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Missouri (MO) Residential Journeyman Electrician Practice Exam
Question 1: Which of the following formulas represents Ohm’s Law? A. P = V × I B. V = I × R C. I = P / V D. R = V / P Answer: B Explanation: Ohm’s Law states that the voltage (V) across a resistor equals the current (I) through it multiplied by its resistance (R): V = I × R. Question 2: If a resistor has a resistance of 10 Ω and a current of 2 A flows through it, what is the voltage across the resistor? A. 5 V B. 10 V C. 20 V D. 40 V Answer: C Explanation: Using Ohm’s Law, V = I × R = 2 A × 10 Ω = 20 V. Question 3: Which formula correctly calculates electrical power? A. P = I / V B. P = V / I C. P = I × R D. P = V × I Answer: D Explanation: Electrical power is the product of voltage and current: P = V × I. Question 4: A device operates at 120 V and draws 5 A. What is its power consumption? A. 600 W B. 240 W C. 125 W D. 720 W Answer: A Explanation: Power P = V × I = 120 V × 5 A = 600 W.
Question 5: In a series circuit, the total resistance is: A. The sum of the individual resistances B. The reciprocal of the sum of the reciprocals C. The product of the individual resistances D. Zero Answer: A Explanation: In a series circuit, resistances add directly; hence, the total resistance is the sum of all individual resistances. Question 6: In a parallel circuit, the voltage across each branch is: A. Different for each branch B. Equal to the supply voltage C. Half of the supply voltage D. Determined by the resistor with the highest value Answer: B Explanation: In a parallel circuit, each branch is directly connected to the supply so the voltage across each branch equals the supply voltage. Question 7: Which unit is used to measure electrical current? A. Volts B. Ohms C. Amperes D. Watts Answer: C Explanation: Electrical current is measured in amperes (A). Question 8: Which unit measures electrical resistance? A. Watts B. Amperes C. Volts D. Ohms Answer: D Explanation: Electrical resistance is measured in ohms (Ω). Question 9: The unit “watt” is a measure of: A. Voltage B. Power C. Current
Explanation: Fuses are typically shown as a rectangle with a gap or break, symbolizing its function to “blow” (open) when excessive current flows. Question 14: What does the term “voltage drop” mean? A. An increase in voltage along a conductor B. A decrease in voltage as current flows through resistance C. The current measured in a circuit D. The power loss in watts Answer: B Explanation: Voltage drop is the reduction in voltage across a conductor or device caused by the resistance within that element. Question 15: When calculating voltage drop for a conductor, which factor is generally NOT considered? A. Conductor length B. Conductor material C. Ambient temperature (in basic calculations) D. Current flowing through the conductor Answer: C Explanation: Basic voltage drop calculations primarily consider conductor length, material (which defines resistance), and current. Ambient temperature is more relevant to ampacity than simple voltage drop. Question 16: When sizing conductors, which factor is most critical? A. Insulation color B. Conductor length and current-carrying requirements C. Voltage drop only D. Manufacturer’s brand Answer: B Explanation: Proper conductor sizing depends on both the length of the conductor and the amount of current it must safely carry. Question 17: In residential load calculations, which of the following loads is typically considered continuous? A. Refrigerator B. Kitchen lighting loads C. Water heater D. Well pump
Answer: B Explanation: Lighting loads are generally treated as continuous loads because they can remain on for extended periods (three hours or more), whereas appliances like refrigerators cycle on and off. Question 18: A circuit operating at 120 V and drawing 15 A is delivering: A. 1,800 W B. 1,200 W C. 15 W D. 120 W Answer: A Explanation: Using P = V × I, the power is 120 V × 15 A = 1,800 W. Question 19: What is a key consideration when selecting a conductor for a circuit? A. Conductor color B. The conductor’s ampacity C. The number of insulation layers D. The distance from the service panel (by itself) Answer: B Explanation: Ampacity, the maximum current a conductor can safely carry, is critical when selecting the proper wire size. Question 20: With a constant voltage, how is current related to resistance? A. Directly proportional B. Inversely proportional C. Unrelated D. Exponentially related Answer: B Explanation: For a constant voltage, Ohm’s Law (I = V/R) shows that current is inversely proportional to resistance. Question 21: What best describes a “load” in an electrical circuit? A. The circuit’s resistance B. A device or component that consumes power C. The supply current D. The insulating material Answer: B
