IEC Electrical Calculations and Theory 5

Question 1

A 120V single-phase circuit runs 250 ft (one-way) using 12 AWG copper (1.98Ω/1000 ft). What is the round-trip voltage drop at 15A?

Accepted Answer

14.85V

Answer

VD = 2 × 250 × (1.98/1000) × 15 = 14.85V.

Question 2

A power factor correction capacitor bank is added to a facility. The primary benefit is:

Accepted Answer

Reducing reactive power demand and improving power factor

Answer

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?

Accepted Answer

120V

Answer

V_rms = V_peak / √2 = 170 / 1.414 ≈ 120V.

Question 4

A delta-connected transformer secondary has a phase voltage of 240V. What is the line voltage?

Accepted Answer

240V

Answer

In a delta connection, line voltage equals phase voltage: V_line = V_phase = 240V.

Question 5

The time constant (τ) of an RC series circuit determines:

Accepted Answer

The rate at which the capacitor charges or discharges

Answer

τ = R × C; after one time constant the capacitor reaches about 63.2% of supply voltage.

IEC - Independent Electrical Contractors Practice Test