Electrical Circuits
15 free questions with worked answers, and 161 more in the full bank.
Free questions
A technician is running a new 50A branch circuit to a range. They pull 8 AWG THHN copper wire through an attic where the ambient temperature reaches 120°F. During inspection, the installation fails. Why?
4 marks · Hard
While remodeling a residential bathroom, an apprentice wires a new 20A receptacle directly to the vanity light switch circuit without GFCI protection. What specific code violation has occurred?
3 marks · Medium
A homeowner complains that a newly installed bedroom outlet trips the breaker whenever a lamp is turned on. The panel has a dual-function AFCI/GFCI breaker. What is the most likely cause?
1 mark · Easy
During an inspection of a commercial kitchen, you notice the 125V, 15A receptacles near the sinks lack GFCI protection. The manager claims they are exempt because they are dedicated for refrigerators. How should you respond?
3 marks · Medium
An electrician installs a 100A subpanel in a detached garage using a 3-wire feeder (two hots, one neutral) and bonds the neutral to the ground bus at the subpanel. What is the primary safety risk here?
4 marks · Hard
A maintenance tech uses a megohmmeter to test a 480V 3-phase pump motor. The reading from phase A to ground is 0.5 MΩ. What should the tech conclude?
3 marks · Medium
You place a clamp-on ammeter around an extension cord powering a running 12A space heater, but the meter reads 0.00A. What is the reason for this reading?
1 mark · Easy
A technician measures voltage at a standard 120V receptacle and gets 120V between hot and neutral, but 0V between neutral and ground, and 60V between hot and ground. What is the most likely fault?
3 marks · Medium
When checking a 3-phase panel, you read 120V from Phase A to Neutral, 120V from Phase B to Neutral, and 208V from Phase C to Neutral. What type of electrical system are you testing?
4 marks · Hard
An industrial oven keeps tripping its breaker intermittently. A tech checks the amp draw and finds it steady at 38A on a 40A breaker. Why is the breaker tripping?
3 marks · Medium
You are replacing a 10 HP, 460V, 3-phase motor. The nameplate lists the Full Load Amps (FLA) as 14.0A and the Service Factor as 1.15. What is the maximum setting for the overload relays according to general NEC rules?
3 marks · Medium
A large induction motor utilizes a star-delta starter to reduce inrush current. During the start sequence, the motor starts in star but trips the main breaker the exact moment it transitions to delta. What is the most likely cause?
4 marks · Hard
An operator reports that a 3-phase conveyor motor is "humming loudly and getting extremely hot" but not turning. Voltage measurements at the motor peckerhead show 480V between L1-L2, 0V between L2-L3, and 0V between L1-L3. What is happening?
3 marks · Medium
When wiring a dual-voltage (230/460V) 9-lead 3-phase motor for high voltage (460V), how should the windings be connected?
1 mark · Easy
A technician is sizing the branch circuit conductors for a single 25 HP, 460V, 3-phase motor with a table FLC of 34A. What is the minimum ampacity required for these conductors?
3 marks · Medium
More Electrical Circuits questions (page 1 of 3)
Answers and explanations are in the full bank.
- A facility installs a large capacitor bank at the main switchgear to improve a lagging power factor of 0.75. After installation, they notice high transient overvoltages and VFDs tripping on overvoltage during off-shift hours. What went wrong? · 4 marks
- An industrial plant has a true power of 400 kW and an apparent power of 500 kVA. What is the plant power factor, and is it lagging or leading if the load is primarily induction motors? · 3 marks
- A consultant recommends adding capacitors to a motor control center. What is the primary financial benefit of correcting a poor power factor in an industrial facility? · 1 mark
- A synchronous motor is operating in a plant. The operator increases the DC excitation field current above the normal level. What effect will this have on the plant electrical system? · 3 marks
- When a system has a power factor of exactly 1.0 (Unity PF), what is the relationship between the voltage and current waveforms? · 3 marks
- You measure the voltage on a 3-phase wye-connected system. The line-to-line voltage is 480V. What should the line-to-neutral voltage measure? · 3 marks
- A 3-phase delta-connected heater has three heating elements, each with a resistance of 10 ohms. The line voltage is 480V. If one element burns out (opens), what happens to the total heat output? · 4 marks
- A 3-phase, 4-wire wye system is supplying a heavily unbalanced lighting load. You measure 15A on Phase A, 10A on Phase B, and 0A on Phase C. Will there be current on the neutral wire? · 3 marks
- In a balanced 3-phase system, what is the phase angle displacement between each of the three voltage waveforms? · 1 mark
- A technician is checking rotation on a 3-phase motor. The motor is spinning backwards. What is the standard procedure to correct this? · 3 marks
- A 45kVA step-down transformer (480V delta primary to 208Y/120V secondary) is loaded with 100A per phase on the secondary. What is the approximate primary line current? · 4 marks
- A transformer nameplate shows a primary voltage of 2400V and a secondary voltage of 240V. If the primary has 1000 turns, how many turns does the secondary have? · 3 marks
- During routine maintenance, a technician notices a loud buzzing noise and high heat coming from a dry-type transformer. Which of the following is the most likely cause of the excess heat? · 1 mark
- A 75kVA transformer is fully loaded. It is feeding a 208V, 3-phase panel. What is the maximum full-load secondary current this transformer can provide before being overloaded? · 3 marks
- You find a liquid-filled transformer at a substation with its pressure relief device deployed and dark oil leaking. A DGA (Dissolved Gas Analysis) reveals high levels of acetylene. What does this indicate? · 4 marks
