Journeyman Electrician Exam (NEC)General Electrical Knowledge and Plan ReadingMedium
A client's industrial facility has a 480V, 3-phase, 3-wire delta electrical service. If a balanced resistive load draws 30A from each line, what is the total apparent power (S) consumed by the load?
- A7.2 kVA
- B41.6 kVA
- C24.9 kVA
- D14.4 kVA
Show answer & explanationAnswer & explanation
Correct answer: C. 24.9 kVA
For a 3-phase system, apparent power (S) = √3 * V_line * I_line. Given V_line = 480V and I_line = 30A. S = √3 * 480V * 30A = 1.732 * 480 * 30 = 24940.8 VA, which is approximately 24.9 kVA.
Why the other options are wrong
- A. This is likely a result of dividing by √3 or other incorrect calculation.
- B. This might result from using 3 * V_phase * I_phase or other incorrect 3-phase formula.
- D. This is calculated as V_line * I_line (single phase) or incorrect use of constants.
3-Phase Apparent Power
Apparent power (S) in a 3-phase system is the product of the line voltage, line current, and the square root of 3. It is measured in Volt-Amperes (VA).
- S = √3 * V_line * I_line
- Includes both real and reactive power components.
- Used for sizing electrical equipment like transformers and generators.
Memory trick: Three-phase power, 'Root Three VI' for apparent, 'Root Three VI Cosine' for real.