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. Per NEC 450.3, when a transformer has primary overcurrent protection set at no more than 125% of rated current, secondary overcurrent protection:

    Answer: May be omitted if primary protection does not exceed 125% of primary rated current

    NEC 450.3 Table 450.3(B) permits omission of secondary overcurrent protection for transformers 600V and under when primary protection does not exceed 125% of rated primary current.

  2. What is the primary difference between a switchboard and a metal-clad switchgear assembly?

    Answer: Metal-clad switchgear is fully enclosed with drawout components providing greater protection than open-construction switchboards

    Metal-clad switchgear uses enclosed drawout circuit breakers with interlocks and bus barriers, providing higher protection levels than switchboards which are typically accessible from the front only.

  3. A 225 kVA, 480V/240V single-phase transformer operates at full load. What is the approximate full-load secondary current?

    Answer: 938A

    Full-load secondary current = kVA × 1,000 ÷ secondary voltage = 225,000 ÷ 240 = 937.5A ≈ 938A.

  4. A K-rated transformer (e.g., K-13) is specifically designed to handle:

    Answer: Non-linear loads that generate harmonic currents

    K-rated transformers use heavier neutral conductors and upgraded windings to tolerate the additional eddy-current heating caused by harmonic currents from non-linear loads such as variable frequency drives and computers.

  5. In a typical commercial distribution system, primary distribution feeders in urban areas operate at what voltage range?

    Answer: 4,160V to 15,000V

    Urban primary distribution feeders typically operate at medium voltages between 4,160V and 15,000V (commonly 4.16 kV, 12.47 kV, or 13.2 kV) before being stepped down to utilization voltage at the service point.

  6. Which transformer cooling designation indicates an oil-immersed, forced-oil-cooled, forced-air-cooled unit?

    Answer: OFAF

    OFAF stands for Oil Forced (pumped oil circulation) / Air Forced (fans on radiators), distinguishing it from ONAN (natural oil, natural air convection cooling).

  7. When two transformers are to be operated in parallel, which condition is NOT required for proper load sharing?

    Answer: Identical kVA ratings

    Transformers operating in parallel do not need identical kVA ratings; however, mismatched kVA ratings cause load sharing to be proportional to each unit's rating based on their impedances.