PC Transformers and Distribution Equipment

Question 1

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

Accepted Answer

4:1

Answer

2:1

Answer

8:1

Answer

6:1

Question 2

Which NEC article governs the installation and protection of transformers?

Accepted Answer

Article 450

Answer

Article 230

Answer

Article 480

Answer

Article 410

Question 3

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

Accepted Answer

Provides voltage transformation and establishes a neutral point on the secondary

Answer

Eliminates the need for overcurrent protection

Answer

Reduces transformer losses to zero

Answer

Requires no grounding on either winding

Question 4

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

Accepted Answer

To adjust secondary voltage output when primary voltage varies from nominal

Answer

To increase the transformer's kVA rating

Answer

To provide multiple secondary voltages simultaneously

Answer

To reduce the transformer's impedance percentage

Question 5

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

Accepted Answer

80%

Answer

60%

Answer

75%

Answer

85%

Question 6

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

Accepted Answer

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

Answer

Maximum ambient temperature only

Answer

Only the primary voltage and kVA rating

Answer

Manufacturer's serial number and country of origin only

Question 7

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

Accepted Answer

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

Answer

The ratio of primary to secondary turns in the winding

Answer

The transformer efficiency at rated full load

Answer

The power factor of the transformer under normal load conditions