Journeyman Electrician Exam (NEC)General Electrical Knowledge and Plan ReadingMedium
A three-phase transformer supplies a balanced resistive load. The line-to-line voltage is 480V, and each phase current is 10A. What is the total apparent power (S) delivered by the transformer?
- A16.63 kVA
- B4.8 kVA
- C8.31 kVA
- D14.4 kVA
Show answer & explanationAnswer & explanation
Correct answer: C. 8.31 kVA
For a three-phase system, the total apparent power (S) is calculated as S = √3 × V_line × I_line. Given V_line = 480V and I_line = 10A, S = √3 × 480V × 10A = 1.732 × 4800 VA = 8313.6 VA, which is approximately 8.31 kVA.
Why the other options are wrong
- A. Incorrect; likely a miscalculation of √3 or other factors.
- B. Incorrect; this would be (V_line * I_line) for a single phase, not three phase with √3.
- D. Incorrect; possibly 3 * V_line * I_line, which is not the correct formula.
Three-Phase Apparent Power
In a three-phase system, apparent power (S) is the total power that appears to be consumed by the load, including both real and reactive power, calculated using line-to-line voltage and line current.
- Formula: S = √3 × V_line × I_line.
- Expressed in Volt-Amperes (VA) or kiloVolt-Amperes (kVA).
- Used for sizing transformers, generators, and OCPDs.
Memory trick: Root 3 for Line values, P for Real, S for Apparent, Q for Reactive.