Explanation: In a split‐phase system, the voltage between the two hot wires is typically 240 V, while each hot to neutral is 120 V. Question 26: Which component is typically located at the service entrance of a residence? A. Lighting fixtures B. Meter socket C. Outlet boxes D. Dimmer switches Answer: B Explanation: The meter socket, where the utility meter is mounted, is a key component at the service entrance. Question 27: What is the primary purpose of a disconnecting means at the service entrance? A. To control lighting circuits B. To safely shut off the electrical supply C. To boost voltage for heavy loads D. To regulate current flow during normal operation Answer: B Explanation: A disconnecting means allows the entire electrical supply to be safely shut off during maintenance or in an emergency. Question 28: Service equipment such as circuit breakers is designed primarily to provide: A. Voltage regulation B. Overcurrent protection C. Enhanced circuit speed D. Power factor correction Answer: B Explanation: Circuit breakers protect circuits by interrupting current flow when it exceeds safe levels, thereby providing overcurrent protection. Question 29: When selecting a service panel, which factor is most critical? A. The panel’s color B. Total ampacity and number of circuits C. Conductor material D. Length of the bus bars Answer: B Explanation: A service panel must have enough ampacity and enough circuit spaces to safely accommodate all the residence’s electrical circuits.
Question 30: Which device is most commonly used for overcurrent protection in residential systems? A. Transformer B. Capacitor C. Circuit breaker D. Relay Answer: C Explanation: Circuit breakers are the primary overcurrent protection devices in residential wiring. Question 31: Which statement best describes the purpose of grounding in electrical systems? A. Connecting the system to the earth to provide a path for fault current B. Insulating all system components C. Increasing system voltage D. Reducing circuit current Answer: A Explanation: Grounding connects the electrical system to the earth, providing a safe path for fault currents and helping to prevent electric shock. Question 32: What is the primary purpose of bonding in electrical installations? A. To insulate electrical components B. To ensure all metallic parts are at the same potential C. To boost current capacity D. To separate circuits from one another Answer: B Explanation: Bonding electrically connects all metallic parts so they share the same potential, reducing shock hazards. Question 33: In a residential service panel, the neutral bus bar is used to: A. Provide a fault current path to ground B. Carry the return current back to the source C. Distribute power equally among circuits D. Protect against overvoltage Answer: B Explanation: The neutral bus bar carries the return current from circuits back to the power source, completing the circuit.
Question 38: Which formula is used to calculate the voltage drop across a conductor? A. Voltage drop = I × R B. Voltage drop = V / I C. Voltage drop = P / I D. Voltage drop = I² × R Answer: A Explanation: The voltage drop is calculated by multiplying the current (I) by the resistance (R) of the conductor. Question 39: In voltage drop calculations, the conductor’s resistance is often expressed in: A. Volts per foot B. Ohms per 1,000 feet C. Amperes per meter D. Watts per kilometer Answer: B Explanation: Conductor resistance is typically given in ohms per 1,000 feet, which is used in voltage drop calculations. Question 40: A split-phase electrical system: A. Uses a single-phase 240 V supply B. Divides a 240 V supply into two 120 V circuits C. Uses three-phase power D. Provides variable voltage levels Answer: B Explanation: A split-phase system divides the 240 V supply into two 120 V circuits (with the two hot conductors 180° out of phase). Question 41: When sizing a service conductor, the most important factor is: A. The number of outlets B. The calculated load in amperes C. The type of lighting used D. The insulation color Answer: B Explanation: The conductor is sized based on the total expected current load (in amperes) to ensure safe operation. Question 42: What is the primary function of a meter socket at the service entrance? A. To regulate voltage
B. To house the utility meter and provide a connection for service conductors C. To act as a junction box for interior circuits D. To convert AC to DC power Answer: B Explanation: The meter socket holds the utility meter and provides the connection point for the conductors entering the building. Question 43: Which device is typically used as a service disconnecting means? A. A fuse holder B. A lockable circuit breaker or switch C. A voltage regulator D. A surge protector Answer: B Explanation: A lockable circuit breaker or switch is used as a service disconnect so that the entire electrical supply can be shut off safely. Question 44: In service equipment, the term “ampacity” refers to: A. The voltage rating B. The maximum current a conductor or device can carry safely C. The physical size of the conductor D. The efficiency of the device Answer: B Explanation: Ampacity is the maximum current that a conductor or device can handle without exceeding its safe temperature limits. Question 45: When installing a service panel, which is a key consideration? A. The distance from the street B. Proximity to combustible materials C. The number of branch circuits available D. The type of insulation on conductors Answer: C Explanation: The service panel must have a sufficient number of circuit spaces (branch circuits) to meet the building’s electrical needs. Question 46: What is the purpose of a bonding jumper in a residential electrical system? A. To connect the hot conductors to the circuit breakers B. To connect the neutral and ground bus bars C. To distribute power to branch circuits
Answer: B Explanation: Copper is widely used because of its high conductivity, durability, and relative affordability.
