Journeyman Electrician’s Exam Transformers and Electrical Theory

Question 1

A single-phase transformer has a primary voltage of 480V and a secondary voltage of 120V. What is the turns ratio?

Accepted Answer

4:1

Answer

Turns ratio = primary voltage ÷ secondary voltage = 480 ÷ 120 = 4:1, confirming this is a step-down transformer.

Question 2

What is the full-load secondary current of a 25 kVA, 480/240V single-phase transformer?

Accepted Answer

104.2 amperes

Answer

I = VA ÷ V = 25,000 ÷ 240 = 104.2 amperes full-load secondary current.

Question 3

Per NEC Table 450.3(B), what is the maximum primary overcurrent protection for a transformer whose primary current is 9 amperes or more (with no secondary protection)?

Accepted Answer

125% of primary current rating

Answer

NEC Table 450.3(B) limits primary-only protection to 125% of the transformer's rated primary current when primary current is ≥9A.

Question 4

Transformer efficiency is calculated by comparing:

Accepted Answer

Output power to input power

Answer

Efficiency (%) = (Output power ÷ Input power) × 100; losses reduce efficiency below 100%.

Question 5

Which 3-phase transformer connection provides a neutral conductor on the secondary side for single-phase loads?

Accepted Answer

Delta-wye

Answer

A delta-wye transformer provides a neutral on the wye secondary, enabling 3-phase 4-wire systems with single-phase branch circuits.

Journeyman Electrician’s Exam Practice Test

Journeyman Electrician’s Exam Practice Test

Journeyman Electrician’s Exam Transformers and Electrical Theory