Journeyman Electrician’s Exam Transformers and Electrical Theory 3

Question 1

Per NEC 450.21(A), dry-type transformers rated 600V or less must maintain a minimum separation of how far from combustible materials?

Accepted Answer

12 inches

Answer

6 inches

Answer

18 inches

Answer

No separation if listed

Question 2

What does a transformer's K-factor rating indicate?

Accepted Answer

Suitability for non-linear (harmonic) loads

Answer

Voltage regulation percentage

Answer

Thermal efficiency at full load

Answer

Short-circuit withstand capability

Question 3

In a 3-phase delta-wye transformer with a 480V secondary, what is the phase-to-neutral (phase) voltage on the wye secondary?

Accepted Answer

277 volts

Answer

480 volts

Answer

240 volts

Answer

120 volts

Question 4

What is the primary purpose of a buck-boost transformer?

Accepted Answer

Make small line voltage corrections of approximately 5–20%

Answer

Step voltage up from low to high in large increments

Answer

Provide galvanic isolation between primary and secondary

Answer

Convert single-phase power to three-phase power

Question 5

Per NEC 210.9, an autotransformer used to supply branch circuits must have:

Accepted Answer

A common neutral conductor connected to the grounded circuit conductor

Answer

A delta-wye internal configuration

Answer

Separate, isolated primary and secondary windings

Answer

A maximum voltage ratio of 2:1

Question 6

Per NEC 250.30, where must the grounding electrode conductor and system bonding jumper for a separately derived system (such as a transformer secondary) be connected?

Accepted Answer

At the source of the separately derived system or its first means of disconnect

Answer

At the main service entrance panel

Answer

At the first downstream overcurrent device only

Answer

At each branch circuit panelboard fed by the transformer