Master Electrician Exam (NEC)General Electrical KnowledgeMedium
A newly installed motor in a commercial facility draws 50A at 480V, 3-phase. The motor nameplate indicates a power factor of 0.85. What is the approximate apparent power (S) consumed by this motor?
- A20.4 kVA
- B24.0 kVA
- C41.6 kVA
- D35.4 kVA
Show answer & explanationAnswer & explanation
Correct answer: C. 41.6 kVA
For a 3-phase circuit, apparent power (S) is calculated as S = (√3 * V * I) / 1000 for kVA. S = (1.732 * 480V * 50A) / 1000 = 41.568 kVA, which approximates to 41.6 kVA. The power factor is used for calculating real power, not apparent power directly from V and I.
Why the other options are wrong
- A. Incorrect. This value might be obtained by multiplying V*I*PF (real power) and then dividing by 1.732.
- B. Incorrect. This value might be obtained from a miscalculation or incorrect formula application.
- D. Incorrect. This value represents the real power (P = √3 * V * I * PF = 41.568 kVA * 0.85 = 35.33 kVA).
Apparent Power in 3-Phase Systems
Apparent power (S) is the total power flowing in an AC circuit, measured in Volt-Amperes (VA), and includes both real and reactive power. For a 3-phase system, it's calculated using the line-to-line voltage and line current.
- S = √3 * V_line * I_line
- Units are Volt-Amperes (VA) or kiloVolt-Amperes (kVA)
- Power factor is not directly used in this calculation but relates apparent to real power.
Memory trick: SARAH: S is Apparent, R is Real, H is Hypotenuse of VAR triangle.