CSLB C-10 Electrical ContractorElectrical Theory and CalculationsHard
A 480V, 3-phase motor has a full-load current of 65 amps. If the motor's power factor is 0.85 and its efficiency is 90%, what is the approximate apparent power drawn by the motor?
- A63.7 kVA
- B31.2 kVA
- C72.4 kVA
- D53.8 kVA
Show answer & explanationAnswer & explanation
Correct answer: D. 53.8 kVA
Apparent power (S) for a 3-phase system is calculated using the formula S = (√3 * V_L * I_L) / 1000 for kVA. S = (1.732 * 480V * 65A) / 1000 = 53.8 kVA. Power factor and efficiency are not needed for apparent power calculation.
Why the other options are wrong
- A. This might come from incorrectly including power factor or efficiency in the calculation for apparent power, or a calculation error.
- B. This value is too low, possibly from an incorrect square root factor or division error.
- C. This value is too high, indicating a possible multiplication error or incorrect formula application.
Three-Phase Apparent Power
Apparent power (S) in a three-phase circuit is the product of the line voltage, line current, and the square root of 3, expressed in volt-amperes (VA) or kilovolt-amperes (kVA).
- Formula: S = √3 * V_L * I_L (for VA) or S = (√3 * V_L * I_L) / 1000 (for kVA).
- V_L is line-to-line voltage, I_L is line current.
- Apparent power includes both real and reactive power.
- Power factor and efficiency are not directly used to calculate apparent power itself.
Memory trick: Apparent is Hypotenuse, Real is Adjacent, Reactive is Opposite.