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 line-to-line voltage on the secondary side is 208V, what is the approximate line-to-neutral voltage on the secondary side?
- A208V
- B240V
- C160V
- D120V
Show answer & explanationAnswer & explanation
Correct answer: D. 120V
In a Wye (star) connected system, the line-to-neutral voltage (Vln) is equal to the line-to-line voltage (Vll) divided by the square root of 3 (√3).
Why the other options are wrong
- A. This option mistakes line-to-line for line-to-neutral voltage.
- B. This is a calculation error, possibly confusing it with a different voltage system.
- C. This is a calculation error, possibly using an incorrect divisor.
Wye Connection Voltages
In a Wye (star) three-phase connection, the line-to-line voltage is √3 times the line-to-neutral voltage, and phases share a common neutral point.
- Vll = √3 * Vln.
- Vln = Vll / √3.
- Provides a neutral point for single-phase loads.
Memory trick: Wye is 'Y' for 'Why' the Lines are Longer than Neutral!