Licensed Electrical Contractor Electrical Contractor Electrical System Design & Installation 2 — Questions and Answers
Question 1: When designing a commercial electrical system, what is the NEC-required minimum height for service entrance conductors over a public roadway?
- 18 feet (Correct answer)
- 15 feet
- 12 feet
- 10 feet
Correct answer: 18 feet
NEC Section 230.24(B) requires service-drop conductors to maintain a minimum clearance of 18 feet over public streets and roadways.
Question 2: In a 208/120V three-phase, four-wire wye system, what is the voltage measured between any two phase conductors?
- 208V (Correct answer)
- 120V
- 240V
- 277V
Correct answer: 208V
In a wye-connected system with 120V phase-to-neutral, the line-to-line voltage is 120 × 1.732 = approximately 208V.
Question 3: What is the maximum allowable voltage drop recommended by the NEC for a combination of feeder and branch circuit conductors?
- 5% (Correct answer)
- 3%
- 8%
- 10%
Correct answer: 5%
NEC recommends a maximum total voltage drop of 5% for the combined feeder and branch circuit, with no more than 3% on either individually.
Question 4: When installing a 400-amp service in a commercial building, what is the minimum size copper conductor required for the grounding electrode conductor?
- 1/0 AWG (Correct answer)
- 2 AWG
- 3/0 AWG
- 250 kcmil
Correct answer: 1/0 AWG
Per NEC Table 250.66, a 400-amp service using copper conductors up to 500 kcmil requires a minimum 1/0 AWG copper grounding electrode conductor.
Question 5: What type of electrical system grounding is required by the NEC for a 480/277V, three-phase, four-wire system supplying a commercial building?
- Solidly grounded (Correct answer)
- Ungrounded
- Resistance grounded
- Corner grounded
Correct answer: Solidly grounded
The NEC requires 480/277V wye systems supplying line-to-neutral loads to be solidly grounded per Section 250.20(B).
Question 6: Per NEC Article 220, what is the general lighting load demand factor applied to the first 10,000 VA of general lighting in a commercial occupancy?
- 100% (Correct answer)
- 50%
- 75%
- 125%
Correct answer: 100%
NEC Table 220.42 requires the first 10,000 VA of general lighting load to be calculated at 100% demand factor.
Licensed Electrical Contractor Electrical Contractor Electrical System Design & Installation 2
When designing a commercial electrical system, what is the NEC-required minimum height for service entrance conductors over a public roadway?