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Journeyman Electrician Exam (NEC-based)

Transformers and generators: practice questions

Our Journeyman Electrician bank has 4 questions on this topic. Here are 4 of them with answers; the full mock exam draws from every topic, like the real one.

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Question 1

What is the full-load current of a 75 kVA, 480-volt three-phase transformer primary (approx.)?

  1. 180 amps
  2. 156 amps
  3. 90 amps
  4. 52 amps
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C. 90 amps — I = kVA × 1000 ÷ (√3 × V) = 75,000 ÷ (1.732 × 480) ≈ 90 A.

Question 2

What does a transfer switch do?

  1. Switches the building's load between the utility and a standby source
  2. Grounds the generator
  3. Increases voltage
  4. Stores energy
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A. Switches the building's load between the utility and a standby source — It prevents both sources being connected at once.

Question 3

What is a 'separately derived system'?

  1. A service
  2. A feeder from the main panel
  3. A system, such as a transformer secondary, with no direct electrical connection to the supply conductors except through grounding
  4. A branch circuit
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C. A system, such as a transformer secondary, with no direct electrical connection to the supply conductors except through grounding — It needs its own system bonding jumper and grounding.

Question 4

What must be provided with a portable generator connected to a building's wiring?

  1. Nothing
  2. A transfer switch to prevent backfeeding the utility
  3. A larger neutral only, so the generator can carry the full building load
  4. A GFCI on the service
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B. A transfer switch to prevent backfeeding the utility — Backfeed can electrocute utility workers.

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