CSLB C-10 Electrical ContractorElectrical Theory and CalculationsHard
A client is planning an installation where a 480V, 3-phase feeder will supply a step-down transformer to provide 208Y/120V power. If the primary winding has 1200 turns, how many turns are required on the secondary winding for the 208V line-to-line output?
- A600 turns
- B450 turns
- C520 turns
- D300 turns
Show answer & explanationAnswer & explanation
Correct answer: C. 520 turns
For a three-phase transformer with a Wye secondary, the line-to-line secondary voltage is 208V, and the line-to-neutral voltage is 120V. The turns ratio is based on the line-to-line voltages. Vp/Vs = Np/Ns. So, 480V / 208V = 1200 turns / Ns. Solving for Ns: Ns = (208V × 1200 turns) / 480V = 520 turns.
Why the other options are wrong
- A. This would result in 240V secondary line-to-line, if the ratio was 2:1 from 480V.
- B. Incorrect calculation, potentially misapplying the Wye voltage relationship or turns ratio.
- D. This would result in a lower secondary voltage, possibly from using 120V instead of 208V in the calculation.
Transformer Turns Ratio (Three-Phase)
The turns ratio for a three-phase transformer relates the primary and secondary line-to-line voltages to the number of turns in each winding, similar to single-phase transformers.
- Vp/Vs = Np/Ns (Vp = primary line voltage, Vs = secondary line voltage).
- For Wye secondary, line-to-line voltage is √3 times line-to-neutral voltage.
- Ensures accurate voltage transformation for balanced three-phase loads.
Memory trick: Voltage Ratio is Turns Ratio, but remember Line-to-Line for 3-Phase.