CLD Electrical Systems & Lighting Control 4 — Questions and Answers
Question 1: A contractor plans to use a standard 600V-rated dimmer on a 277V circuit. What is the safety concern?
- The dimmer will overheat due to higher current
- The dimmer voltage rating is adequate; no concern exists
- A 277V circuit requires a dimmer rated for at least 347V or the specific 277V application (Correct answer)
- 277V circuits cannot be dimmed
Correct answer: A 277V circuit requires a dimmer rated for at least 347V or the specific 277V application
Dimmers must be rated for the specific voltage of the circuit; a 277V application requires a 277V-listed dimmer, and 600V ratings do not automatically imply 277V compatibility.
Question 2: In emergency lighting design, what is the minimum duration that a battery-backed unit must maintain illumination per NFPA 101?
- 30 minutes
- 90 minutes (Correct answer)
- 2 hours
- 4 hours
Correct answer: 90 minutes
NFPA 101 (Life Safety Code) requires emergency lighting to operate for a minimum of 90 minutes after normal power failure.
Question 3: What control technique does 'forward phase' (leading edge) dimming use to reduce light output?
- It extends the AC waveform's OFF period at the beginning of each half-cycle
- It chops the beginning of each AC half-cycle, delaying current flow (Correct answer)
- It adjusts the frequency of the AC supply
- It reduces the RMS voltage via a transformer tap
Correct answer: It chops the beginning of each AC half-cycle, delaying current flow
Forward phase (leading edge) dimming uses a TRIAC to cut off the start of each AC half-cycle, reducing the RMS power delivered to the load.
Question 4: A BACnet-integrated lighting control system allows the building automation system (BAS) to override lighting. Which scenario best justifies this integration?
- To allow occupants to manually adjust light levels
- To enable demand response — the BAS dims lighting during peak utility demand periods (Correct answer)
- To set lamp color temperatures automatically
- To replace the lighting control panel entirely
Correct answer: To enable demand response — the BAS dims lighting during peak utility demand periods
BAS-to-lighting integration via BACnet enables demand response strategies where the BAS automatically reduces lighting loads during peak demand, cutting utility costs.
Question 5: Which wire gauge has a higher ampacity: 12 AWG or 10 AWG copper conductor in conduit at 75°C rating?
- 12 AWG — smaller number means larger wire
- 10 AWG — larger number means larger wire
- 10 AWG — lower AWG number indicates larger wire diameter and higher ampacity (Correct answer)
- They are identical in ampacity
Correct answer: 10 AWG — lower AWG number indicates larger wire diameter and higher ampacity
In the AWG system, lower numbers indicate larger wire diameters; 10 AWG copper is rated for 35A versus 20A for 12 AWG at 75°C per NEC Table 310.16.
Question 6: A specifier requires 'Class 2' wiring for low-voltage control runs. What is the maximum voltage permitted for a Class 2 circuit per NEC Article 725?
- 24V DC only
- 60V DC / 30V AC (Correct answer)
- 150V DC / 120V AC
- 300V AC/DC
Correct answer: 60V DC / 30V AC
NEC Article 725 defines Class 2 circuits as limited to 60V DC or 30V AC, which limits shock and fire hazard and allows simplified wiring methods.
Question 7: What is 'phantom load' (also called standby power) in the context of lighting control devices?
- The load drawn by luminaires operating at maximum output
- The electrical energy consumed by a dimmer or driver while the light output is at 0% (Correct answer)
- The load caused by harmonics in the distribution system
- The load from emergency lighting circuits on standby battery
Correct answer: The electrical energy consumed by a dimmer or driver while the light output is at 0%
Phantom or standby load is the power consumed by electronic devices (dimmers, drivers, sensors) while the connected luminaire is off, which adds up across large installations.
CLD Electrical Systems & Lighting Control 4
A contractor plans to use a standard 600V-rated dimmer on a 277V circuit.
What is the safety concern?