IEC Electrical Calculations and Theory 4 — Questions and Answers
Question 1: A 120/240V, single-phase, 3-wire service has a 20A load on phase A and a 15A load on phase B. What is the neutral current?
- 35A
- 5A (Correct answer)
- 17.5A
- 20A
Correct answer: 5A
In a single-phase 3-wire system, neutral current equals the difference between the two hot-leg currents: 20 − 15 = 5A.
Question 2: Kirchhoff's Voltage Law (KVL) states that:
- The sum of currents entering a node equals the sum leaving
- Voltage is the same across all parallel branches
- The algebraic sum of all voltages around any closed loop equals zero (Correct answer)
- Resistance is proportional to voltage
Correct answer: The algebraic sum of all voltages around any closed loop equals zero
KVL is based on conservation of energy: all voltage rises and drops around a closed loop sum to zero.
Question 3: A capacitor in an AC circuit causes current to:
- Lag voltage by 90°
- Lead voltage by 90° (Correct answer)
- Be in phase with voltage
- Lag voltage by 45°
Correct answer: Lead voltage by 90°
In a purely capacitive circuit, current leads voltage by 90° (mnemonic: ICE — current leads in capacitors).
Question 4: What is the resonant frequency of a series LC circuit with L = 50mH and C = 50µF?
- 100.7 Hz
- 31.8 Hz (Correct answer)
- 200 Hz
- 50 Hz
Correct answer: 31.8 Hz
f = 1 / (2π√LC) = 1 / (2π√(0.05 × 0.00005)) ≈ 31.8 Hz.
Question 5: A 480V, 3-phase motor produces 50 HP output at 90% efficiency. What is the input power in kW?
- 33.6 kW
- 37.3 kW
- 41.4 kW (Correct answer)
- 55.9 kW
Correct answer: 41.4 kW
Output = 50 × 0.746 = 37.3 kW; Input = 37.3 / 0.90 ≈ 41.4 kW.
Question 6: Kirchhoff's Current Law (KCL) states that:
- Voltage at a node equals the sum of branch voltages
- The sum of currents entering a node equals the sum of currents leaving it (Correct answer)
- Total resistance equals the sum of individual resistances
- Power factor is the ratio of real to apparent power
Correct answer: The sum of currents entering a node equals the sum of currents leaving it
KCL is based on conservation of charge: current flowing into any node must equal current flowing out.
Question 7: What is the inductive reactance of a 0.5H inductor at 60Hz?
- 30Ω
- 188.5Ω (Correct answer)
- 120Ω
- 376.9Ω
Correct answer: 188.5Ω
XL = 2πfL = 2π × 60 × 0.5 ≈ 188.5Ω.
IEC Electrical Calculations and Theory 4
A 120/240V, single-phase, 3-wire service has a 20A load on phase A and a 15A load on phase B.
What is the neutral current?