CSLB C-10 Electrical ContractorElectrical Theory and CalculationsMedium
A 480V, 3-phase, 60 Hz transformer is connected Delta on the primary side and Wye on the secondary side. If the secondary line-to-line voltage is 208V, what is the approximate phase voltage on the secondary side?
- A480V
- B240V
- C208V
- D120V
Show answer & explanationAnswer & explanation
Correct answer: D. 120V
In a Wye (star) connected secondary, the line-to-line voltage (V_L) is √3 times the phase voltage (V_P). Therefore, V_P = V_L / √3. V_P = 208V / 1.732 ≈ 120V.
Why the other options are wrong
- A. This is the primary voltage and not relevant to the secondary phase voltage calculation.
- B. This is a common single-phase voltage, but not directly related to 208V Wye phase voltage.
- C. This is the line-to-line voltage, not the phase voltage.
Wye (Star) Connection Voltages
In a Wye (star) three-phase connection, the line-to-line voltage is √3 times the phase voltage, and the line current is equal to the phase current.
- V_L = √3 * V_P (Line voltage equals √3 times phase voltage).
- I_L = I_P (Line current equals phase current).
- Provides a neutral point for single-phase loads.
- Commonly used for distribution systems (e.g., 208Y/120V).
Memory trick: Wye is Root 3, Delta is Direct.