CSLB C-10 Electrical ContractorElectrical Theory and CalculationsMedium
A 2400V primary transformer drawing 5 amps is used to step down voltage to 120V on the secondary. What is the approximate current in the secondary winding, assuming an ideal transformer?
- A50 A
- B25 A
- C100 A
- D200 A
Show answer & explanationAnswer & explanation
Correct answer: C. 100 A
For an ideal transformer, the power in the primary equals the power in the secondary (Pp = Ps), so Vp × Ip = Vs × Is. Therefore, Is = (Vp × Ip) / Vs = (2400V × 5A) / 120V = 12000VA / 120V = 100A.
Why the other options are wrong
- A. This would be if the voltage ratio was 1:48 instead of 1:20.
- B. This is a significant underestimation, likely a calculation error.
- D. This would be if the voltage ratio was much higher, or a miscalculation.
Transformer Current Ratio
For an ideal transformer, the ratio of primary current to secondary current is inversely proportional to the ratio of primary voltage to secondary voltage (Ip/Is = Vs/Vp).
- Based on the principle of power conservation (VpIp = VsIs).
- Step-down transformers increase current.
- Assumes no losses (ideal transformer).
Memory trick: Current Ratio is Inverse to Voltage Ratio, Power Stays the Same.