Practice Test Geeks(PC) Professional Electrical Contractor Practice Test
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Transformers and Distribution Equipment Flashcards

7 cards from real PC practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

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  1. A transformer has a primary voltage of 480V and a secondary voltage of 120V. What is the turns ratio (primary to secondary)?

    Answer: 4:1

    The turns ratio equals primary voltage divided by secondary voltage: 480V ÷ 120V = 4:1.

  2. Which NEC article governs the installation and protection of transformers?

    Answer: Article 450

    NEC Article 450 covers transformers, including overcurrent protection, installation requirements, and marking.

  3. A delta-wye transformer configuration is commonly used in electrical distribution primarily because it:

    Answer: Provides voltage transformation and establishes a neutral point on the secondary

    A delta-wye transformer steps voltage up or down and provides a neutral conductor on the wye secondary, essential for 208Y/120V or 480Y/277V distribution systems.

  4. What is the primary purpose of taps on a transformer primary winding?

    Answer: To adjust secondary voltage output when primary voltage varies from nominal

    Taps allow the primary turns to be adjusted so the secondary voltage remains correct even when utility supply voltage is slightly above or below nominal.

  5. A 75 kVA single-phase transformer supplies a 60 kW resistive load at unity power factor. What is the approximate load percentage?

    Answer: 80%

    At unity power factor, kVA equals kW, so the load percentage is 60 kW ÷ 75 kVA = 80%.

  6. Per NEC 450.8, which marking must appear on a transformer nameplate?

    Answer: Rated kVA, frequency, primary and secondary voltage, and impedance

    NEC 450.8 requires transformer nameplates to include rated kVA, frequency, primary and secondary voltage, impedance (on transformers 25 kVA and larger), and other information.

  7. What does a transformer's impedance percentage (%Z) represent?

    Answer: The percentage of rated primary voltage required to circulate full-load current with the secondary shorted

    %Z is determined by measuring what percentage of rated primary voltage is needed to drive rated current through a bolted secondary short circuit, indicating voltage regulation and fault current contribution.