IEC Electrical Calculations and Theory 2 — Questions and Answers
Question 1: A 240V single-phase circuit supplies a 4,800W resistive load. What is the current draw?
- 10A
- 20A (Correct answer)
- 24A
- 48A
Correct answer: 20A
I = P / V = 4,800 / 240 = 20A.
Question 2: What is the total resistance of a 10Ω and 15Ω resistor connected in parallel?
- 25Ω
- 6Ω (Correct answer)
- 12.5Ω
- 150Ω
Correct answer: 6Ω
1/R = 1/10 + 1/15 = 5/30, so R = 6Ω.
Question 3: A 3-phase, 480V motor draws 30A at 0.85 power factor. What is the true (real) power consumed?
- 12,470W
- 21,168W (Correct answer)
- 24,941W
- 43,200W
Correct answer: 21,168W
P = √3 × V × I × PF = 1.732 × 480 × 30 × 0.85 ≈ 21,168W.
Question 4: What happens to total capacitance when two capacitors are wired in series?
- It equals the sum of both values
- It is greater than either individual value
- It is less than the smallest individual value (Correct answer)
- It doubles
Correct answer: It is less than the smallest individual value
Series capacitors combine via the reciprocal formula, always yielding a total less than the smallest capacitor.
Question 5: In AC circuits, the term 'impedance' refers to:
- Pure resistance only
- Inductive reactance only
- The total opposition to current including resistance and reactance (Correct answer)
- Capacitive reactance only
Correct answer: The total opposition to current including resistance and reactance
Impedance (Z) is the vector sum of resistance (R) and reactance (X) in an AC circuit.
Question 6: A single-phase transformer has a 480V primary and 120V secondary. If the secondary delivers 10A, what is the primary current (ignoring losses)?
- 2.5A (Correct answer)
- 10A
- 40A
- 0.25A
Correct answer: 2.5A
I_primary = I_secondary × (V_secondary / V_primary) = 10 × (120/480) = 2.5A.
Question 7: Using NEC guidelines, what is the maximum recommended voltage drop for a feeder circuit?
- 1%
- 2%
- 3% (Correct answer)
- 5%
Correct answer: 3%
NEC recommends no more than 3% voltage drop on feeders and 5% total for feeders plus branch circuits combined.
IEC Electrical Calculations and Theory 2
A 240V single-phase circuit supplies a 4,800W resistive load.
What is the current draw?