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System Design and Installation Flashcards

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  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?

    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.

  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?

    Answer: 1/0 AWG

    NEC Table 250.66 requires a 1/0 AWG copper grounding electrode conductor for service conductors of 350–600 kcmil.

  3. In a fire pump installation, what is the minimum voltage drop permitted at the motor terminals under locked-rotor conditions?

    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.

  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)?

    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.

  5. A master electrician is designing a switchboard installation. Per NEC 408.18, what is the minimum headroom required in front of a switchboard?

    Answer: 6.5 feet

    NEC 408.18(A) requires a minimum headroom of 6.5 feet in front of switchboards.

  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)?

    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.

  7. Which NEC article governs the design and installation requirements for emergency systems in commercial buildings?

    Answer: Article 700

    NEC Article 700 covers emergency systems that are legally required and essential for safety to human life.