Services and Service Equipment (Service Entrance, Panels, Grounding,
Overcurrent Protection)
Question 51: In the context of a service entrance, “disconnecting means” refers to: A. A device that automatically adjusts voltage B. A device that allows the power to be disconnected from the building C. A tool for splicing conductors D. A grounding rod Answer: B Explanation: “Disconnecting means” is a device (such as a circuit breaker or switch) that provides a safe way to disconnect all power entering the building. Question 52: Which factor can affect the ampacity of a conductor? A. Insulation color B. Ambient temperature C. Conductor length D. Voltage drop Answer: B Explanation: Ambient temperature affects a conductor’s ability to dissipate heat and thus its ampacity. Question 53: To ensure proper grounding when installing a service panel, it is essential to: A. Install a backup battery B. Connect the grounding electrode conductor to a grounding electrode system C. Use double-insulated conductors D. Paint the panel a bright color Answer: B Explanation: Proper grounding is achieved by connecting the service panel’s grounding conductor to an approved grounding electrode system (such as a ground rod). Question 54: Which device is used to protect individual branch circuits from overcurrent? A. Main disconnect switch B. Circuit breaker C. Transformer
D. Surge protector Answer: B Explanation: Circuit breakers protect individual branch circuits by tripping (opening) when current exceeds safe levels. Question 55: In residential wiring, “service entrance conductors” are: A. Wires connecting the service panel to branch circuits B. Conductors that bring electrical power from the utility to the building C. Wires connecting lighting fixtures D. Grounding conductors only Answer: B Explanation: Service entrance conductors carry power from the utility’s service point (or meter) into the building’s service panel. Question 56: One primary safety reason for proper bonding in an electrical system is to: A. Minimize energy consumption B. Reduce electromagnetic interference C. Ensure all metallic parts remain at the same electrical potential D. Increase power distribution efficiency Answer: C Explanation: Proper bonding ensures that all exposed metal parts share the same potential, thereby reducing the risk of electric shock. Question 57: Which of the following is NOT part of a service entrance installation? A. Meter socket B. Service disconnecting means C. Distribution panel D. Interior lighting fixtures Answer: D Explanation: Interior lighting fixtures are part of the branch circuits, not the service entrance installation. Question 58: When calculating voltage drop over a long conductor run, which factor is most important? A. Insulation color B. Conductor cross-sectional area C. Wire age D. Manufacturer’s name
Explanation: Load diversity acknowledges that not all devices will run at their maximum rating at the same time, allowing for more efficient system design. Question 63: When selecting a circuit breaker for a branch circuit, which rating is most important? A. Physical size B. Ampere rating relative to the circuit load C. Color of the breaker D. Number of poles Answer: B Explanation: The breaker’s ampere rating must be appropriate for the circuit’s expected load to provide proper overcurrent protection. Question 64: In a split‐phase residential system, the voltage between one hot conductor and neutral is typically: A. 60 V B. 120 V C. 240 V D. 480 V Answer: B Explanation: In split‐phase systems, each hot-to-neutral voltage is typically 120 V, with 240 V between the two hots. Question 65: A “main service disconnect” in residential wiring is used to: A. Control individual circuits B. Disconnect the entire electrical supply C. Regulate appliance voltage D. Convert AC to DC Answer: B Explanation: The main service disconnect is intended to shut off all electrical power to the residence for safety during maintenance or emergencies. Question 66: In an overcurrent condition, a circuit breaker will: A. Increase voltage B. Open the circuit to interrupt current C. Divert current to another circuit D. Permanently damage the circuit Answer: B
