- 5,100 W
Correct answer: 3,672 W
True power P = V × I × PF = 240 × 18 × 0.85 = 3,672 W.
Question 5: Under NEC 230.70, the service disconnecting means must be located:
- Inside the building as close as practicable to the point of service entry (Correct answer)
- At the utility transformer
- On the exterior of the building only
- In the main panelboard regardless of location
Correct answer: Inside the building as close as practicable to the point of service entry
NEC 230.70 requires the service disconnect to be inside or outside the building nearest the point where the service conductors enter.
Question 6: In a parallel circuit, if one branch has a resistance of 10 Ω and another has 15 Ω, what is the total equivalent resistance?
- 6 Ω (Correct answer)
- 25 Ω
- 12.5 Ω
- 7.5 Ω
Correct answer: 6 Ω
Parallel resistance: 1/Rt = 1/10 + 1/15 = 3/30 + 2/30 = 5/30, so Rt = 6 Ω.
Question 7: NEC Article 500 classifies hazardous locations. A Class I, Division 1 location is defined as one where:
- Flammable gases or vapors are present under normal operating conditions (Correct answer)
- Combustible dust is present intermittently
- Ignitable fibers are stored but not processed
- Flammable liquids are present only during maintenance
Correct answer: Flammable gases or vapors are present under normal operating conditions
Class I, Division 1 locations are those where flammable gases or vapors exist in sufficient quantities under normal operating conditions to produce explosive mixtures.
Licensed Electrical Contractor Code, Theory, and Calculations 2
According to NEC Article 310, what is the ampacity of a 12 AWG copper conductor with THHN insulation installed in a conduit with an ambient temperature of 30°C?