CME System Design and Installation 2 — Questions and Answers
Question 1: When designing a service entrance for a commercial building, what is the minimum clearance required above a flat roof (not readily accessible) for service-drop conductors per NEC 230.24?
- 8 feet
- 10 feet
- 3 feet (Correct answer)
- 18 inches
Correct answer: 3 feet
NEC 230.24(A) Exception No. 2 permits 3 feet above a roof not readily accessible where the voltage does not exceed 300V between conductors.
Question 2: A 480/277V three-phase four-wire system is being designed for a large office building. Which grounding electrode conductor size is required for a 500 kcmil copper service conductor per NEC Table 250.66?
- 1/0 AWG (Correct answer)
- 2/0 AWG
- 3/0 AWG
- 4/0 AWG
Correct answer: 1/0 AWG
NEC Table 250.66 requires a 1/0 AWG copper grounding electrode conductor for service conductors of 350–600 kcmil.
Question 3: In a fire pump installation, what is the minimum voltage drop permitted at the motor terminals under locked-rotor conditions?
- 5% voltage drop
- Voltage must not drop below 65% of motor nameplate voltage (Correct answer)
- Voltage must not drop below 70% of motor nameplate voltage
- 15% voltage drop
Correct answer: Voltage must not drop below 65% of motor nameplate voltage
NFPA 20 and NEC 695.7 require that the voltage at fire pump motor terminals shall not drop below 65% of nameplate voltage under locked-rotor conditions.
Question 4: When installing a paralleled conductor set for a feeder, what is the minimum conductor size permitted for each parallel conductor per NEC 310.10(H)?
- 2/0 AWG (Correct answer)
- 3/0 AWG
- 1/0 AWG
- 250 kcmil
Correct answer: 2/0 AWG
NEC 310.10(H)(1) requires each parallel conductor to be at least 1/0 AWG for copper or 1/0 AWG for aluminum, but practical parallel sets begin at 1/0; the minimum is 1/0 AWG.
Question 5: A master electrician is designing a switchboard installation. Per NEC 408.18, what is the minimum headroom required in front of a switchboard?
- 6 feet
- 6.5 feet (Correct answer)
- 7 feet
- 8 feet
Correct answer: 6.5 feet
NEC 408.18(A) requires a minimum headroom of 6.5 feet in front of switchboards.
Question 6: When sizing a transformer for a commercial HVAC system with a 75 kVA transformer, what is the maximum overcurrent protection device size permitted for the primary at 480V three-phase per NEC 450.3(B)?
- 125% of rated primary current
- 150% of rated primary current (Correct answer)
- 200% of rated primary current
- 250% of rated primary current
Correct answer: 150% of rated primary current
NEC 450.3(B) Table permits OCPD up to 125% of rated current for transformers over 600V; for transformers 600V or less, with secondary protection, up to 250%; without secondary OCPD, 150% for supervised locations.
Question 7: Which NEC article governs the design and installation requirements for emergency systems in commercial buildings?
- Article 700 (Correct answer)
- Article 701
- Article 702
- Article 708
Correct answer: Article 700
NEC Article 700 covers emergency systems that are legally required and essential for safety to human life.
CME System Design and Installation 2
When designing a service entrance for a commercial building, what is the minimum clearance required above a flat roof (not readily accessible) for service-drop conductors per NEC 230.24?