Licensed Electrical Contractor Electrical System Design & Installation 3 — Questions and Answers
Question 1: When installing conduit through fire-rated walls, what is required at each penetration per NEC 300.21?
- Conduit bushings
- Fire stopping material (Correct answer)
- Expansion fittings
- Grounding bushings
Correct answer: Fire stopping material
NEC 300.21 requires fire-rated wall penetrations to be firestopped to prevent the spread of fire through electrical installations.
Question 2: A panelboard rated 200A is fed by a 200A breaker. Per NEC 408.36, what is the maximum size overcurrent protection allowed for a panelboard rated for individual protection?
- Must not exceed the panelboard busbar rating (Correct answer)
- 125% of the panelboard rating
- 150% of the panelboard rating
- The next standard size above the panel rating
Correct answer: Must not exceed the panelboard busbar rating
Per NEC 408.36, the overcurrent protection for a panelboard shall not exceed the rating of the panelboard.
Question 3: Which grounding electrode is considered the most effective and is preferred when available at a new construction site?
- Ground rod
- Concrete-encased electrode (Ufer ground) (Correct answer)
- Ground ring
- Water pipe
Correct answer: Concrete-encased electrode (Ufer ground)
The concrete-encased electrode (Ufer ground) provides excellent contact with earth and is preferred due to its large surface area and stable impedance.
Question 4: What does the term 'demand factor' mean in electrical system design?
- The ratio of maximum demand to total connected load (Correct answer)
- The power factor of the load
- The efficiency rating of electrical equipment
- The ratio of reactive power to apparent power
Correct answer: The ratio of maximum demand to total connected load
Demand factor is the ratio of maximum demand to total connected load, accounting for the fact that not all loads operate simultaneously at full capacity.
Question 5: When installing Type NM cable in a wood-framed wall, the cable must be protected by a steel plate when the edge of the cable is within how many inches of the edge of a stud?
- 1 inch
- 1.25 inches (Correct answer)
- 1.5 inches
- 2 inches
Correct answer: 1.25 inches
NEC 300.4(A)(1) requires a steel plate or bushing at least 1/16 inch thick when NM cable is within 1-1/4 inches of the edge of a stud.
Question 6: In electrical system design, what is the purpose of using a 'high-leg delta' (wild leg) configuration?
- To provide 277V single-phase for lighting
- To supply both 240V 3-phase and 120V single-phase from one transformer bank (Correct answer)
- To reduce harmonic distortion in the system
- To allow connection of both delta and wye loads
Correct answer: To supply both 240V 3-phase and 120V single-phase from one transformer bank
A high-leg delta provides 240V 3-phase power along with 120V single-phase from two legs, with the high leg (B phase) measuring approximately 208V to neutral.
Question 7: What is the minimum burial depth for Schedule 80 PVC conduit containing 120V branch circuit conductors under a residential driveway?
- 6 inches
- 12 inches (Correct answer)
- 18 inches
- 24 inches
Correct answer: 12 inches
NEC Table 300.5 allows Schedule 80 PVC conduit burial at 12 inches under a residential driveway for circuits up to 120V.
Licensed Electrical Contractor Electrical System Design & Installation 3
When installing conduit through fire-rated walls, what is required at each penetration per NEC 300.21?