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Transformers and Distribution Systems Flashcards

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  1. Per NEC Article 100, which of the following best defines a 'separately derived system'?

    Answer: A system with power derived from a source having no direct electrical connection to the supply conductors of another system

    A separately derived system, per NEC Article 100, derives its power from a source with no direct electrical connection to circuit conductors of any other system, such as a transformer secondary.

  2. When connecting two single-phase transformers in parallel, which of the following is NOT a requirement for proper parallel operation?

    Answer: Identical kVA ratings

    Transformers operating in parallel do not need identical kVA ratings; they must have the same voltage ratios, polarity, and impedance percentages for proper load sharing.

  3. Which formula correctly calculates transformer efficiency?

    Answer: (Output Power ÷ Input Power) × 100

    Efficiency = (Output Power ÷ Input Power) × 100; input power equals output power plus all internal losses (core losses and copper losses).

  4. A 45 kVA, single-phase, 240V/120V transformer is operating at full load. What is the rated primary current?

    Answer: 187.5A

    Primary rated current = kVA × 1,000 ÷ primary voltage = 45,000 ÷ 240 = 187.5A.

  5. According to NEC 450.11, which of the following information is required on a transformer nameplate?

    Answer: Rated kVA, frequency, and primary and secondary voltages

    NEC 450.11 requires transformer nameplates to show rated kVA, frequency, primary and secondary voltages, temperature class, and other electrical ratings needed for proper installation.

  6. Transformer inrush current occurs when power is first applied and can typically reach how many times the rated full-load current?

    Answer: 6–10 times

    Transformer inrush current, caused by residual flux and core saturation when energized, can reach 6–10 times the rated full-load current for a brief period.

  7. A 480V/120V step-down transformer supplies a 2,400W load at 120V. What is the approximate primary current drawn from the 480V supply, assuming 100% efficiency?

    Answer: 5A

    Primary current = load power ÷ primary voltage = 2,400W ÷ 480V = 5A, since input VA equals output VA at 100% efficiency.