Building Engineering Electrical Systems in Buildings 2 — Questions and Answers
Question 1: What is the purpose of a transfer switch in a building's electrical system?
- To switch between utility power and generator power safely (Correct answer)
- To transfer loads between phases
- To step voltage up or down
- To distribute power to sub-panels
Correct answer: To switch between utility power and generator power safely
A transfer switch prevents backfeeding by safely isolating utility power before connecting generator power during an outage.
Question 2: Which electrical system configuration is most common in US commercial buildings?
- Three-phase 4-wire 208Y/120V (Correct answer)
- Single-phase 2-wire 120V
- Three-phase 3-wire 480V delta
- Single-phase 3-wire 240V
Correct answer: Three-phase 4-wire 208Y/120V
Three-phase 208Y/120V wye systems supply both 120V single-phase loads and 208V three-phase equipment in commercial facilities.
Question 3: What is the maximum allowed voltage drop for branch circuits in building electrical design per NEC recommendations?
- 3% (Correct answer)
- 5%
- 10%
- 1%
Correct answer: 3%
NEC recommends no more than 3% voltage drop on branch circuits and 5% total from the service entrance to the farthest outlet.
Question 4: Which lighting control device reduces energy use by dimming lights based on available daylight?
- Daylight harvesting sensor (Correct answer)
- Occupancy sensor
- Time clock relay
- Demand controller
Correct answer: Daylight harvesting sensor
Daylight harvesting sensors measure ambient light and automatically dim or switch off artificial lighting when natural light is sufficient.
Question 5: What ampacity is a standard 12 AWG copper conductor rated for in a 60°C environment per NEC?
- 20A (Correct answer)
- 15A
- 25A
- 30A
Correct answer: 20A
A 12 AWG copper conductor has a 20A ampacity rating at 60°C, making it suitable for 20A branch circuits.
Question 6: What is the function of a surge protective device (SPD) installed at a building's main panel?
- To limit transient voltage spikes from entering building wiring (Correct answer)
- To regulate steady-state voltage
- To filter harmonic distortion
- To balance phase loads
Correct answer: To limit transient voltage spikes from entering building wiring
An SPD clamps transient voltage spikes caused by lightning or utility switching before they can damage sensitive equipment throughout the building.
Building Engineering Electrical Systems in Buildings 2
What is the purpose of a transfer switch in a building's electrical system?