IEC Electrical Calculations and Theory 4

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?

Accepted Answer

5A

Answer

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:

Accepted Answer

The algebraic sum of all voltages around any closed loop equals zero

Answer

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:

Accepted Answer

Lead voltage by 90°

Answer

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?

Accepted Answer

31.8 Hz

Answer

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?

Accepted Answer

41.4 kW

Answer

Output = 50 × 0.746 = 37.3 kW; Input = 37.3 / 0.90 ≈ 41.4 kW.

IEC - Independent Electrical Contractors Practice Test