CEP Installation & Maintenance Practices 5 — Questions and Answers
Question 1: During commissioning of a 3-phase motor, the technician measures 480V L1-L2, 480V L2-L3, but only 277V L1-L3. What is the MOST likely problem?
- The motor is running in reverse rotation
- One phase conductor is open (open phase condition) (Correct answer)
- The motor windings are shorted phase-to-phase
- The power factor capacitors are incorrectly sized
Correct answer: One phase conductor is open (open phase condition)
An open phase in a 3-phase system causes the voltage across the open leg's endpoints to appear as a phase-to-neutral voltage (277V on a 480Y/277V system) rather than a phase-to-phase voltage.
Question 2: What NEC article governs the installation of electrical wiring in hazardous (classified) locations involving flammable gases or vapors?
- NEC Article 500 (Correct answer)
- NEC Article 400
- NEC Article 550
- NEC Article 680
Correct answer: NEC Article 500
NEC Article 500 covers classified locations for flammable gases, vapors, and dusts, establishing the Class/Division and Zone classification system for hazardous area wiring.
Question 3: When replacing a failed contactor in a motor control center (MCC) bucket, what step must be taken BEFORE removing the old contactor?
- Label the coil voltage on the new contactor
- Photograph or document the existing wiring connections (Correct answer)
- Test the new contactor coil with a megohmmeter
- Verify the motor nameplate horsepower rating
Correct answer: Photograph or document the existing wiring connections
Documenting or photographing existing wiring before removal ensures correct reconnection, preventing misconnections that could damage the motor or create hazardous conditions.
Question 4: A 200A, 240/120V single-phase service uses 4/0 AWG aluminum service entrance conductors. What is the minimum size copper equipment grounding conductor required?
- 8 AWG copper
- 6 AWG copper
- 4 AWG copper (Correct answer)
- 2 AWG copper
Correct answer: 4 AWG copper
NEC Table 250.66 specifies that for a 200A service with 4/0 AWG aluminum service entrance conductors, the grounding electrode conductor must be at least 4 AWG copper.
Question 5: When installing a receptacle in a commercial kitchen within 6 feet of a sink, what protection is required per NEC 210.8(B)?
- A weatherproof cover plate
- A GFCI-protected receptacle (Correct answer)
- A dedicated 20A circuit for each receptacle
- An AFCl breaker on the circuit
Correct answer: A GFCI-protected receptacle
NEC 210.8(B) requires GFCI protection for all receptacles installed within 6 feet of sinks in commercial kitchens and other commercial locations.
Question 6: What is the significance of torquing terminations to the manufacturer's specified values during installation?
- It ensures the conductor insulation is not damaged during termination
- It prevents loose connections that cause overheating and ensures code compliance (Correct answer)
- It is only required for connections above 600V
- It is a best practice with no code backing
Correct answer: It prevents loose connections that cause overheating and ensures code compliance
NEC 110.14(D) (2020 NEC) requires that terminals be tightened to the torque values specified by the equipment manufacturer to prevent resistance heating from loose connections.
Question 7: During preventive maintenance, a technician uses a clamp-on ammeter and measures significantly higher current on the neutral conductor than on any individual phase of a 3-phase, 4-wire wye system. What is the MOST likely cause?
- A ground fault on one of the phase conductors
- Excessive non-linear loads (harmonics) causing triplen harmonic currents in the neutral (Correct answer)
- An overloaded phase-to-phase circuit
- A broken neutral at the service entrance panel
Correct answer: Excessive non-linear loads (harmonics) causing triplen harmonic currents in the neutral
Non-linear loads (computers, VFDs, LED drivers) produce triplen harmonic currents (3rd, 9th, 15th, etc.) that add arithmetically in the neutral rather than canceling, causing neutral current to exceed phase current.
CEP Installation & Maintenance Practices 5
During commissioning of a 3-phase motor, the technician measures 480V L1-L2, 480V L2-L3, but only 277V L1-L3.
What is the MOST likely problem?