Master Electrician Exam (NEC)General Electrical KnowledgeMedium
A master electrician is troubleshooting a transformer in a 480V primary, 208Y/120V secondary system. The transformer has a turns ratio of 2:1. If the primary voltage is correctly measured at 480V, what would be the expected line-to-line voltage on the secondary side of an ideal transformer with this ratio?
- A240V
- B120V
- C208V
- D277V
Show answer & explanationAnswer & explanation
Correct answer: A. 240V
For an ideal transformer, the voltage ratio is directly proportional to the turns ratio. A 2:1 turns ratio means the secondary voltage is half of the primary voltage. 480V primary / 2 = 240V secondary (line-to-line). The 208Y/120V rating indicates the intended system voltage, but the ideal transformer's turns ratio directly determines the voltage transformation.
Why the other options are wrong
- B. Incorrect. 120V is the line-to-neutral voltage of a 208Y/120V system, not the line-to-line voltage from a 2:1 ratio.
- C. Incorrect. 208V is the line-to-line voltage of the intended system, but an ideal transformer with a 2:1 ratio from 480V would produce 240V.
- D. Incorrect. 277V is typically found in a 480Y/277V system, not a 2:1 ratio from 480V to a 208Y/120V system.
Transformer Turns Ratio and Voltage
For an ideal transformer, the ratio of the number of turns in the primary coil to the number of turns in the secondary coil is equal to the ratio of the primary voltage to the secondary voltage.
- N_p / N_s = V_p / V_s
- Step-down transformer: N_p > N_s, V_p > V_s
- Step-up transformer: N_p < N_s, V_p < V_s
- This relationship assumes an ideal transformer with no losses.
Memory trick: TURNS to VOLTS: Direct Ratio Rules.