CEI Commercial & Industrial Systems 3 — Questions and Answers
Question 1: In an industrial plant, a 200A feeder supplies a subpanel. The feeder conductors are 3/0 AWG THWN copper in conduit. What is the allowable ampacity of these conductors at 75°C?
- 165A
- 200A (Correct answer)
- 225A
- 260A
Correct answer: 200A
Per NEC Table 310.16, 3/0 AWG THWN copper has an allowable ampacity of 200A at 75°C.
Question 2: Which of the following grounding and bonding requirements applies to metal raceways in a commercial building?
- Metal raceways are optional as an equipment grounding conductor
- Metal raceways that qualify must be bonded to form a continuous EGC path (Correct answer)
- Only rigid conduit qualifies as an EGC
- Metal raceways never serve as an EGC
Correct answer: Metal raceways that qualify must be bonded to form a continuous EGC path
NEC 250.118 lists metal raceways such as RMC, IMC, and EMT as acceptable equipment grounding conductors when properly installed and bonded.
Question 3: A commercial kitchen uses a 208V, single-phase, 20A branch circuit for a commercial dishwasher. The circuit conductors must be at least what AWG size (copper, THHN, 75°C terminals)?
- 14 AWG
- 12 AWG (Correct answer)
- 10 AWG
- 8 AWG
Correct answer: 12 AWG
A 20A branch circuit requires conductors with an ampacity of at least 20A; 12 AWG copper at 75°C has an ampacity of 20A per NEC Table 310.16.
Question 4: Per NEC Article 440, what determines the branch-circuit rating for a hermetic refrigerant motor-compressor?
- Rated-load current (RLC) or branch-circuit selection current (BCSC), whichever is greater (Correct answer)
- Full-load current from NEC Table 430.248
- Locked-rotor current divided by 6
- Nameplate horsepower only
Correct answer: Rated-load current (RLC) or branch-circuit selection current (BCSC), whichever is greater
NEC 440.22 requires that the branch-circuit short-circuit protection be based on the greater of the RLC or the BCSC as marked on the nameplate.
Question 5: In a commercial building, what is the minimum burial depth for rigid metal conduit (RMC) installed under a parking lot?
- 6 in
- 12 in (Correct answer)
- 18 in
- 24 in
Correct answer: 12 in
NEC Table 300.5 requires RMC to be buried a minimum of 6 in, but under a parking lot (not a road subject to heavy vehicles), the standard minimum of 6 in for RMC applies; however, NEC 300.5 Column 1 sets 6 in for RMC in all locations.
Question 6: What is the purpose of a transfer switch in a commercial building's standby power system?
- To regulate generator voltage to utility levels
- To prevent simultaneous connection of the utility and generator to the load (Correct answer)
- To increase available fault current for overcurrent devices
- To balance loads between utility phases
Correct answer: To prevent simultaneous connection of the utility and generator to the load
A transfer switch ensures the load is connected to only one source at a time, preventing backfeed from the generator onto the utility line.
Question 7: Under NEC 517, a Type 1 Essential Electrical System in a hospital must restore power to life safety branch circuits within what time frame?
- 5 seconds
- 10 seconds (Correct answer)
- 30 seconds
- 60 seconds
Correct answer: 10 seconds
NEC 517.31 and 517.34 require the life safety branch of the Essential Electrical System to be restored within 10 seconds of a power interruption.
CEI Commercial & Industrial Systems 3
In an industrial plant, a 200A feeder supplies a subpanel.
The feeder conductors are 3/0 AWG THWN copper in conduit.
What is the allowable ampacity of these conductors at 75°C?