Explanation: When an overcurrent is detected, the circuit breaker trips (opens the circuit) to stop the current flow, protecting the wiring and devices. Question 67: One primary safety purpose of proper grounding is to: A. Improve system efficiency B. Provide a safe path for fault current to reduce shock risk C. Lower circuit breaker ratings D. Balance circuit loads Answer: B Explanation: Grounding ensures that fault currents have a low-resistance path to earth, reducing the risk of shock and equipment damage. Question 68: Which overcurrent protection device is designed to be reset after tripping? A. Fuse B. Circuit breaker C. GFCI D. Surge protector Answer: B Explanation: Circuit breakers can be reset after they trip, whereas fuses must be replaced once blown. Question 69: In conductor sizing, why is it important to consider voltage drop? A. To prevent conductor overheating B. To keep the voltage at the load within acceptable limits C. To increase energy efficiency D. To reduce conductor size Answer: B Explanation: Adequate sizing helps ensure that voltage drop remains low enough so that the connected equipment receives the correct voltage. Question 70: With aluminum conductors in residential wiring, it is recommended to: A. Use them in all circuits without modification B. Use special connectors or anti-oxidation compounds C. Omit grounding D. Ignore voltage drop concerns Answer: B Explanation: Aluminum requires special connectors and, at times, antioxidant compounds to prevent oxidation and ensure reliable electrical connections.
Question 75: A key requirement for a service disconnecting means per code is that it must be: A. Installed in a locked cabinet B. Readily accessible (exterior or interior as allowed) C. Painted red D. Installed in the basement only Answer: B Explanation: Codes require that the service disconnect be easily accessible to allow for rapid shutdown in emergencies. Question 76: When calculating a conductor’s ampacity, which temperature rating is typically used? A. The insulation’s maximum operating temperature B. The ambient installation temperature C. The conductor’s color temperature D. The service panel temperature Answer: A Explanation: The insulation’s rated maximum operating temperature is used to determine the safe ampacity of a conductor. Question 77: The “duty cycle” of an electrical device refers to: A. The percentage of time it operates at full load B. Its voltage rating C. Its operating frequency D. Its operating resistance Answer: A Explanation: Duty cycle is the proportion of time a device is actively operating at full load compared to its total operating time. Question 78: In a residential electrical service, which conductor is typically bonded to ground? A. Hot conductor B. Neutral conductor C. Both hot and neutral D. None Answer: B Explanation: The neutral conductor is normally bonded to ground at the service entrance, establishing a reference potential for the system.
Question 79: An acceptable method for calculating load requirements for branch circuits is to: A. Assume a fixed 100 W per circuit B. Sum device wattages and apply demand factors C. Only consider lighting wattage D. Use the largest device’s wattage for the whole circuit Answer: B Explanation: The proper method is to total the wattages of connected devices and then apply demand factors because not all devices run simultaneously at full capacity. Question 80: When selecting a fuse for a branch circuit, what is most important? A. Physical size B. Current rating, slightly above normal operating current C. Fuse holder color D. Circuit breaker voltage rating Answer: B Explanation: The fuse’s current rating should be chosen so it is just above the circuit’s normal current but will still protect against overcurrent. Question 81: If a service panel is not properly bonded to the grounding electrode system, the potential consequence is: A. Increased power efficiency B. Shock hazards and possible malfunction of overcurrent devices C. Lower circuit breaker ratings D. Improved load balancing Answer: B Explanation: Improper bonding can create dangerous voltage differences between metal parts, leading to shock hazards and interfering with protective devices. Question 82: In residential wiring, the term “branch circuit” refers to: A. The service conductor from the utility B. Wiring from the service panel to outlets, lighting, and appliances C. Wiring from the panel to the meter D. Grounding wiring only Answer: B Explanation: A branch circuit is the set of wiring that runs from the service panel to the various devices and outlets within the residence.
