IEC Transformers and Distribution Systems 3

Question 1

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

Accepted Answer

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

Answer

A system with a dedicated ground fault circuit interrupter

Answer

A system that uses a separate grounding electrode conductor only

Answer

A system powered by a generator with its own neutral bus

Question 2

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

Accepted Answer

Identical kVA ratings

Answer

Same voltage ratios (turns ratio)

Answer

Same polarity

Answer

Same impedance percentages

Question 3

Which formula correctly calculates transformer efficiency?

Accepted Answer

(Output Power ÷ Input Power) × 100

Answer

(Input Power ÷ Output Power) × 100

Answer

(Core Losses ÷ Copper Losses) × 100

Answer

(kVA ÷ kW) × 100

Question 4

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

Accepted Answer

187.5A

Answer

93.75A

Answer

375A

Answer

750A

Question 5

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

Accepted Answer

Rated kVA, frequency, and primary and secondary voltages

Answer

Weight, dimensions, and country of manufacture

Answer

Core material type and winding wire gauge

Answer

Manufacturing date and serial number only

Question 6

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

Accepted Answer

6–10 times

Answer

1–2 times

Answer

3–5 times

Answer

15–20 times

Question 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?

Accepted Answer

5A

Answer

10A

Answer

20A

Answer

40A