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

2:1

Answer

8:1

Answer

1:4

Question 2

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

Accepted Answer

104.2 amperes

Answer

52.1 amperes

Answer

26 amperes

Answer

208 amperes

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

100% of primary current rating

Answer

150% of primary current rating

Answer

250% of primary current rating

Question 4

Transformer efficiency is calculated by comparing:

Accepted Answer

Output power to input power

Answer

Input voltage to output voltage

Answer

Primary current to secondary current

Answer

Primary turns to secondary turns

Question 5

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

Accepted Answer

Delta-wye

Answer

Delta-delta

Answer

Wye-delta

Answer

Open delta

Question 6

In a 3-phase, 4-wire wye system where the line-to-line voltage is 208V, what is the line-to-neutral (phase) voltage?

Accepted Answer

120 volts

Answer

208 volts

Answer

360 volts

Answer

240 volts