IEC Electrical Calculations and Theory 5 โ Questions and Answers
Question 1: A 120V single-phase circuit runs 250 ft (one-way) using 12 AWG copper (1.98ฮฉ/1000 ft). What is the round-trip voltage drop at 15A?
- 3.71V
- 7.43V
- 14.85V (Correct answer)
- 1.49V
Correct answer: 14.85V
VD = 2 ร 250 ร (1.98/1000) ร 15 = 14.85V.
Question 2: A power factor correction capacitor bank is added to a facility. The primary benefit is:
- Increasing system voltage above rated levels
- Reducing true (real) power consumption
- Reducing reactive power demand and improving power factor (Correct answer)
- Eliminating harmonic distortion
Correct answer: Reducing reactive power demand and improving power factor
Capacitor banks supply reactive power locally, reducing reactive current from the utility and improving power factor.
Question 3: What is the RMS voltage of a sine wave with a peak voltage of 170V?
- 120V (Correct answer)
- 141V
- 155V
- 170V
Correct answer: 120V
V_rms = V_peak / โ2 = 170 / 1.414 โ 120V.
Question 4: A delta-connected transformer secondary has a phase voltage of 240V. What is the line voltage?
- 138V
- 240V (Correct answer)
- 415V
- 480V
Correct answer: 240V
In a delta connection, line voltage equals phase voltage: V_line = V_phase = 240V.
Question 5: The time constant (ฯ) of an RC series circuit determines:
- The steady-state current
- The rate at which the capacitor charges or discharges (Correct answer)
- The resonant frequency of the circuit
- The maximum voltage across the resistor
Correct answer: The rate at which the capacitor charges or discharges
ฯ = R ร C; after one time constant the capacitor reaches about 63.2% of supply voltage.
Question 6: In a balanced 3-phase wye (Y) system, the line current equals:
- โ3 times the phase current
- Phase current divided by โ3
- The phase current (Correct answer)
- 3 times the phase current
Correct answer: The phase current
In a wye connection each line conductor carries only one phase winding's current, so I_line = I_phase.
Question 7: A 10kW continuous resistive load on 240V single-phase requires a circuit breaker sized at 125%. What is the minimum standard breaker size?
- 40A
- 50A
- 60A (Correct answer)
- 70A
Correct answer: 60A
I = 10,000/240 = 41.67A; ร 1.25 = 52.08A; the next standard breaker size above 52A is 60A.
IEC Electrical Calculations and Theory 5
A 120V single-phase circuit runs 250 ft (one-way) using 12 AWG copper (1.98ฮฉ/1000 ft).
What is the round-trip voltage drop at 15A?