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

NEC 450.21(A) requires at least 12 inches of separation between a dry-type transformer and combustible material unless a fire-resistant barrier is installed.

Question 2

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

Accepted Answer

Suitability for non-linear (harmonic) loads

Answer

A transformer's K-factor rating indicates how well it can handle harmonic currents generated by non-linear loads such as computers and VFDs.

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

Phase voltage = 480 ÷ √3 = 277V; the 480V wye secondary is a 277/480V system used for commercial lighting and power.

Question 4

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

Accepted Answer

Make small line voltage corrections of approximately 5–20%

Answer

Buck-boost transformers are used to raise (boost) or lower (buck) line voltage by small amounts—typically 5–20%—to correct for voltage drop or overvoltage.

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

NEC 210.9 requires autotransformers supplying branch circuits to have a common neutral bonded to the grounded (neutral) conductor of the system.

Journeyman Electrician’s Exam Practice Test

Journeyman Electrician’s Exam Practice Test

Journeyman Electrician’s Exam Transformers and Electrical Theory 3