Transformers and Electrical Theory Flashcards
6 cards from real Journeyman Electrician’s Exam practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 6 Transformers and Electrical Theory flashcards as text
A single-phase transformer has a primary voltage of 480V and a secondary voltage of 120V. What is the turns ratio?
Answer: 4:1
Turns ratio = primary voltage ÷ secondary voltage = 480 ÷ 120 = 4:1, confirming this is a step-down transformer.
What is the full-load secondary current of a 25 kVA, 480/240V single-phase transformer?
Answer: 104.2 amperes
I = VA ÷ V = 25,000 ÷ 240 = 104.2 amperes full-load secondary current.
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)?
Answer: 125% of primary current rating
NEC Table 450.3(B) limits primary-only protection to 125% of the transformer's rated primary current when primary current is ≥9A.
Transformer efficiency is calculated by comparing:
Answer: Output power to input power
Efficiency (%) = (Output power ÷ Input power) × 100; losses reduce efficiency below 100%.
Which 3-phase transformer connection provides a neutral conductor on the secondary side for single-phase loads?
Answer: Delta-wye
A delta-wye transformer provides a neutral on the wye secondary, enabling 3-phase 4-wire systems with single-phase branch circuits.
In a 3-phase, 4-wire wye system where the line-to-line voltage is 208V, what is the line-to-neutral (phase) voltage?
Answer: 120 volts
Phase voltage = line voltage ÷ √3 = 208 ÷ 1.732 ≈ 120V, the standard single-phase voltage in a 208Y/120V system.