- A main 1200A breaker trips simultaneously with a downstream 100A branch breaker during a short circuit on the branch circuit. This is an example of a failure in what design principle? · 3 marks
- Why are time-delay (dual-element) fuses commonly used in motor circuits instead of fast-acting fuses? · 1 mark
- An electrician needs to rack out a 480V air circuit breaker. The arc flash label indicates an incident energy of 12 cal/cm² at the working distance. What PPE category is required minimum? · 4 marks
- While performing a Lockout/Tagout (LOTO) procedure on a pump motor, an operator shuts off the local disconnect, applies a lock, and tags it. What critical step MUST be performed next before starting work? · 3 marks
- According to NFPA 70E, what is the boundary called where unqualified personnel are strictly prohibited from crossing unless escorted by a qualified person and wearing proper PPE? · 1 mark
- A hospital IT system utilizes an isolated power system (IPS) in an operating room. The line isolation monitor (LIM) alarms at 5mA. What does this alarm indicate? · 4 marks
- In an IEC-style TN-C-S earthing system, what happens to the PEN (Protective Earth & Neutral) conductor when it enters the customer premises? · 3 marks
- What is the primary purpose of the equipment grounding conductor (bare copper or green wire) in a standard 120V branch circuit? · 1 mark
- A commercial office building with hundreds of LED drivers and computers has a neutral conductor running very hot in the 3-phase wye feeder. The phase currents are perfectly balanced. What is causing the overheating? · 3 marks
- You hook up a power quality analyzer to a Variable Frequency Drive (VFD) input and notice a Total Harmonic Distortion (THD-V) of 8%. The facility manager says it is fine. What is the standard IEEE 519 recommendation for voltage THD at the point of common coupling? · 4 marks
- During routine technical work, which statement about Kirchhoff's voltage law should guide the technician? · 3 marks
- During routine technical work, which statement about Kirchhoff's current law should guide the technician? · 3 marks
- During routine technical work, which statement about current in a series circuit should guide the technician? · 3 marks
- During routine technical work, which statement about voltage in a parallel circuit should guide the technician? · 3 marks
- During routine technical work, which statement about a gold tolerance band on a resistor should guide the technician? · 3 marks
- During routine technical work, which statement about fuse placement in a single-phase load circuit should guide the technician? · 3 marks
- During routine technical work, which statement about a circuit breaker should guide the technician? · 3 marks
- During routine technical work, which statement about residual-current protection should guide the technician? · 3 marks
- During routine technical work, which statement about RMS voltage for a sine wave should guide the technician? · 3 marks
- During routine technical work, which statement about electrical impedance should guide the technician? · 3 marks
- During routine technical work, which statement about inductive reactance should guide the technician? · 3 marks
- During routine technical work, which statement about capacitive reactance should guide the technician? · 3 marks
- During routine technical work, which statement about an ideal transformer turns ratio should guide the technician? · 3 marks
- During routine technical work, which statement about an electromagnetic relay should guide the technician? · 3 marks
- During routine technical work, which statement about a contactor should guide the technician? · 3 marks
- During routine technical work, which statement about three-phase power should guide the technician? · 3 marks
- During routine technical work, which statement about a star-connected three-phase load should guide the technician? · 3 marks
- During routine technical work, which statement about a protective earthing conductor should guide the technician? · 3 marks
- During a workshop assessment, the reported condition is: A series lighting circuit is completely dark, and a continuity test finds one lamp filament open. What diagnosis best explains the whole circuit symptom? · 1 mark
- A component drops 28.6 V while carrying 1.3 A. What is its resistance? · 1 mark
- Equipment in a Kigali district office runs at 400 V and draws 12.7 A. What power does it consume? · 1 mark
- A cable of 55 Ω carries 0.7 A. How much power is dissipated as heat in the cable? · 3 marks
- Three resistors of 113 Ω, 57 Ω and 88 Ω are connected in series. What is the total resistance? · 1 mark
- Two resistors of 104 Ω and 22 Ω are wired in parallel. What is the combined resistance? · 3 marks
- A 1200 W appliance runs for 8 hours. At 105 RWF per kWh, what does that cost? · 3 marks
- In a divider across 48 V, R₁ = 76 Ω and R₂ = 31 Ω. What voltage appears across R₂? · 3 marks
- A sinusoidal supply has a peak value of 43 V. What is its RMS value? · 3 marks
- In a TVET training workshop four loads are measured: 12 V across 52 Ω, 400 V across 22 Ω, 24 V across 108 Ω, 48 V across 12 Ω. Which dissipates the least power? · 3 marks
- A 3 kW heater runs from a 230 V supply on a circuit protected by a 6 A breaker. Will that circuit carry it? · 4 marks
- If the cross-sectional area of a conductor is doubled, what happens to its resistance? · 3 marks
- A resistor of 117 Ω carries a current of 2.2 A. What voltage appears across it? · 1 mark
- Equipment in a sector office network runs at 230 V and draws 9.2 A. What power does it consume? · 1 mark
- In a health-centre server room four loads are measured: 24 V across 23 Ω, 230 V across 21 Ω, 12 V across 49 Ω, 400 V across 58 Ω. Which draws the largest current? · 3 marks
- Equipment in a school computer lab runs at 230 V and draws 9.5 A. What power does it consume? · 1 mark
- In a sector office network four loads are measured: 400 V across 20 Ω, 48 V across 113 Ω, 48 V across 105 Ω, 12 V across 10 Ω. Which dissipates the most power? · 3 marks