IEC Electrical Calculations and Theory 3 — Questions and Answers
Question 1: A 120V circuit has a 15Ω load. How much power does the load consume?
- 480W
- 960W (Correct answer)
- 8W
- 1,800W
Correct answer: 960W
P = V² / R = 120² / 15 = 14,400 / 15 = 960W.
Question 2: Inductive reactance (XL) increases when:
- Frequency decreases
- Inductance decreases
- Frequency increases (Correct answer)
- Resistance increases
Correct answer: Frequency increases
XL = 2πfL, so inductive reactance is directly proportional to frequency.
Question 3: What is the apparent power (VA) of a 3-phase, 208V system drawing 25A per phase?
- 5,200 VA
- 9,004 VA (Correct answer)
- 15,600 VA
- 18,009 VA
Correct answer: 9,004 VA
S = √3 × V × I = 1.732 × 208 × 25 ≈ 9,004 VA.
Question 4: Two 100W, 120V incandescent lamps are connected in series across 120V. What is the total power consumed?
- 200W
- 100W
- 50W (Correct answer)
- 25W
Correct answer: 50W
Each lamp's resistance = 120²/100 = 144Ω; series total = 288Ω; P = 120²/288 = 50W.
Question 5: The power factor of a purely resistive circuit is:
- 0
- 0.5
- 0.707
- 1.0 (Correct answer)
Correct answer: 1.0
A purely resistive circuit has no reactive power, so PF = real power / apparent power = 1.0.
Question 6: A conductor's resistance increases when its:
- Cross-sectional area increases
- Length decreases
- Temperature increases (Correct answer)
- Conductivity increases
Correct answer: Temperature increases
Most conductors have a positive temperature coefficient, meaning resistance rises as temperature increases.
Question 7: What is the skin effect in AC conductors?
- Current flows uniformly throughout the conductor cross-section
- Current concentrates near the outer surface, increasing effective AC resistance (Correct answer)
- DC resistance is higher than AC resistance
- Magnetic fields cancel inside the conductor at low frequency
Correct answer: Current concentrates near the outer surface, increasing effective AC resistance
At AC frequencies, current density is highest near the conductor surface, reducing usable cross-section and increasing resistance.
IEC Electrical Calculations and Theory 3
A 120V circuit has a 15Ω load.
How much power does the load consume?