Journeyman Electrician Exam (NEC)General Electrical Knowledge and Plan ReadingHard
A three-phase motor draws 20A per phase at 480V and has a power factor of 0.85. What is the approximate apparent power (S) drawn by the motor?
- A28.8 kVA
- B16.6 kVA
- C33.2 kVA
- D14.1 kVA
Show answer & explanationAnswer & explanation
Correct answer: B. 16.6 kVA
For a three-phase circuit, apparent power (S) = √3 × V_L × I_L, where V_L is line-to-line voltage and I_L is line current. S = √3 × 480V × 20A = 1.732 × 480 × 20 = 16,627.2 VA, which is approximately 16.6 kVA. The power factor is used for calculating real power (P = S × PF), but not apparent power.
Why the other options are wrong
- A. This might be 2 * V * I (single-phase) or an incorrect application of factors.
- C. This is an incorrect calculation, possibly from using 2 * V * I and then multiplying by 0.85, or a miscalculation of √3.
- D. This might be a calculation for real power (P = √3 * V * I * PF = 16.6 kVA * 0.85 = 14.1 kVA).
3-Phase Apparent Power
Apparent power (S) in a 3-phase system is the total power that appears to be supplied by the source, measured in Volt-Amperes (VA).
- Formula: S = √3 × V_L × I_L, where V_L is line-to-line voltage and I_L is line current.
- It is the vector sum of real power (P) and reactive power (Q).
- Power factor (PF) is the ratio of real power to apparent power (PF = P/S).
Memory trick: S: 'S'quare root of 3, 'V'olts, 'I'nside.